function.dd

Ddoc

$(SPEC_S Functions,

$(GRAMMAR
$(GNAME FunctionBody):
    $(GLINK2 statement, BlockStatement)
    $(GLINK BodyStatement)
    $(GLINK InStatement) $(GLINK BodyStatement)
    $(GLINK OutStatement) $(GLINK BodyStatement)
    $(GLINK InStatement) $(GLINK OutStatement) $(GLINK BodyStatement)
    $(GLINK OutStatement) $(GLINK InStatement) $(GLINK BodyStatement)

$(GNAME InStatement):
    $(D in) $(GLINK2 statement, BlockStatement)

$(GNAME OutStatement):
    $(D out) $(GLINK2 statement, BlockStatement)
    $(D out) $(D $(LPAREN)) $(I Identifier) $(D $(RPAREN)) $(GLINK2 statement, BlockStatement)

$(GNAME BodyStatement):
    $(D body) $(GLINK2 statement, BlockStatement)
)


$(H4 Function Return Values)

        $(P Function return values are considered to be rvalues.
        This means they cannot be passed by reference to other functions.
        )

$(H4 Functions Without Bodies)

	$(P Functions without bodies:)

---
int foo();
---

	$(P that are not declared as $(D abstract) are expected to have their implementations
	elsewhere, and that implementation will be provided at the link step.
	This enables an implementation of a function to be completely hidden from the user
	of it, and the implementation may be in another language such as C, assembler, etc.
	)

$(H4 $(LNAME2 pure-functions, Pure Functions))

        $(P Pure functions are functions which cannot access global or static, mutable
        state save through their arguments. This can enable optimizations based on the fact
        that a pure function is guaranteed to mutate nothing which isn't passed to it,
        and in cases where the compiler can guarantee that a pure function cannot
        alter its arguments, it can enable full, functional purity (i.e. the guarantee
        that the function will always return the same result for the same arguments).
        To that end, a pure function:
        )

        $(UL
        $(LI does not read or write any global or static mutable state)
        $(LI cannot call functions that are not pure)
        $(LI can override an impure function, but an impure function
        cannot override a pure one)
        $(LI is covariant with an impure function)
        $(LI cannot perform I/O)
        )

        $(P As a concession to practicality, a pure function can:)

        $(UL
        $(LI allocate memory via a $(GLINK2 expression, NewExpression))
        $(LI terminate the program)
        $(LI read and write the floating point exception flags)
        $(LI read and write the floating point mode flags, as long as those flags
        are restored to their initial state upon function entry)
        $(LI perform impure operations in statements that are in a
        $(GLINK2 version, ConditionalStatement)
        controlled by a $(GLINK2 version, DebugCondition).)
        )

        $(P A pure function can throw exceptions.)

---
import std.stdio;
int x;
immutable int y;
const int* pz;

pure int foo(int i,
             char* p,
             const char* q,
             immutable int* s)
{
  debug writeln("in foo()"); // ok, impure code allowed in debug statement
  x = i;   // error, modifying global state
  i = x;   // error, reading mutable global state
  i = y;   // ok, reading immutable global state
  i = *pz; // error, reading const global state
  return i;
}
---


$(H4 $(LNAME2 nothrow-functions, Nothrow Functions))

        $(P Nothrow functions do not throw any exceptions derived
        from class $(I Exception).
        )

        $(P Nothrow functions are covariant with throwing ones.)

$(H4 $(LNAME2 ref-functions, Ref Functions))

        $(P Ref functions allow functions to return by reference.
        This is analogous to ref function parameters.
        )

---
ref int foo() {
  auto p = new int;
  return *p;
}
...
foo() = 3;  // reference returns can be lvalues
---

$(H4 $(LNAME2 auto-functions, Auto Functions))

        $(P Auto functions have their return type inferred from any
        $(GLINK2 statement, ReturnStatement)s
        in the function body.
        )

        $(P An auto function is declared without a return type.
        If it does not already have a storage class, use the
        $(D_KEYWORD auto) storage class.
        )

        $(P If there are multiple $(I ReturnStatement)s, the types
        of them must match exactly. If there are no $(I ReturnStatement)s,
        the return type is inferred to be $(D_KEYWORD void).
        )

---
auto foo(int i) {
  return i + 3;  // return type is inferred to be int
}
---

$(H4 $(LNAME2 auto-ref-functions, Auto Ref Functions))

        $(P Auto ref functions infer their return type just as
        $(LINK2 auto-functions, auto functions) do.
        In addition, they become $(LINK2 ref-functions, ref functions)
        if the return expression is an lvalue,
        and it would not be a reference to a local or a parameter.
        )

---
auto ref foo(int x)     { return x; }  // value return
auto ref foo()          { return 3; }  // value return
auto ref foo(ref int x) { return x; }  // ref return
auto ref foo(out int x) { return x; }  // ref return
auto ref foo() { static int x; return x; }  // ref return
---

        $(P The lexically first $(GLINK2 statement, ReturnStatement)
        determines the ref-ness of a function:
        )

---
auto ref foo(ref int x) { return 3; return x; }  // ok, value return
auto ref foo(ref int x) { return x; return 3; }  // error, ref return, 3 is not an lvalue
---


$(H4 $(LNAME2 inout-functions, Inout Functions))

    $(P Functions that deal with mutable, const, or immutable types with
        equanimity often need to transmit their type to the return value:

        ---
        int[] foo(int[] a, int x, int y) { return a[x .. y]; }

        const(int)[] foo(const(int)[] a, int x, int y) { return a[x .. y]; }

        immutable(int)[] foo(immutable(int)[] a, int x, int y) { return a[x .. y]; }
        ---
    )

    $(P The code generated by these three functions is identical.
        To indicate that these can be one function, the $(D_KEYWORD inout)
        type constructor is employed:

        ---
        inout(int)[] foo(inout(int)[] a, int x, int y) { return a[x .. y]; }
        ---
    )

    $(P The $(D_KEYWORD inout) forms a wildcard that stands in for
        any of mutable, const, immutable, inout, or inout const. When the
        function is called, the inout of the return type is changed to whatever
        the mutable, const, immutable, inout, or inout const status of the
        argument type to the parameter inout was.
    )

    $(P Inout types can be implicitly converted to const or inout const,
        but to nothing else. Other types cannot be implicitly converted to inout.
        Casting to or from inout is not allowed in @safe functions.
    )

