"auto" can be used to derive the type of many things, but it has limitations. For example, what is the type of this?:
template <typename T, typename V>
auto add_two_template (const T a, const V b) {
return a + b;
}Is it T or V? It could be either, or some derived type. This is where decltype comes to the rescue"
template <typename T, typename V>
auto add_two_template (const T a, const V b) -> decltype(a + b) {
return a + b;
}You can apply this to lambdas too:
auto lambda_add_two = ( [](const int a, const int b) { return a + b; } );
std::cout << typeid(decltype(lambda_add_two)).name() << std::endl;We are using typeid(...).name() here to print the information, however, this is compiler specific and so the output will be mangled by the compiler.
In the following example we have a demangler (which you can ignore) but it prints slightly more verbose output using g++ abi interface:
#include <cxxabi.h> // needed for abi::__cxa_demangle
#include <iostream>
#include <memory>
//
// See
// https://stackoverflow.com/questions/4939636/function-to-mangle-demangle-functions
//
auto cppDemangle(const char *abiName)
{
//
// This function allocates and returns storage in ret
//
int status;
char *ret = abi::__cxa_demangle(abiName, 0 /* output buffer */, 0 /* length */, &status);
auto deallocator = ([](char *mem) {
if (mem)
free((void *) mem);
});
if (status) {
// 0: The demangling operation succeeded.
// -1: A memory allocation failure occurred.
// -2: mangled_name is not a valid name under the C++ ABI mangling rules.
// -3: One of the arguments is invalid.
std::unique_ptr< char, decltype(deallocator) > retval(nullptr, deallocator);
}
//
// Create a unique pointer to take ownership of the returned string so it
// is freed when that pointers goes out of scope
//
std::unique_ptr< char, decltype(deallocator) > retval(ret, deallocator);
return retval;
}
class TheUniverse
{
private:
int size;
public:
constexpr TheUniverse(int size) : size(size) {}
constexpr int get_size() const { return (size); }
};
constexpr auto what_is_the(int meaning, int of)
{
meaning++;
return meaning * of;
}
// const works just as well here for all the below
constexpr auto meaning = 5;
constexpr auto of = 7;
constexpr auto life = what_is_the(meaning, of);
constexpr auto planets = 100000;
constexpr char the_earth[ life ] {};
//
// Note using constexpr will make this symbol visible in the linker
//
constexpr auto the_universe = TheUniverse(sizeof(the_earth) * planets);
static int add_two(const int a, const int b) { return a + b; }
template < typename T, typename V > auto add_two_template(const T a, const V b) -> decltype(a + b) { return a + b; }
int main(void)
{
// What is the meaning...:
std::cout << "the meaning is " << life << std::endl;
// What type is life...:
std::cout << typeid(decltype(life)).name() << std::endl;
std::cout << cppDemangle(typeid(decltype(life)).name()) << std::endl;
// How big is the earth...:
std::cout << sizeof(the_earth) << std::endl;
// What type is the earth...:
std::cout << typeid(decltype(the_earth)).name() << std::endl;
std::cout << cppDemangle(typeid(decltype(the_earth)).name()) << std::endl;
// How big is the universe...:
std::cout << the_universe.get_size() << std::endl;
// What type is the universe...:
std::cout << typeid(decltype(the_universe)).name() << std::endl;
std::cout << cppDemangle(typeid(decltype(the_universe)).name()) << std::endl;
auto lambda_add_two = ([](const int a, const int b) { return a + b; });
// What type is a lambda_add_two...:
std::cout << typeid(decltype(lambda_add_two)).name() << std::endl;
std::cout << cppDemangle(typeid(decltype(lambda_add_two)).name()) << std::endl;
// What type is a lambda_add_two(1, 2)...:
std::cout << cppDemangle(typeid(decltype(lambda_add_two(1, 2))).name()) << std::endl;
// What type is add_two()...:
std::cout << typeid(decltype(add_two)).name() << std::endl;
std::cout << cppDemangle(typeid(decltype(add_two)).name()) << std::endl;
// What type is add_two(1, 2)...:
std::cout << cppDemangle(typeid(decltype(add_two(1, 2))).name()) << std::endl;
add_two(1, 2); // just to silence compiler warnings
// What type is add_two_template<int, int>()...:
std::cout << cppDemangle(typeid(decltype(add_two_template< int, int >)).name()) << std::endl;
add_two(1, 2); // just to silence compiler warnings
// What type is add_two_template<float, int>()...:
std::cout << cppDemangle(typeid(decltype(add_two_template< float, int >)).name()) << std::endl;
add_two(1, 2); // just to silence compiler warnings
// What type is add_two_template<int, float>()...:
std::cout << cppDemangle(typeid(decltype(add_two_template< int, float >)).name()) << std::endl;
add_two(1, 2); // just to silence compiler warnings
// What type is add_two_template<float, float>()...:
std::cout << cppDemangle(typeid(decltype(add_two_template< float, float >)).name()) << std::endl;
add_two(1, 2); // just to silence compiler warnings
// What type is add_two_template((int)1, (int)2)...:
std::cout << cppDemangle(typeid(decltype(add_two_template((int) 1, (int) 2))).name()) << std::endl;
add_two(1, 2); // just to silence compiler warnings
// What type is add_two_template((float)1.1, (int)2)...:
std::cout << cppDemangle(typeid(decltype(add_two_template((float) 1.1, (int) 2))).name()) << std::endl;
add_two(1, 2); // just to silence compiler warnings
// What type is add_two_template((int)1, (float)2.2)...:
std::cout << cppDemangle(typeid(decltype(add_two_template((int) 1, (float) 2.2))).name()) << std::endl;
add_two(1, 2); // just to silence compiler warnings
// What type is add_two_template((float)1.1, (float)2.2)...:
std::cout << cppDemangle(typeid(decltype(add_two_template((int) 1.1, (float) 2.2))).name()) << std::endl;
add_two(1, 2); // just to silence compiler warnings
return 0;
}To build:
cd decltype rm -f *.o example clang -std=c++2a -Werror -g -O3 -fstack-protector-all -ggdb3 -Wall -c -o main.o main.cpp clang main.o -lstdc++ -o example ./example
Expected output:
�[31;1;4mWhat is the meaning...:�[0m the meaning is 42 �[31;1;4mWhat type is life...:�[0m i int �[31;1;4mHow big is the earth...:�[0m 42 �[31;1;4mWhat type is the earth...:�[0m A42_c char [42] �[31;1;4mHow big is the universe...:�[0m 4200000 �[31;1;4mWhat type is the universe...:�[0m 11TheUniverse TheUniverse �[31;1;4mWhat type is a lambda_add_two...:�[0m Z4mainE3$_1 main::$_1 �[31;1;4mWhat type is a lambda_add_two(1, 2)...:�[0m int �[31;1;4mWhat type is add_two()...:�[0m FiiiE int (int, int) �[31;1;4mWhat type is add_two(1, 2)...:�[0m int �[31;1;4mWhat type is add_two_template()...:�[0m int (int, int) �[31;1;4mWhat type is add_two_template()...:�[0m float (float, int) �[31;1;4mWhat type is add_two_template()...:�[0m float (int, float) �[31;1;4mWhat type is add_two_template()...:�[0m float (float, float) �[31;1;4mWhat type is add_two_template((int)1, (int)2)...:�[0m int �[31;1;4mWhat type is add_two_template((float)1.1, (int)2)...:�[0m float �[31;1;4mWhat type is add_two_template((int)1, (float)2.2)...:�[0m float �[31;1;4mWhat type is add_two_template((float)1.1, (float)2.2)...:�[0m float