#include <iostream>
#include <vector>
 
struct A;
 
struct B
{
    B() : _a(nullptr) {}
 
    void setA(A*a) { _a = a; }
    void genericFunction();
 
private:
    A* _a;
};
 
struct A
{
    void addB(B*b);
    void iterate();
    void specialFunction();
 
private:
    std::vector<B*> _b;
};
 
void B::genericFunction()
{
    std::cout << "From B::genericFunction: ";
    if (_a)
        _a->specialFunction();
}
 
void A::addB(B* b)
{
    _b.push_back(b);
    b->setA(this);
}
 
void A::iterate()
{
    for (auto b : _b)
        b->genericFunction();
}
 
void A::specialFunction()
{
    std::cout << "A::specialFunction()\n";
}
 
int main()
{
    B b1;
    B b2;
 
    A a;
    a.addB(&b1);
    a.addB(&b2);
 
    a.iterate();
}

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