The std::function class template is a general purpose polymorphic function wrapper. Instances of std::function can store, copy and invoke any target: lambda expressions, binding expressions or other function objects, as well as pointers to member functions and pointers to data members.

std::function is a type of disposal object. This means that it erases the details of how some operations happen and provides a uniform runtime interface for them.

Usage

To use std::function it is necessary to include the header: <functional>.

Example 01

The most basic usage would be the example below:

In this case it stores the square function without having to inform the parameters, in this case it takes an int inside the parentheses and returns an int inside the angle brackets.

#include <iostream>
#include <functional>

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

int main(){
   std::function<int(int)> fn = square;
   std::cout << fn(3) << '\n';
}

Example 02

In this case it stores two int and returns a double:

#include <iostream>
#include <functional>

int sum(int x, int y){
   return x + y;
}

int main(){
   std::function<double(int, int)> fn = sum;
   std::cout << fn(6, 9) << '\n';
}

Example 03

Similar to above, but now it’s a function pointer to the address of &sum:

#include <iostream>
#include <functional>

int sum(int x, int y){
   return x + y;
}

int main(){
   std::function<double(int, int)> fn = &sum;
   std::cout << fn(6, 9) << '\n';
}

Example 04

In the <functional> header is also available std::negate, in this case using std::negate it inverts the output value:

#include <iostream>
#include <functional>

int main(){
   std::function<int(int)> fn = std::negate<int>();
   std::cout << fn(42) << '\n';
}

Output: -42.

Example 05

It can also be used in conjunction with lambda, so its signature makes it possible to identify the lambda:

#include <iostream>
#include <functional>

int main(){
   std::function<int(int)> fn = [&](int x){
     return x * 2;
   };
   std::cout << fn(9) << '\n';
}

That is, it is another STL feature that can be useful in several cases where performance is fundamental, but debugging is also important. For more information see this link.

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