    $(P A set of arguments to a function with inout parameters is considered
        a match if any inout argument types match exactly, or:)

    $(OL
        $(LI No argument types are composed of inout types.)
        $(LI A mutable, const or immutable argument type can be matched against each
        corresponding parameter inout type.)
    )

    $(P If such a match occurs, the inout is considered the common qualifier of
        the matched qualifiers. If more than two parameters exist, the common
        qualifier calculation is recursively applied.
    )

    $(TABLE2 Common qualifier of the two type qualifiers,
        $(TROW , $(I mutable), $(D const), $(D immutable), $(D inout), $(D inout const))
        $(TROW $(I mutable) $(LPAREN)= m$(RPAREN), m, c, c, c, c)
        $(TROW $(D const) $(LPAREN)= c$(RPAREN), c, c, c, c, c)
        $(TROW $(D immutable) $(LPAREN)= i$(RPAREN), c, c, i, wc, wc)
        $(TROW $(D inout) $(LPAREN)= w$(RPAREN), c, c, wc, w, wc)
        $(TROW $(D inout const) $(LPAREN)= wc$(RPAREN), c, c, wc, wc, wc)
    )

    $(P The inout in the return type is then rewritten to be the inout matched
        qualifiers:

        ---
        int[] ma;
        const(int)[] ca;
        immutable(int)[] ia;

        inout(int)[] foo(inout(int)[] a) { return a; }
        void test1()
        {
            // inout matches to mutable, so inout(int)[] is
            // rewritten to int[]
            int[] x = foo(ma);

            // inout matches to const, so inout(int)[] is
            // rewritten to const(int)[]
            const(int)[] y = foo(ca);

            // inout matches to immutable, so inout(int)[] is
            // rewritten to immutable(int)[]
            immutable(int)[] z = foo(ia);
        }

        inout(const(int))[] bar(inout(int)[] a) { return a; }
        void test2()
        {
            // inout matches to mutable, so inout(const(int))[] is
            // rewritten to const(int)[]
            const(int)[] x = foo(ma);

            // inout matches to const, so inout(const(int))[] is
            // rewritten to const(int)[]
            const(int)[] y = foo(ca);

            // inout matches to immutable, so inout(int)[] is
            // rewritten to immutable(int)[]
            immutable(int)[] z = foo(ia);
        }
        ---
    )

    $(P $(B Note:) Shared types are not overlooked. Shared types cannot
        be matched with inout.
    )


$(H4 $(LNAME2 property-functions, Property Functions))

        $(P Property functions are tagged with the $(D @property)
        attribute. They cannot be called with parentheses (hence
        they act like fields except in some cases).
        )

        $(P If a property function has no parameters, it works as a getter.
        If has exactly one parameter, it works as a setter.
        )

        ---
        struct S {
          int m_x;
          @property {
            int x() { return m_x; }
            int x(int newx) { return m_x = newx; }

            int function(int, int) adder() {
              return function int(int a, int b) { return a + b; }
            }
          }
        }
        void main() {
          S s;
          s.x;        // lowered to s.x()
          s.x = 3;    // lowered to s.x(3)

          //s.x();    // NG: lowered to s.x()(), but the int value
                      //     returned by s.x() is not callable
          //s.x(3);   // NG: lowered to s.x()(3), but the int value
                      //     returned by s.x() is not callable
          assert(s.adder(1, 2) == 3);
                      // OK lowered to s.adder()(1, 2)
        }
        ---

        $(P If a getter property function returns a reference to other storage,
        it also works as a setter.
        )

        ---
        struct S {
          int m_x;
          @property ref int x() { return m_x; }
        }
        void main() {
          S s;
          int n = s.x;  // s.x()
          assert(s.m_x == n);
          s.x = 2;      // s.x() = 2;
          assert(s.m_x == 2);
        }
        ---

        $(P In most places, getter property functions are called immediately.
        One exceptional case is the address operator.
        )

        ---
        struct S {
          int m_x;
          @property     int x1() { return m_x; }
          @property ref int x2() { return m_x; }
        }
        void main() {
          S s;
          auto x1 = &s.x1;
          auto x2 = &s.x2;
          static assert(is(typeof(x1) == delegate));
          static assert(is(typeof(x2) == delegate));
          // Both x1 and x2 are delegate, not int pointer.

          int n = x1();
          assert(s.m_x == n);
          x2() = 1;
          assert(s.m_x == 1);
        }
        ---

        $(P Even if the given operand is a property function, the address operator
        returns the address of the property function rather than the address of
        its return value.
        )


$(H4 $(LNAME2 optional-parenthesis, Optional parenthesis))

        $(P If a function call does not take any arguments syntactically,
        it is callable without parenthesis, like a getter $(RELATIVE_LINK2
        property-functions, property functions).
        )

        ---
        void foo() {}   // no arguments
        void bar(int[] arr) {}  // for UFCS

        void main() {
          foo();        // OK
          foo;          // also OK

          int[] arr;
          arr.bar();    // OK
          arr.bar;      // also OK
        }
        ---

        $(P However, assignment syntax is disallowed unlike with property functions.
        )

        ---
        struct S {
          void foo(int) {}  // one argument
        }
        void main() {
          S s;
          s.foo(1);     // OK
          //s.foo = 1;  // disallowed
        }
        ---

$(H4 $(LNAME2 virtual-functions, Virtual Functions))

        $(P Virtual functions are functions that are called indirectly through a
        function pointer table, called a vtbl[], rather than directly. All
        $(D public) and $(D protected) member functions which are non-static and
        aren't templatized are virtual unless the compiler can determine that
        they will never be overridden (e.g. they're marked with $(D final) and
        don't override any functions in a base class), in which case, it will
        make them non-virtual. This results in fewer bugs caused by not
        declaring a function virtual and then overriding it anyway.
        )

        $(P Member functions which are $(D private) or $(D package) are never
        virtual, and hence cannot be overridden.
        )

        $(P Functions with non-D linkage cannot be virtual and hence cannot be
        overridden.
        )

        $(P Member template functions cannot be virtual and hence cannot be
        overridden.
        )

        $(P Functions marked as $(D final) may not be overridden in a
        derived class, unless they are also $(D private).
        For example:
        )

