Error passing a method to another method. C++

Question

I am a beginner in programming and decides to venture into a project with some friends, in it I need the program descending which the order of the routine based on a factor, I am passing to a smaller chained list to facilitate the organization and have to deal with the whole class.
However I am finding problem to pass the method to function. Follow an example code:

#include <iostream>
#include <string>

using namespace std;

class Txt
{
    string palavra;
    int tam;
public:
    void load(string txt) { palavra = txt; tam = txt.length(); }
    void print() { cout << palavra << endl; };
    int w_tam() { return tam; }
};

class Info2
{
public:
    int tam;
    void (*print)();
};

Info2 redirecionar(Txt txt)
{
    Info2 info;
    info.tam = txt.w_tam();
    info.print = &(txt.print); // aqui acontece um erro
    return info;
}

int main()
{
    Txt A, B;
    A.load("Amor");
    B.load("Bolota");

    Info2 AA, BB;
    AA = redirecionar(A);
    BB = redirecionar(B);

    if (AA.tam > BB.tam)
    {
        AA.print();
    }
    else
    {
        BB.print();
    }
}

I use VS2017, and error has the following description: '&' invalid operation in the associated member function expression. But I could not understand, I saw the same idea in another topical in C.

Answer 1

The guy from Txt::print is not void (*)(), and yes void (Txt:: *)().

Change the type of print in class Info2:

class Info2
{
public:
    int tam;
    void (Txt:: *print)();
};

And assign the address of the member function:

info.print = &Txt::print;

Also save the instance of a Txt, because a member function can only be called with a valid class instance.

To call the member function, use the Operator .* (or ->* in the case of Info2 be accessible by a pointer only) together with the instance of a Txt:

auto print = AA.print;
(A.*print)(); // Ou (A.*AA.print)();

Note the parentheses around A.*print: the precedence of a function call is greater than that of the operator .*, then parentheses are needed.

If you want to avoid this syntax and not mind dynamic allocations, you can make use of std::function to store the method call inside a lambda:

#include <functional>

class Info2
{
public:
    int tam;
    std::function<void()> print;
};

Info2 redirecionar(Txt &txt)
{
    Info2 info;
    // ...
    info.print = [&txt] { txt.print(); };
    // ...
}

// ..
AA.print();