Passing a class-member function to a global function as argument

Question

I am trying to pass a member function of class A to a global function as parameter. What must I do to make this work? Also, is this a good idea? Context: I want to do this because (the synonym) doSomething(...) is a very general function that is used in main() as well as in different classes. Thus I can avoid to have several copies of the same code in my Project. What are alternatives (if it's not optimal)?

#include <iostream>
using namespace std;

double doSomething(int i, double (*f)(double)) { return (*f)(i); }

class A{
public:
    A(double x) : number(x) {}
    double times(double i) { return ::doSomething(i, &A::multiply);} //calles the global function and gives a member function as parameter
    double multiply(double i) {return number*i;}
private:
    double number;
};

int main() {

    A obj(5.0);
    cout << obj.times(3.5) <<endl;

    return 0;
}

Compiler complains:

../src/test5.cpp: In member function ‘double A::times(double)’:
../src/test5.cpp:17:63: error: cannot convert ‘double (A::*)(double)’ to ‘double (*)(double)’ for argument ‘2’ to ‘double doSomething(int, double (*)(double))’
  double times(double i) { return ::doSomething(i, &A::multiply);} //calles the global function and gives a parameter member function as parameter
                                                               ^
../src/test5.cpp:17:65: warning: control reaches end of non-void function [-Wreturn-type]
  double times(double i) { return ::doSomething(i, &A::multiply);} //calles the global function and gives a parameter member function as parameter

Answer 1

With C++11 you might do:

#include <functional>

typedef std::function<double (double)> do_something_function;
double doSomething(double i, do_something_function f) { return f(i); }

class A{
    public:
    A(double x) : number(x) {}
    double times(double i) {
        do_something_function f = [this] (double i) {
            return this->multiply(i);
        };
        return ::doSomething(i, f);
    }
    double multiply(double i) {
        return number * i;
    }

    private:
    double number;
};

Answer 2

The way you call a non-static function member of a class via pointer differs from the way you do it on freestanding function because a pointer to instance is needed.

#include <iostream>
using namespace std;

class A;
typedef double (A::*f)(double);
double doSomething(double i, f p, A* ap);

class A {
public:

    A(double x) : number(x) {
    }

    double times(double i) {
        return ::doSomething(i, &A::multiply, this);
    } //calles the global function and gives a member function and pointer to A

    double multiply(double i) {
        return number*i;
    }
private:
    double number;
};

double doSomething(double i, double (A::*fp)(double), A* ap) {
    return (ap->*fp)(i);  // call *fp on *ap
}

int main() {

    A obj(5.0);
    cout << obj.times(3.5) << endl;

    return 0;
}

---

example

Answer 3

it is tricky of programing -- convert a non-static member function into a global function refer to the source code : https://github.com/shenxiaolong-code/processInternalExplorer/blob/master/sources/SystemExplorer/include/SystemExplorer/callback_cpp2c.h