------
class A {
  int def() { ... }
  final int foo() { ... }
  final private int bar() { ... }
  private int abc() { ... }
}

class B : A {
  override int def() { ... }  // ok, overrides A.def
  override int foo() { ... }  // error, A.foo is final
  int bar() { ... }  // ok, A.bar is final private, but not virtual
  int abc() { ... }  // ok, A.abc is not virtual, B.abc is virtual
}

void test(A a) {
  a.def();    // calls B.def
  a.foo();    // calls A.foo
  a.bar();    // calls A.bar
  a.abc();    // calls A.abc
}

void func() {
  B b = new B();
  test(b);
}
------

        $(P Covariant return types
        are supported, which means that the
        overriding function in a derived class can return a type
        that is derived from the type returned by the overridden function:
        )

------
class A { }
class B : A { }

class Foo {
  A test() { return null; }
}

class Bar : Foo {
  override B test() { return null; } // overrides and is covariant with Foo.test()
}
------

        $(P Virtual functions all have a hidden parameter called the
        $(I this) reference, which refers to the class object for which
        the function is called.
        )

        $(P To avoid dynamic binding on member function call, insert
        base class name before the member function name. For example:
        )

------
class B {
  int foo() { return 1; }
}
class C : B {
  override int foo() { return 2; }

  void test() {
    assert(B.foo() == 1);  // translated to this.B.foo(), and
                           // calls B.foo statically.
    assert(C.foo() == 2);  // calls C.foo statically, even if
                           // the actual instance of 'this' is D.
  }
}
class D : C {
  override int foo() { return 3; }
}
void main() {
  auto d = new D();
  assert(d.foo() == 3);    // calls D.foo
  assert(d.B.foo() == 1);  // calls B.foo
  assert(d.C.foo() == 2);  // calls C.foo
  d.test();
}
------

$(H4 $(LNAME2 function-inheritance, Function Inheritance and Overriding))

        A function in a derived class with the same name and parameter
        types as a function in a base class overrides that function:

------
class A {
  int foo(int x) { ... }
}

class B : A {
  override int foo(int x) { ... }
}

void test() {
  B b = new B();
  bar(b);
}

void bar(A a) {
  a.foo(1);   // calls B.foo(int)
}
------

        $(P However, when doing overload resolution, the functions in the base
        class are not considered:
        )

------
class A {
  int foo(int x) { ... }
  int foo(long y) { ... }
}

class B : A {
  override int foo(long x) { ... }
}

void test() {
  B b = new B();
  b.foo(1);  // calls B.foo(long), since A.foo(int) not considered
  A a = b;

   a.foo(1);    // issues runtime error (instead of calling A.foo(int))
}
------

        $(P To consider the base class's functions in the overload resolution
        process, use an $(I AliasDeclaration):
        )

------
class A {
  int foo(int x) { ... }
  int foo(long y) { ... }
}

class B : A {
  $(CODE_HIGHLIGHT alias foo = A.foo;)
  override int foo(long x) { ... }
}

void test() {
  B b = new B();
  bar(b);
}

void bar(A a) {
  a.foo(1);      // calls A.foo(int)
  B b = new B();
  b.foo(1);      // calls A.foo(int)
}
------

        $(P If such an $(I AliasDeclaration) is not used, the derived
        class's functions completely override all the functions of the
        same name in the base class, even if the types of the parameters
        in the base class functions are different. If, through
        implicit conversions to the base class, those other functions do
        get called, a $(CODE core.exception.HiddenFuncError) exception is raised:
        )
---
import core.exception;

class A {
   void $(CODE_HIGHLIGHT set)(long i) { }
   void set(int i)  { }
}
class B : A {
   void set(long i) { }
}

void foo(A a) {
  int i;
  try {
    a.set(3);   // error, throws runtime exception since
                // A.set(int) should not be available from B
  }
  catch ($(CODE_HIGHLIGHT HiddenFuncError) o) {
     i = 1;
  }
  assert(i == 1);
}

void main() {
  foo(new B);
}
---
        $(P Note that the this current runtime behavior is deprecated.
        The compiler will currently emit a compile-time error for the
        above test-case unless the ($(B -d)) deprecation switch is enabled.
        In the future this deprecated runtime-only checking feature will be removed.)

        $(P If an $(CODE HiddenFuncError) exception is thrown in your program,
        the use of overloads and overrides needs to be reexamined in the
        relevant classes.)

        $(P The $(CODE HiddenFuncError) exception is not thrown if the
        hidden function is disjoint, as far as overloading is concerned,
        from all the other virtual functions is the inheritance hierarchy.)


        $(P A function parameter's default value is not inherited:)

------
class A {
  void $(CODE_HIGHLIGHT foo)(int x = 5) { ... }
}

class B : A {
  void foo(int $(CODE_HIGHLIGHT x = 7)) { ... }
}

class C : B {
  void foo(int $(CODE_HIGHLIGHT x)) { ... }
}


void test() {
  A a = new A();
  a.foo();       // calls A.foo(5)

  B b = new B();
  b.foo();       // calls B.foo(7)

  C c = new C();
  c.foo();       // error, need an argument for C.foo
}
------

        $(P If a derived class overrides a base class member function with diferrent
        $(GLINK2 declaration, FunctionAttributes), the missing attributes will be
        automatically compensated by the compiler.)

------
class B {
  void foo() pure nothrow @safe {}
}
class D : B {
  override void foo() {}
}
void main() {
  auto d = new D();
  pragma(msg, typeof(&d.foo));
  // prints "void delegate() pure nothrow @safe" in compile time
}
------


$(H4 Inline Functions)

        $(P There is no inline keyword. The compiler makes the decision whether to
        inline a function or not, analogously to the register keyword no
        longer being relevant to a
        compiler's decisions on enregistering variables.
        (There is no register keyword either.))

        $(P If a $(GLINK2 expression, FunctionLiteral) is immediately called,
        its inlining would be enforced normally.)


$(H3 $(LNAME2 function-overloading, Function Overloading))

        $(P Functions are overloaded based on how well the arguments
        to a function can match up with the parameters.
        The function with the $(I best) match is selected.
        The levels of matching are:
        )

        $(OL
        $(LI no match)
        $(LI match with implicit conversions)
        $(LI match with conversion to const)
        $(LI exact match)
        )

        $(P Each argument (including any $(CODE this) pointer) is
        compared against the function's corresponding parameter, to
        determine the match level for that argument. The match level
        for a function is the $(I worst) match level of each of its
        arguments.)

