Lambdas are like inline functions and very useful in a number of places where you need a one off function. For example:
std::initializer_list< int > i = {42, 6, 7, 1, 4, 2, 3};
std::vector< int > v(i);
std::sort(v.begin(), v.end(), [](const int &a, const int &b) { return a < b; });This invokes a lambda at each point in the sort algorighm and is identical to:
static int my_sorter (const int &a, const int &b) { return a < b; };
std::initializer_list< int > i = {42, 6, 7, 1, 4, 2, 3};
std::vector< int > v(i);
std::sort(v.begin(), v.end(), my_sorter);
}Now the syntax is a bit obscure. "[]" is defined as the capture set and indicates what kind of things are accessible within the lambda:
- [] means capture everything
- [&zaphod] means capture the lvalue zaphod by reference so it can be modified
- [=zaphod] capture by value
- "()" are the arguements to the lambda
- "->" is used to specify the return type if that cannot be derived or is not clear to the reader.
- "mutable" to allow modification of pass-by-value; see further below.
Here is an example of pass by reference and modification:
auto zaphod_head = 1;
auto new_head = [&zaphod_head](){zaphod_head++;};
new_head();
std::cout << "Zaphod has " << zaphod_head << "heads" << std::endl;However the following will NOT compile:
auto marvin_head = 1;
auto new_head = [=]() {marvin_head++;};
new_head();
std::cout << "Marvin has " << marvin_head << " heads" << std::endl;The compiler will detect we are trying to modify a pass-by-value and will complain. If we want to this, we must add "mutable"
auto marvin_head = 1;
auto new_head = [=]() mutable {marvin_head++;};
new_head();
std::cout << "Marvin has " << marvin_head << " heads" << std::endl;Lambas can also be generic. This is very similar to templating e.g.:
auto lambda = [](auto x, auto y) {return x + y;};You can also do odd things like the following. "life" here is created on the fly as a local variable and can be incremented each time "deep_thought()" is invoked.
auto the_meaning = 42;
auto deep_thought = [&life = the_meaning, the_meaning = the_meaning + 1] () {
life += 2;
return the_meaning + 2;
};
auto the_answer = deep_thought();
deep_thought();Here is a full example:
#include <algorithm>
#include <iomanip>
#include <iostream>
#include <vector>
int main()
{
// Create a vector of unsorted ints:
std::initializer_list< int > init1 = {42, 6, 7, 1, 4, 2, 3};
std::vector< int > vec1(init1);
// Sort via lamda sorter:
std::sort(vec1.begin(), vec1.end(), [](const int &a, const int &b) { return a < b; });
for (auto &i : vec1) {
std::cout << "vec1: walk " << i << std::endl;
}
// Modify vector via lamda modifier:
std::for_each(vec1.begin(), vec1.end(), [](int &a) { a++; });
for (auto &i : vec1) {
std::cout << "vec1: walk " << i << std::endl;
}
// Modify vector via lamda modifier that specifies return type:
std::for_each(vec1.begin(), vec1.end(), [](int &a) -> void { a++; });
for (auto &i : vec1) {
std::cout << "vec1: walk " << i << std::endl;
}
// Modify vector via lamda modifier that specifies return type:
std::transform(vec1.begin(), vec1.end(), vec1.begin(), [](int &a) -> int { return a + 1; });
for (auto &i : vec1) {
std::cout << "vec1: walk " << i << std::endl;
}
// Modify vector via lamda modifier with [=] capture:
auto delta = 1;
std::for_each(vec1.begin(), vec1.end(), [ = ](int &a) { a += delta; });
for (auto &i : vec1) {
std::cout << "vec1: walk " << i << std::endl;
}
// Modify vector via lamda modifier with [&] capture:
std::for_each(vec1.begin(), vec1.end(), [ & ](int &a) {
a += delta;
delta++;
});
for (auto &i : vec1) {
std::cout << "vec1: walk " << i << std::endl;
}
// Auto lambda:
auto lambda = [](auto x, auto y) { return x + y; };
std::initializer_list< float > init2 = {42.1, 6.2, 7.3, 1.0, 4.4, 2.5, 3.6};
std::vector< float > vec2(init2);
std::sort(vec2.begin(), vec2.end(), lambda);
for (auto &i : vec2) {
std::cout << "vec2: walk " << std::setprecision(4) << i << std::endl;
}
// Lambda with modification:
{
auto zaphod_head = 1;
auto new_head = [ &zaphod_head ]() { zaphod_head++; };
new_head();
std::cout << "Zaphod has " << zaphod_head << " heads" << std::endl;
}
// Lambda with modification:
{
auto marvin_head = 1;
auto new_head = [ = ]() mutable { marvin_head++; };
new_head();
std::cout << "Marvin has " << marvin_head << " heads" << std::endl;
}
// Lambda with modification in the capture:
auto the_meaning = 36;
auto deep_thought = [ &life = the_meaning, the_meaning = the_meaning + 1 ]() {
life += 2;
std::cout << "life = " << life << std::endl;
return the_meaning / 6;
};
deep_thought();
deep_thought();
auto the_answer = deep_thought();
std::cout << "the_meaning = " << the_meaning << std::endl;
std::cout << "the_answer = " << the_answer << std::endl;
}To build:
cd lambda 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;4mCreate a vector of unsorted ints:�[0m �[31;1;4mSort via lamda sorter:�[0m vec1: walk 1 vec1: walk 2 vec1: walk 3 vec1: walk 4 vec1: walk 6 vec1: walk 7 vec1: walk 42 �[31;1;4mModify vector via lamda modifier:�[0m vec1: walk 2 vec1: walk 3 vec1: walk 4 vec1: walk 5 vec1: walk 7 vec1: walk 8 vec1: walk 43 �[31;1;4mModify vector via lamda modifier that specifies return type:�[0m vec1: walk 3 vec1: walk 4 vec1: walk 5 vec1: walk 6 vec1: walk 8 vec1: walk 9 vec1: walk 44 �[31;1;4mModify vector via lamda modifier that specifies return type:�[0m vec1: walk 4 vec1: walk 5 vec1: walk 6 vec1: walk 7 vec1: walk 9 vec1: walk 10 vec1: walk 45 �[31;1;4mModify vector via lamda modifier with [=] capture:�[0m vec1: walk 5 vec1: walk 6 vec1: walk 7 vec1: walk 8 vec1: walk 10 vec1: walk 11 vec1: walk 46 �[31;1;4mModify vector via lamda modifier with [&] capture:�[0m vec1: walk 6 vec1: walk 8 vec1: walk 10 vec1: walk 12 vec1: walk 15 vec1: walk 17 vec1: walk 53 �[31;1;4mAuto lambda:�[0m vec2: walk 3.6 vec2: walk 2.5 vec2: walk 4.4 vec2: walk 1 vec2: walk 7.3 vec2: walk 6.2 vec2: walk 42.1 �[31;1;4mLambda with modification:�[0m Zaphod has 2 heads �[31;1;4mLambda with modification:�[0m Marvin has 1 heads �[31;1;4mLambda with modification in the capture:�[0m life = 38 life = 40 life = 42 the_meaning = 42 the_answer = 6