C++ Class 未初始化

Question

In C++ you can't include Header Files in each other because of recursion. So you have to make use of Predefined Classes right? So I want to get the value of num in C.hpp and print it out in B.hpp .

C Class gets initialized before B Class which is expected.

Console Output

But when I'm trying to call A::instance->c it is NULL !

B Class with unitialized C Class

A.hpp

#include <iostream>
class C;                        // PREDEFINED CLASS
class B;                        // PREDEFINED CLASS
class A {
public:
    inline static A* instance;
    C* c{ 0 };
    B* b{ 0 };

    A() {
        instance = this;
    }

    void start(B* b,C* c) {
        this->c = c;
        this->b = b;
    }
};

B.hpp

#include "A.hpp"
#include "C.hpp"

class B {
public:

    void call() {
        if (A::instance->c)
            std::cout << "C Num: " << A::instance->c->num << "\n";
        else std::cout << "C Class is NULL!\n";
    }

    B() {
        std::cout << "B init!\n";
        call();
    }
};

C.hpp

class C {
public:
    int num = 0;

    C() {
        std::cout << "C init!\n";
        num = 69;
    }
};

Source.cpp

#include "A.hpp"
#include "B.hpp"
#include "C.hpp"

void main() {
    A a;
    a.start(new B(), new C());
}

Answer 1

C Class gets initialized before B Class which is expected.

No, it's not expected, the order of evaluation of function arguments is unspecified.

But even if that's the case as in your example output, you've created new C class object, and now you are trying to create B class object and during that, you are trying to access A::instance->c which is not set yet .

You set c for class A object in the start function, but you are still evaluating its parameters during this calls and didn't reach the actual body of the function.

Thus, the c in the class A is the default 0 as you set by C* c{ 0 }; .