        $(P Literals do not match $(CODE ref) or $(CODE out) parameters.)


        $(P If two or more functions have the same match level,
        then $(LNAME2 partial-ordering, $(I partial ordering))
        is used to try to find the best match.
        Partial ordering finds the most specialized function.
        If neither function is more specialized than the other,
        then it is an ambiguity error.
        Partial ordering is determined for functions $(CODE f())
        and $(CODE g()) by taking the parameter types of $(CODE f()),
        constructing a list of arguments by taking the default values
        of those types, and attempting to match them against $(CODE g()).
        If it succeeds, then $(CODE g()) is at least as specialized
        as $(CODE f()).
        For example:
        )
---
class A { }
class B : A { }
class C : B { }
void foo(A);
void foo(B);

void test() {
  C c;
  /* Both foo(A) and foo(B) match with implicit conversion rules.
   * Applying partial ordering rules,
   * foo(B) cannot be called with an A, and foo(A) can be called
   * with a B. Therefore, foo(B) is more specialized, and is selected.
   */
  foo(c); // calls foo(B)
}
---
        $(P A function with a variadic argument is considered less
        specialized than a function without.
        )


        $(P Functions defined with non-D linkage cannot be overloaded.
        This is because the name mangling might not take the parameter types
        into account.
        )

$(H3 $(LNAME2 overload-sets, Overload Sets))

        $(P Functions declared at the same scope overload against each
        other, and are called an $(I Overload Set).
        A typical example of an overload set are functions defined
        at module level:
        )

---
module A;
void foo() { }
void foo(long i) { }
---

        $(P $(CODE A.foo()) and $(CODE A.foo(long)) form an overload set.
        A different module can also define functions with the same name:
        )

---
module B;
class C { }
void foo(C) { }
void foo(int i) { }
---

        $(P and A and B can be imported by a third module, C.
        Both overload sets, the $(CODE A.foo) overload set and the $(CODE B.foo)
        overload set, are found. An instance of $(CODE foo) is selected
        based on it matching in exactly one overload set:
        )

---
import A;
import B;

void bar(C c) {
  foo();    // calls A.foo()
  foo(1L);  // calls A.foo(long)
  foo(c);   // calls B.foo(C)
  foo(1,2); // error, does not match any foo
  foo(1);   // error, matches A.foo(long) and B.foo(int)
  A.foo(1); // calls A.foo(long)
}
---

        $(P Even though $(CODE B.foo(int)) is a better match than $(CODE
        A.foo(long)) for $(CODE foo(1)),
        it is an error because the two matches are in
        different overload sets.
        )

        $(P Overload sets can be merged with an alias declaration:)

---
import A;
import B;

alias foo = A.foo;
alias foo = B.foo;

void bar(C c) {
  foo();    // calls A.foo()
  foo(1L);  // calls A.foo(long)
  foo(c);   // calls B.foo(C)
  foo(1,2); // error, does not match any foo
  foo(1);   // calls B.foo(int)
  A.foo(1); // calls A.foo(long)
}
---

$(H4 $(LNAME2 parameters, Function Parameters))

        $(P Parameter storage classes are $(D in), $(D out),
        $(D ref), $(D lazy), $(D const), $(D immutable), $(D shared),
	$(D inout) or
        $(D scope).
        For example:
	)
------
int foo(in int x, out int y, ref int z, int q);
------

        $(P x is $(D in), y is $(D out), z is $(D ref), and q is none.
        )

        $(UL
        $(LI The function declaration makes it clear what the inputs and
        outputs to the function are.)
        $(LI It eliminates the need for IDL (interface description language) as a separate language.)
        $(LI It provides more information to the compiler, enabling more
        error checking and
        possibly better code generation.)
        )

	$(TABLE_2COLS Parameter Storage Classes,
	$(THEAD Storage Class, Description)
	$(TROW $(I none), parameter becomes a mutable copy of its argument)

	$(TROW $(D in), equivalent to $(D const scope))
	$(TROW $(D out), parameter is initialized upon function entry with the default value
	for its type)

	$(TROW $(D ref),   parameter is passed by reference)
	$(TROW $(D scope), references in the parameter
	cannot be escaped (e.g. assigned to a global variable))
	$(TROW $(D lazy), argument is evaluated by the called function and not by the caller)
		$(TROW $(D const), argument is implicitly converted to a const type)
	$(TROW $(D immutable), argument is implicitly converted to an immutable type)
	$(TROW $(D shared), argument is implicitly converted to a shared type)
	$(TROW $(D inout), argument is implicitly converted to an inout type)
	)

------
void foo(out int x) {
  // x is set to int.init,
  // which is 0, at start of foo()
}

int a = 3;
foo(a);
// a is now 0


void abc(out int x) {
  x = 2;
}

int y = 3;
abc(y);
// y is now 2


void def(ref int x) {
  x += 1;
}

int z = 3;
def(z);
// z is now 4
------------

        $(P For dynamic array and object parameters, which are passed
        by reference, in/out/ref
        apply only to the reference and not the contents.
        )

        $(P $(D lazy) arguments are evaluated not when the function is called,
        but when the parameter is evaluated within the function. Hence,
        a $(D lazy) argument can be executed 0 or more times. A $(D lazy) parameter
        cannot be an lvalue.)

---
void dotimes(int n, lazy void exp) {
  while (n--)
    exp();
}

void test() {
  int x;
  dotimes(3, writeln(x++));
}
---

        $(P prints to the console:)

$(CONSOLE
0
1
2
)

        $(P A $(D lazy) parameter of type $(D void) can accept an argument
        of any type.)

$(H4 Function Default Arguments)

        $(P Function parameter declarations can have default values:)

---
void foo(int x, int y = 3) {
  ...
}
...
foo(4);   // same as foo(4, 3);
---

        $(P Default parameters are evaluated in the context of the
        function declaration.
        If the default value for a parameter is given, all following
        parameters must also have default values.
        )

$(H3 $(LNAME2 variadic, Variadic Functions))

        Functions taking a variable number of arguments are called
        variadic functions. A variadic function can take one of
        three forms:

        $(OL
        $(LI C-style variadic functions)
        $(LI Variadic functions with type info)
        $(LI Typesafe variadic functions)
        )


$(H4 C-style Variadic Functions)

        A C-style variadic function is declared as taking
        a parameter of ... after the required function parameters.
        It has non-D linkage, such as $(D extern (C)):

------
extern (C) int foo(int x, int y, ...);

foo(3, 4);      // ok
foo(3, 4, 6.8); // ok, one variadic argument
foo(2);         // error, y is a required argument
------

        There must be at least one non-variadic parameter declared.

