Thread: Can you call a constructor within a destructor, or vise vesa?

folks, I came across questions about calling constructor or destructor.
I am curious whether you can:

Code:

1. call a constructor in a constructor
2. call a destructor in a constructor
3. call a constructor in a destructor
4. call a destructor in a destructor
5. call a constructor in a member function
6. call a destructor in a member function

So I wrote the following code and want to share with you what I got. Comments are welcomed! Please check if I miss anything.

1. call a constructor in a constructor

Code:

class A{
public: 
    A(){
        A();
        x = 100;
        cout<<"A() "<<x<<endl;
    }
    int x;
    ~A(){
        x = 0;
        cout<<"~A() "<<x<<endl;
    }
};
int main()
{
    A obj;
    return 0; 
}

Basically you are not actually calling the constructor, you are just generating a temp object.
If you use this->A() to call then you will get an error.

Code:

error: calling type `A' like a method

Also this brings a recursive calling. Soon you will use up memory and crash.

2. call a destructor in a constructor

Code:

class A{
public: 
    A(){
        this->~A();
        x = 100;
        cout<<"A() "<<x<<endl;
    }
    int x;
    ~A(){
        x = 0;
        cout<<"~A() "<<x<<endl;
    }
};

int main()
{
    A obj;
    return 0; 
}

You can't call ~A() without qualifying: this->~A(). At least I didn't figure out a way to avoid it.
Yes, you can call destructor in the class, and you can see it destructs the member variable x.

3. call a constructor in a destructor

Code:

class A{
public: 
    A(){
        x = 100;
        cout<<"A() "<<x<<endl;
    }
    int x;
    ~A(){
        A();
        x = 0;
        cout<<"~A() "<<x<<endl;
    }
};

int main()
{
    A obj;
    return 0; 
}

Again, you are not calling constructor, just a temp obj.

4. call a destructor in a destructor

Code:

class A{
public: 
    A(){
        x = 100;
        cout<<"A() "<<x<<endl;
    }
    int x;

    ~A(){
        this->~A();
        x = 0;
        cout<<"~A() "<<x<<endl;
    }
};

int main()
{
    A obj;
    return 0; 
}

This also gives you a dead loop.

5. call a constructor in a member function

Code:

class A{
public: 
    A(){
        x = 100;
        cout<<"A() "<<x<<endl;
    }
    int x;
    void foo()
    {
        cout<<"BEFORE foo"<<x<<endl;
        A();
        cout<<"AFTER foo"<<x<<endl;
    }
    ~A(){
        x = 0;
        cout<<"~A() "<<x<<endl;
    }
};

int main()
{
    A obj;
    obj.foo();
    return 0; 
}

It works. just a temp obj.

6. call a destructor in a member function

Code:

class A{
public: 
    A(){
        x = 100;
        cout<<"A() "<<x<<endl;
    }
    int x;
    void foo()
    {
        cout<<"BEFORE foo"<<x<<endl;
        A();
        cout<<"AFTER foo"<<x<<endl;
    }
    ~A(){
        x = 0;
        cout<<"~A() "<<x<<endl;
    }
};

int main()
{
    A obj;
    obj.foo();
    return 0; 
}

It works.

Your analysis seems basically correct. The only one that you would ever consider using is calling the destructor in a member function, but you would only use that in rare occasions when you are managing memory yourself and are freeing the memory used by the object elsewhere.

Otherwise doing these things is impossible, useless, and/or dangerous. The constructor and destructor are generally called automatically, if you want to do it explicitly there are almost always better solutions.