C++: Call function of Class B when a function of Class A is called (Pointer to Class A is a Member of Class B)

Question

class A {
  void functionA();
};

class B {
  A* A_;
  void functionB();
};

How can I automatically call functionB() in a class B instance, if functionA() is called outside of the class B instance? (Pointer to a Class A instance is member of the class B instance). I am looking for something like the SIGNAL/SLOT method in Qt.

Answer 1

One option is to use a function object as a callback. In the following example, B's constructor registers a lambda function in the A instance which will be called by A whenever functionA is called. This lambda, in turn, simply calls functionB.

#include <functional>
#include <iostream>

class A {
public:
    void functionA() {
        std::cout << "Function A called" << std::endl;
        if (callback_) {
            callback_();
        }
    }

    void setCallback(std::function<void(void)> callback) {
        this->callback_ = callback;
    }

private:
    std::function<void(void)> callback_;
};

class B {
public:
    B(A* a) : A_(a) {
        A_->setCallback([this](){this->functionB();});
    }

    void functionB() {
        std::cout << "Function B called" << std::endl;
    }

private:
    A* A_;
};

int main() {
    A a;
    B b = B(&a);

    a.functionA();
}

The output:

Function A called
Function B called

As you can see in the output, when a.functionA() is called in main, functionB is also called automatically.