C++ reference calls 'wrong' destructor

August 22, 2022

Why does below snippet print:

Constructing Working on this
Constructing working on that
Destructing working on that
Destructing working on that

I would expect "constructing this + that" and "destructing this that".

So the idea here is to have to have a reference to a workingItem and then do something in the destructor if something goes out of scope. It seems that this does not work, and my question is why? Does this cause a memory leak? Also why don’t I get "destructing this" because I did not overwrite a?

I can get this working by making workingItem a pointer, but my question is: is there also a way to get this working by using a ref?

#include <iostream>

class A {
 private:
   std::string str;
 public:
   A(const std::string& s) : str(s) { std::cout << "Constructing "<<s<<std::endl; };
   ~A() { std::cout << "Destructing "<<str<<std::endl; };
};

int main() {
    A a("Working on this");
    A& workingItem=a;
    A a2("working on that");
    workingItem=a2;
}

>Solution :

Implement a operator= to get the full picture:

#include <iostream>

class A {
 private:
   std::string str;
 public:
   A(const std::string& s) : str(s) { std::cout << "Constructing "<<s<<std::endl; };
   ~A() { std::cout << "Destructing "<<str<<std::endl; };
   A& operator=(const A& other){
       std::cout << "assign " << other.str << " to " << str <<"\n";
       str = other.str;
       return *this;
   }
}; 

int main() {
    A a("Working on this");
    A& workingItem=a;
    A a2("working on that");
    workingItem=a2;
}

Output is:

Constructing Working on this
Constructing working on that
assign working on that to Working on this
Destructing working on that
Destructing working on that

You cannot rebind references. This line A& workingItem=a; initializes the reference workingItem to refer to a.

This line is something completely different: workingItem=a2; it assigns a2 to the object refered to by workingItem, and that is a1. It is the same as a1 = a2;. Hence you end up with two identical As that get destroyed when main returns.

In general it is important to be aware of the difference between initialization, T x = y;, and assignment, x = y;.