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Angel Angel
Asked: 2020-11-05 16:26:45 +0800 CST 2020-11-05 16:26:45 +0800 CST

Is std::function equivalent to a pointer?

  • 772

I was practicing with lambdas and came across the following code:

auto make_fibo() {
    return [](int n) {
        std::function<int(int)> recurse;

        recurse = [&](int n) {
            return (n <= 2) ? 1 : recurse(n - 1) + recurse(n - 2);
        };

        return recurse(n);
    };
}

I did not know what it was functionor how it worked after searching and reading several texts, for example the following:

http://www.cplusplus.com/reference/functional/function/

My question is yes, it std::functionis similar to the following for example:

typedef int (*FredMemFn)(int i);
c++

2 Answers

  1. Best Answer

    The answer is yes. The point is that function<> is capable of handling more cases than just being a simple pointer to a function. For example, it can also cover the case of functor , that is, the case of a class with the operator () overloaded. Take a look at the following code:

    #include <iostream>
#include <functional>
using namespace std;

int doble(int x) {
    return x * 2;
}

class Doblador {
public:
    int operator()(int x) {
        return x * 2;
    }
};

int main() {
    Doblador d;
    function<int(int)> f = doble;

    cout << "Doble de 2 = " << f( 2 ) << endl;

    f = d;
    cout << "Doble de 2 = " << f( 2 ) << endl;

    return 0;
}

    You have the code here: http://ideone.com/5lALLH I hope you find it useful.

    • 20

  2. std::functionis a class that wraps any element that can be called, for example:

    • Pointers to functions (what is mentioned in the question).

    • Objects of a class that have the operator ()overloaded.

    • Lambda expressions.

    • bind expressions ( std::bind, basically a function pointer with one or more predefined arguments).

    Source: cppreference.com - std::function

    Note that template functions are not callable as such, only a particular instance of a template function with a type already attached can be combined with a std::function.

    • 4