How to pass a Lambda Function as a parameter [duplicate]

Question

I'm having some issues working with c++ recently, its basically this:

Inside a function (lets say int main), I declared a variable Y = 5, and I have this lambda function that receives a value and adds up Y;

My problem is: I need to pass this lambda function to an already existent function, so it can be called inside the other function.

I tried a couple of things, but none of them worked as I intended (some not even worked):

double receives( double (*f)(double) )
{
    return f(5);
}

int main() 
{

    int y = 5;

    auto fun = [](double x) {
      return x + y;
    };

    cout << receives(&fun);

    return 0;
}

Another problem is that I cant change my receives function signature, the parameter must be double (*f)(double) because of the remainder of the code. I also need the lambda function to carry over the y value, without using additional parameters.

Anyone can help me on this?

Answer 1

Use std::function like for example

#include <iostream>
#include <functional>

double receives( const std::function<double( double )> &f )
{
    return f( 5 );
}

int main() 
{
    int y = 5;

    auto fun = [y](double x) 
    {
      return x + y;
    };

    std::cout << receives( fun ) << '\n';

    return 0;
}