Function pointer to member function

int (*x)() is not a pointer to member function. A pointer to member function is written like this: int (A::*x)(void) = &A::f;.

Call member function on string command

#include <iostream>
#include <string>


class A 
{
public: 
    void call();
private:
    void printH();
    void command(std::string a, std::string b, void (A::*func)());
};

void A::printH()
{
    std::cout<< "H\n";
}

void A::call()
{
    command("a","a", &A::printH);
}

void A::command(std::string a, std::string b, void (A::*func)())
{
    if(a == b)
    {
        (this->*func)();
    }
}

int main()
{
    A a;
    a.call();
    return 0;
}

Pay attention to (this->*func)(); and the way to declare the function pointer with class name void (A::*func)()

The syntax is wrong. A member pointer is a different type category from a ordinary pointer. The member pointer will have to be used together with an object of its class:

class A {
public:
 int f();
 int (A::*x)(); // <- declare by saying what class it is a pointer to
};

int A::f() {
 return 1;
}


int main() {
 A a;
 a.x = &A::f; // use the :: syntax
 printf("%d\n",(a.*(a.x))()); // use together with an object of its class
}

a.x does not yet say on what object the function is to be called on. It just says that you want to use the pointer stored in the object a. Prepending a another time as the left operand to the .* operator will tell the compiler on what object to call the function on.