------
extern (C) int def(...); // error, must have at least one parameter
------

        $(P
        C-style variadic functions match the C calling convention for
        variadic functions, and is most useful for calling C library
        functions like $(D printf).
	)

	$(P Access to variadic arguments is done using the standard library
	module $(D core.stdc.stdarg).
	)

------
import core.stdc.stdarg;

void test()
{
    foo(3, 4, 5);   // first variadic argument is 5
}

int foo(int x, int y, ...)
{
    va_list ap;

    version (X86)
        va_start(args, y);  // y is the last named parameter
    else
    version (Win64)
        va_start(args, y);  // ditto
    else
    version (X86_64)
        va_start(args, __va_argsave);
    else
    static assert(0, "Platform not supported.");

    int z;
    va_arg(ap, z);  // z is set to 5
}
------


$(H4 D-style Variadic Functions)

        Variadic functions with argument and type info are declared as taking
        a parameter of ... after the required function parameters.
        It has D linkage, and need not have any non-variadic parameters
        declared:

------
int abc(char c, ...);   // one required parameter: c
int def(...);           // ok
------

	To access them, the following import is required:

------
import core.vararg;
------

        These variadic functions have a special local variable declared for
        them,
        $(D _argptr), which is a $(D core.vararg)
	reference to the first of the variadic
        arguments. To access the arguments, $(D _argptr) must be used
	in conjuction with $(D va_arg):

------
import core.vararg;

void test() {
  foo(3, 4, 5);   // first variadic argument is 5
}

int foo(int x, int y, ...) {

  int z;

  z = va_arg!int(_argptr); // z is set to 5
}
------

        An additional hidden argument
        with the name $(D _arguments) and type $(D TypeInfo[])
        is passed to the function.
        $(D _arguments) gives the number of arguments and the type
        of each, enabling type safety to be checked at run time.

------
import std.stdio;
import core.vararg;

class Foo { int x = 3; }
class Bar { long y = 4; }

void printargs(int x, ...) {
  writefln("%d arguments", _arguments.length);
  for (int i = 0; i < _arguments.length; i++)
  {
    writeln(_arguments[i]);

    if (_arguments[i] == typeid(int))
    {
      int j = va_arg!(int)(_argptr);
      writefln("\t%d", j);
    }
    else if (_arguments[i] == typeid(long))
    {
      long j = va_arg!(long)(_argptr);
      writefln("\t%d", j);
    }
    else if (_arguments[i] == typeid(double))
    {
      double d = va_arg!(double)(_argptr);
      writefln("\t%g", d);
    }
    else if (_arguments[i] == typeid(Foo))
    {
      Foo f = va_arg!(Foo)(_argptr);
      writefln("\t%s", f);
    }
    else if (_arguments[i] == typeid(Bar))
    {
      Bar b = va_arg!(Bar)(_argptr);
      writefln("\t%s", b);
    }
    else
      assert(0);
  }
}

void main() {
  Foo f = new Foo();
  Bar b = new Bar();

  writefln("%s", f);
  printargs(1, 2, 3L, 4.5, f, b);
}
------

        which prints:

------
0x00870FE0
5 arguments
int
        2
long
        3
double
        4.5
Foo
        0x00870FE0
Bar
        0x00870FD0
------


$(H4 Typesafe Variadic Functions)

        $(P Typesafe variadic functions are used when the variable argument
        portion of the arguments are used to construct an array or
        class object.)

        $(P For arrays:)

------
int test() {
  return sum(1, 2, 3) + sum(); // returns 6+0
}

int func() {
  int[3] ii = [4, 5, 6];
  return sum(ii);             // returns 15
}

int sum(int[] ar ...) {
  int s;
  foreach (int x; ar)
    s += x;
  return s;
}
------

        For static arrays:

------
int test() {
  return sum(2, 3);   // error, need 3 values for array
  return sum(1, 2, 3); // returns 6
}

int func() {
  int[3] ii = [4, 5, 6];
  int[] jj = ii;
  return sum(ii); // returns 15
  return sum(jj); // error, type mismatch
}

int sum(int[3] ar ...) {
  int s;
  foreach (int x; ar)
    s += x;
  return s;
}
------

        For class objects:

------
class Foo {
  int x;
  string s;

  this(int x, string s) {
    this.x = x;
    this.s = s;
  }
}

void test(int x, Foo f ...);

...

Foo g = new Foo(3, "abc");
test(1, g);         // ok, since g is an instance of Foo
test(1, 4, "def");  // ok
test(1, 5);         // error, no matching constructor for Foo
------

        An implementation may construct the object or array instance
        on the stack. Therefore, it is an error to refer to that
        instance after the variadic function has returned:

------
Foo test(Foo f ...) {
  return f;   // error, f instance contents invalid after return
}

int[] test(int[] a ...) {
  return a;       // error, array contents invalid after return
  return a[0..1]; // error, array contents invalid after return
  return a.dup;   // ok, since copy is made
}
------

        For other types, the argument is built with itself, as in:

------
int test(int i ...) {
  return i;
}

...
test(3);    // returns 3
test(3, 4); // error, too many arguments
int[] x;
test(x);    // error, type mismatch
------

$(H4 Lazy Variadic Functions)

        $(P If the variadic parameter is an array of delegates
        with no parameters:
        )

---
void foo(int delegate()[] dgs ...);
---

        $(P Then each of the arguments whose type does not match that
        of the delegate is converted to a delegate.
        )

---
int delegate() dg;
foo(1, 3+x, dg, cast(int delegate())null);
---

        $(P is the same as:)

---
foo( { return 1; }, { return 3+x; }, dg, null );
---

$(H3 $(LNAME2 Local Variables, Local Variables))

        $(P It is an error to use a local variable without first assigning it a
        value. The implementation may not always be able to detect these
        cases. Other language compilers sometimes issue a warning for this,
        but since it is always a bug, it should be an error.
        )

