我需要知道编译器如何与 cpp 中的析构函数一起工作

Question

Lets say I have 2 diffrent classes.

Why when I put class A object as data member inside class B , the class that getting destroyed first is class B and then class A but when i put class A object in the builder of class B then class A getting destroyed first and then class B ? for example:

class A {
public:
    ~A() { std::cout << "Deleting A\n"; }
};

class B {
private:
    A object;
public:
    ~B() { std::cout << "Deleting B\n"; }
};

int main( void ) {
    B test;
    return 0;
}

result:

Deleting B
Deleting A

But the next example gives me the opposite result :

class A {
public:
    ~A() { std::cout << "Deleting A\n"; }
};

class B {
public:
    B() { A object; }
    ~B() { std::cout << "Deleting B\n"; }
};

int main( void ) {
    B test;
    return 0;
}

result:

Deleting A
Deleting B

Answer 1

In your second example B test; calls the constructor of B.

The constructor B is just a function and it has a local variable A object; . This object is created and at the end of the scope (end of the constructor of B) it is detroyed.

Back in main at the end of main the object test is destroyed.

So in that case the object A object; is destroyed first, later object B test; is destroyed.

In your first example this is something different. There A object; is a member of class B, so the constructor of B is constructing the member automatically. The order of construction is: base classes first (not present in your example), then members, then the body of the constructor. Destruction is the opposite order: body of the destructor first, then the members (including your A object; ), then the base classes (not present in your example).

As already said in a comment, you can easily step through the code in a debugger to get a better understanding of the order of execution.

Answer 2

因为类 A 的对象在类 B 的构造函数完成后立即销毁（因为它是函数内部的局部变量），并且在调用类 B 的析构函数之前。

Answer 3

In the second example you've got your object (instance of Class A), local in your Constructor. When you now instantiate B in your main function, the constructor is called, creates object , and destroys it, as sonn as the constructor has finished. In your first example, object is a class member and lives as long as your class object in your main function. When the programm is finished it calls your costum Destructor , this is then executed first, then it destructs all members ( standard Constructor ).