        $(P It is an error to declare a local variable that is never referred to.
        Dead variables, like anachronistic dead code, are just a source of
        confusion for maintenance programmers.
        )

        $(P It is an error to declare a local variable that hides another local
        variable in the same function:
        )

------
void func(int x) {
   int x;     // error, hides previous definition of x
   double y;
   ...
   { char y;  // error, hides previous definition of y
     int z;
   }
   { wchar z; // legal, previous z is out of scope
   }
}
------

        $(P While this might look unreasonable, in practice whenever
        this is done it either is a
        bug or at least looks like a bug.
        )

        $(P It is an error to return the address of or a reference to a
        local variable.
        )

        $(P It is an error to have a local variable and a label with the same
        name.
        )

$(H3 $(LNAME2 Local Static Variables, Local Static Variables))

	$(P Local variables in functions can be declared as static
	or $(D __gshared) in which case they are statically allocated
	rather than being allocated on the stack.
	As such, their value persists beyond the exit of the function.
	)

---
void foo() {
  static int n;
  if (++n == 100)
    writeln("called 100 times");
}
---

	$(P The initializer for a static variable must be evaluatable at
	compile time, and they are initialized upon the start of the thread
	(or the start of the program for $(D __gshared)).
	There are no static constructors or static destructors
	for static local variables.
	)

	$(P Although static variable name visibility follows the usual scoping
	rules, the names of them must be unique within a particular function.
	)

---
void main() {
  { static int x; }
  { static int x; } // error
  { int i; }
  { int i; } // ok
}
---

$(H3 $(LNAME2 variadicnested, Nested Functions))

        $(P Functions may be nested within other functions:)

------
int bar(int a) {
  int foo(int b) {
    int abc() { return 1; }

    return b + abc();
  }
  return foo(a);
}

void test() {
  int i = bar(3); // i is assigned 4
}
------

        $(P Nested functions can be accessed only if the name is in scope.)

------
void foo()
{
  void A()
  {
    B(); // error, B() is forward referenced
    C(); // error, C undefined
  }
  void B()
  {
    A(); // ok, in scope
    void C()
    {
      void D()
      {
        A();      // ok
        B();      // ok
        C();      // ok
        D();      // ok
      }
    }
  }
  A(); // ok
  B(); // ok
  C(); // error, C undefined
}
------

        $(P and:)

------
int bar(int a) {
  int foo(int b) { return b + 1; }
  int abc(int b) { return foo(b); }   // ok
  return foo(a);
}

void test() {
  int i = bar(3);     // ok
  int j = bar.foo(3); // error, bar.foo not visible
}
------

        $(P Nested functions have access to the variables and other symbols
        defined by the lexically enclosing function.
        This access includes both the ability to read and write them.
        )

------
int bar(int a) {
  int c = 3;

  int foo(int b) {
    b += c;       // 4 is added to b
    c++;          // bar.c is now 5
    return b + c; // 12 is returned
  }
  c = 4;
  int i = foo(a); // i is set to 12
  return i + c;   // returns 17
}

void test() {
  int i = bar(3); // i is assigned 17
}
------

        $(P This access can span multiple nesting levels:)

------
int bar(int a) {
  int c = 3;

  int foo(int b) {
      int abc() {
          return c;   // access bar.c
      }
      return b + c + abc();
  }
  return foo(3);
}
------

        $(P Static nested functions cannot access any stack variables of
        any lexically enclosing function, but can access static variables.
        This is analogous to how static member functions behave.
        )

------
int bar(int a) {
  int c;
  static int d;

  static int foo(int b) {
    b = d;          // ok
    b = c;          // error, foo() cannot access frame of bar()
    return b + 1;
  }
  return foo(a);
}
------

        $(P Functions can be nested within member functions:)

------
struct Foo {
  int a;

  int bar() {
    int c;

    int foo() {
      return c + a;
    }
    return 0;
  }
}
------

        $(P Nested functions always have the D function linkage type.
        )

        $(P Unlike module level declarations, declarations within function
        scope are processed in order. This means that two nested functions
        cannot mutually call each other:
        )

------
void test() {
  void foo() { bar(); } // error, bar not defined
  void bar() { foo(); } // ok
}
------

        $(P There are several workarounds for this limitation:

        $(LI You can declare the functions to be static members of a nested struct:)

------
void test() {
  static struct S {
      static void foo() { bar(); } // ok
      static void bar() { foo(); } // ok
  }

  S.foo();  // compiles (but note the infinite runtime loop)
}
------

        $(LI You can declare one or more of the functions to be function templates
        even if they take no specific template arguments:)

------
void test() {
  void foo()() { bar(); } // ok (foo is a function template)
  void bar()   { foo(); } // ok
}
------

        $(LI You can declare the functions inside of a mixin template:)

------
mixin template T()
{
  void foo() { bar(); } // ok
  void bar() { foo(); } // ok
}

void test()
{
  mixin T!();
}
------

        $(LI Or you can use a delegate:)

------
void test() {
  void delegate() fp;
  void foo() { fp(); }
  void bar() { foo(); }
  fp = &bar;
}
------

    )

	$(P Nested functions cannot be overloaded.)

$(H4 $(LNAME2 closures, Delegates, Function Pointers, and  Closures))

        $(P A function pointer can point to a static nested function:)

------
int function() fp;

void test() {
  static int a = 7;
  static int foo() { return a + 3; }

  fp = &foo;
}

void bar() {
  test();
  int i = fp();       // i is set to 10
}
------

        $(P A delegate can be set to a non-static nested function:)

------
int delegate() dg;

void test() {
  int a = 7;
  int foo() { return a + 3; }

  dg = &foo;
  int i = dg(); // i is set to 10
}
------


        $(P The stack variables referenced by a nested function are
        still valid even after the function exits (this is different
        from D 1.0). This is called a $(I closure).
        Returning addresses of stack variables, however, is not
        a closure and is an error.
        )

------
int* bar() {
  int b;
  test();
  int i = dg(); // ok, test.a is in a closure and still exists
  return &b;    // error, bar.b not valid after bar() exits
}
------


        $(P Delegates to non-static nested functions contain two pieces of
        data: the pointer to the stack frame of the lexically enclosing
        function (called the $(I frame pointer)) and the address of the
        function. This is analogous to struct/class non-static member
        function delegates consisting of a $(I this) pointer and
        the address of the member function.
        Both forms of delegates are interchangeable, and are actually
        the same type:
        )

------
struct Foo {
  int a = 7;
  int bar() { return a; }
}

int foo(int delegate() dg) {
  return dg() + 1;
}

void test() {
  int x = 27;
  int abc() { return x; }
  Foo f;
  int i;

  i = foo(&abc);   // i is set to 28
  i = foo(&f.bar); // i is set to 8
}
------

        $(P This combining of the environment and the function is called
        a $(I dynamic closure).
        )

        $(P The $(D .ptr) property of a delegate will return the
        $(I frame pointer) value as a $(D void*).
        )

        $(P The $(D .funcptr) property of a delegate will return the
        $(I function pointer) value as a function type.
        )

        $(P $(B Future directions:) Function pointers and delegates may merge
        into a common syntax and be interchangeable with each other.
        )

$(H4 Anonymous Functions and Anonymous Delegates)

        $(P See $(GLINK2 expression, FunctionLiteral)s.
        )

$(H3 $(D main()) Function)

        $(P For console programs, $(D main()) serves as the entry point.
        It gets called after all the module initializers are run, and
        after any unittests are run.
        After it returns, all the module destructors are run.
        $(D main()) must be declared using one of the following forms:
        )

----
void main() { ... }
void main(string[] args) { ... }
int main() { ... }
int main(string[] args) { ... }
----

$(H3 $(LNAME2 interpretation, Compile Time Function Execution (CTFE)))

    $(P Functions which are both portable and free of side-effects can be
    executed at compile time. This is useful when constant folding
    algorithms need to include recursion and looping. Compile time function
    execution is subject to the following restrictions:
    )

    $(OL
    $(LI The function source code must be available to the compiler. Functions
        which exist in the source code only as $(D_KEYWORD extern) declarations
        cannot be executed at compile time.)

    $(LI Executed expressions may not reference any global or local
        static variables.)

    $(LI $(D_KEYWORD asm) statements are not permitted)

    $(LI Non-portable casts (eg, from $(D int[]) to $(D float[])), including
        casts which depend on endianness, are not permitted.
        Casts between signed and unsigned types are permitted)
    )

    $(P Pointers are permitted in CTFE, provided they are used safely:)

    $(UL
        $(LI
        C-style semantics on pointer arithmetic are strictly enforced.
        Pointer arithmetic is permitted only on pointers which point to static
        or dynamic array elements. Such pointers must point to an element of
        the array, or to the first element past the array.
        Pointer arithmetic is completely forbidden on pointers which are null,
        or which point to a non-array.
        )

        $(LI
        The memory location of different memory blocks is not defined.
        Ordered comparison ($(D <), $(D <)$(D =), $(D >), $(D >=)) between two pointers is permitted
        when both pointers point to the same array, or when at least one
        pointer is $(D null).
        )

        $(LI
        Pointer comparisons between independent memory blocks will generate
        a compile-time error, unless two such comparisons are combined
        using $(D &&) or $(CODE_PIPE)$(CODE_PIPE) to yield a result which is independent of the
        ordering of memory blocks. Each comparison must consist of two pointer
        expressions compared with $(D <), $(D <)$(D =), $(D >),
        or $(D >)$(D =), and may optionally be
        negated with $(D !).

        $(P
        For example, the expression $(D (p1 > q1 && p2 <= q2))
        is permitted when $(D p1), $(D p2) are expressions yielding pointers
        to memory block $(I P), and $(D q1), $(D q2) are expressions yielding
        pointers to memory block $(I Q), even when $(I P) and $(I Q) are
        unrelated memory blocks.
        It returns true if $(D [p1..p2]) lies inside $(D [q1..q2]), and false otherwise.
        Similarly, the expression $(D (p1 < q1 || p2 > q2)) is true if
        $(D [p1..p2]) lies outside $(D [q1..q2]), and false otherwise.
        )
        )

        $(LI
        Equality comparisons (==, !=, $(D_KEYWORD is), $(D_KEYWORD !is)) are
        permitted between all pointers, without restriction.
        )

        $(LI
        Any pointer may be cast to $(D void *) and from $(D void *) back to
        its original type. Casting between pointer and non-pointer types is
        prohibited.
        )
    )

    $(P Note that the above restrictions apply only to expressions which are
        actually executed. For example:
    )
---
static int y = 0;

int countTen(int x) {
  if (x > 10)
    ++y;
  return x;
}

static assert(countTen(6) == 6); // OK
static assert(countTen(12) == 12);  // invalid, modifies y.
---
    $(P The $(D __ctfe) boolean pseudo-variable, which evaluates to $(D_KEYWORD true)
        at compile time, but $(D_KEYWORD false) at run time, can be used to provide
        an alternative execution path to avoid operations which are forbidden
        at compile time. Every usage of $(D __ctfe) is evaluated before
        code generation and therefore has no run-time cost, even if no optimizer
        is used.
    )


	$(P In order to be executed at compile time, the function
	must appear in a context where it must be so executed, for
	example:)

	$(UL
	$(LI initialization of a static variable)
	$(LI dimension of a static array)
	$(LI argument for a template value parameter)
	)

---
template eval( A... ) {
  const typeof(A[0]) eval = A[0];
}

int square(int i) {
  return i * i;
}

void foo() {
  static j = square(3);     // compile time
  writeln(j);
  writeln(square(4));      // run time
  writeln(eval!(square(5))); // compile time
}
---

    $(P Executing functions at compile time can take considerably
    longer than executing it at run time.
    If the function goes into an infinite loop, it will hang at
    compile time (rather than hanging at run time).
    )

    $(P Non-recoverable errors (such as $(D_KEYWORD assert) failures) do not
    throw exceptions; instead, they end interpretation immediately.
    )

    $(P Functions executed at compile time can give different results
    from run time in the following scenarios:
    )

    $(UL

    $(LI floating point computations may be done at a higher
    precision than run time)
    $(LI dependency on implementation defined order of evaluation)
    $(LI use of uninitialized variables)

    )

    $(P These are the same kinds of scenarios where different
    optimization settings affect the results.)

$(H4 String Mixins and Compile Time Function Execution)

        $(P Any functions that execute at compile time must also
        be executable at run time. The compile time evaluation of
        a function does the equivalent of running the function at
        run time. This means that the semantics of a function cannot
        depend on compile time values of the function. For example:)

---
int foo(char[] s) {
  return mixin(s);
}

const int x = foo("1");
---

        $(P is illegal, because the runtime code for foo() cannot be
        generated. A function template would be the appropriate
        method to implement this sort of thing.)

$(H3 $(LNAME2 function-safety, Function Safety))

        $(P $(I Safe functions) are functions that are statically checked
        to exhibit no possibility of
        $(DPLLINK glossary.html#undefined_behavior, $(I undefined behavior)).
        Undefined behavior is often used as a vector for malicious
        attacks.
        )

$(H4 $(LNAME2 safe-functions, Safe Functions))

        $(P Safe functions are marked with the $(CODE @safe) attribute.)

        $(P The following operations are not allowed in safe
        functions:)

        $(UL
        $(LI No casting from a pointer type to any type other than $(CODE void*).)
        $(LI No casting from any non-pointer type to a pointer type.)
        $(LI No modification of pointer values.)
        $(LI Cannot access unions that have pointers or references overlapping
        with other types.)
        $(LI Calling any system functions.)
        $(LI No catching of exceptions that are not derived from $(CODE class Exception).)
        $(LI No inline assembler.)
        $(LI No explicit casting of mutable objects to immutable.)
        $(LI No explicit casting of immutable objects to mutable.)
        $(LI No explicit casting of thread local objects to shared.)
        $(LI No explicit casting of shared objects to thread local.)
        $(LI No taking the address of a local variable or function parameter.)
        $(LI Cannot access $(D __gshared) variables.)
        )

        $(P Functions nested inside safe functions default to being
        safe functions.
        )

        $(P Safe functions are covariant with trusted or system functions.)

        $(P $(B Note:) The verifiable safety of functions may be compromised by
        bugs in the compiler and specification. Please report all such errors
        so they can be corrected.
        )

$(H4 $(LNAME2 trusted-functions, Trusted Functions))

        $(P Trusted functions are marked with the $(CODE @trusted) attribute.)

        $(P Trusted functions are guaranteed by the programmer to not exhibit
        any undefined behavior if called by a safe function.
        Generally, trusted functions should be kept small so that they are
        easier to manually verify.
        )

        $(P Trusted functions may call safe, trusted, or system functions.
        )

        $(P Trusted functions are covariant with safe or system functions.)

$(H4 $(LNAME2 system-functions, System Functions))

        $(P System functions are functions not marked with $(CODE @safe) or
        $(CODE @trusted)
        and are not nested inside $(CODE @safe) functions.
        System functions may be marked with the $(CODE @system) attribute.
        A function being system does not mean it actually is unsafe, it just
        means that the compiler is unable to verify that it cannot exhibit
        undefined behavior.
        )

        $(P System functions are $(B not) covariant with trusted or safe functions.
        )


$(H3 $(LNAME2 function-attribute-inference, Function Attribute Inference))

        $(P $(GLINK2 expression, FunctionLiteral)s and
        $(DDSUBLINK template, function-templates, function template)s, since their function bodies
        are always present, infer the
        $(RELATIVE_LINK2 pure-functions, $(D pure)),
        $(RELATIVE_LINK2 nothrow-functions, $(D nothrow)), and
        $(RELATIVE_LINK2 safe-functions, $(D @safe)) attributes unless
        specifically overridden.
        )

        $(P Attribute inference is not done for other functions, even if the function
        body is present.
        )

        $(P The inference is done by determining if the function body follows the
        rules of the particular attribute.
        )

        $(P Cyclic functions (i.e. functions that wind up directly or indirectly
        calling themselves) are inferred as being impure, throwing, and @system.
        )

        $(P If a function attempts to test itself for those attributes, then
        the function is inferred as not having those attributes.
        )

$(H3 $(LNAME2 pseudo-member, Uniform Function Call Syntax (UFCS)))

        $(P A free function can be called with a syntax that looks as if the function
        were a member function of its first parameter type.
        )

        ---
        void func(X thisObj);

        X obj;
        obj.func();
        // If 'obj' does not have regular member 'func',
        // it's automatically rewritten to 'func(obj)'
        ---

	$(P This provides a way to add functions to a class
	externally as if they were public final member functions.
	)

        $(P It also works with $(D @property) functions:)

        ---
        @property prop(X thisObj);
        @property prop(X thisObj, int value);

        X obj;
        obj.prop;      // Rewrites to: prop(obj);
        obj.prop = 1;  // Rewrites to: prop(obj, 1);
        ---

        $(P Syntactically parenthesis-less check for $(D @property)
        functions is done at the same time as UFCS rewrite.)

        $(P When UFCS rewrite is necessary, compiler searches the name
        on accessible module level scope, in order from the innermost scope.)

        ---
        module a;
        void foo(X);
        alias boo = foo;
        void main() {
          void bar(X);
          import b : baz;  // void baz(X);

          X obj;
          obj.foo();    // OK, calls a.foo;
          //obj.bar();  // NG, UFCS does not see nested functions
          obj.baz();    // OK, calls b.baz, because it is declared at the
                        // top level scope of module b

          import b : boo = baz;
          obj.boo();    // OK, calls aliased b.baz instead of a.boo (== a.foo),
                        // because the declared alias name 'boo' in local scope
                        // overrides module scope name
        }
        class C {
          void mfoo(X);
          static void sbar(X);
          import b : ibaz = baz;  // void baz(X);
          void test() {
            X obj;
            //obj.mfoo();  // NG, UFCS does not see member functions
            //obj.sbar();  // NG, UFCS does not see static member functions
            obj.ibaz();    // OK, ibaz is an alias of baz which declared at
                           //     the top level scope of module b
          }
        }
        ---

        $(P The reason why local symbols are not considered by UFCS, is
        to avoid unexpected name conflicts. See below problematic examples.)

        ---
        int front(int[] arr) { return arr[0]; }

        void main() {
          int[] a = [1,2,3];
          auto x = a.front();   // call .front by UFCS

          auto front = x;       // front is now a variable
          auto y = a.front();   // Error, front is not a function
        }

        class C {
          int[] arr;
          int front() {
            return arr.front(); // Error, C.front is not callable
                                // using argument types (int[])
          }
        }
        ---


)

Macros:
        TITLE=Functions
        WIKI=Function
        CATEGORY_SPEC=$0