为什么这个赋值运算符不返回复制的 object？

Question

I have a class called Stack. I've implemented the copy constructor but for the copy assignment constructor I just have it call the copy constructor since the logic is the same. But for some reason it just returns a default constructed object. I don't understand why.

The copy constructor:

Stack::Stack( const Stack& s )
{
    if ( s.Empty() )
    {
        this->entry = nullptr;
    }
    else
    {
        this->entry = new Entry();
        Entry* i = this->entry;
        for ( Entry* p = s.entry; p != nullptr; p = p->next )
        {
            i->number = p->number;
            i->next = p->next == nullptr ? nullptr : new Entry();
            i = i->next;
        }
    }
}

The copy assignment constructor:

Stack& Stack::operator=( const Stack& s )
{
    return Stack( s );
}

The calling code:

Stack s;
s.Push( 5 );
s.Push( 3 );

Stack s2;
s2 = s; //s2 just ends up defaulted constructed instead of copied from s

If I replace the lines:

Stack s2;
s2 = s;

with:

Stack s2(s);

everything works just fine. What am I doing wrong here?

EDIT --

So the lesson here is to implement the assignment ctor and leverage that in the copy ctor, not the other way around as I had done.

Answer 1

The behaviour of your assignment operator is undefined.

You are returning a dangling reference .

The idiomatic way of writing the assignment operator is std::move the instance of the object passed ( s in your case, which you should pass by value) to self, and return *this as a reference.

Answer 2

Your assignment operator is incorrect. It is returning a reference to a temporary object. It should return *this instead:

Stack& Stack::operator=( const Stack& s )
{
   // copy s to members
   return *this;
}

If you want to avoid implementing things twice its usually easier to implement the assignment operator and use that in the copy constructor:

Stack::Stack( const Stack& s )
:Stack()
{
  *this = s;
}

Stack& Stack::operator=( const Stack& s )
{
    if (&s == this) return *this;
    // TODO: free current members?
    if ( s.Empty() )
    {
        this->entry = nullptr;
    }
    else
    {
        this->entry = new Entry();
        Entry* i = this->entry;
        for ( Entry* p = s.entry; p != nullptr; p = p->next )
        {
            i->number = p->number;
            i->next = p->next == nullptr ? nullptr : new Entry();
            i = i->next;
        }
    }
    return *this;
}

Answer 3

When you do

Stack& Stack::operator=( const Stack& s )
{
    return Stack( s );
}

Stack(s) creates an object with a scope limited to the current function. You then return a reference to this local object.

So, when doing x = y , x is not modified to become a copy of y and a reference to an object that is out of scope is returned, then discarded.

The job of the Stack& Stack::operator=( const Stack& s ) is not to return a copy. Its job is to modify x (called this ) so that it becomes equal to y (called s ).

So, the proper way would look something like:

Stack& Stack::operator=( const Stack& s )
{
    this->entry = new Entry();
    // then all the code to make `this` equal to `s`
}

Answer 4

Since you have a working copy constructor and working destructor for Stack , you can simply use copy / swap to implement the assignment operator:

Stack& Stack::operator=( const Stack& s )
{
    if ( this != &s)
    {
       Stack temp(s);
       std::swap(temp.entry, entry);
    }
    return *this;
}

If you have other member variables, you will need to swap those also.

This works by creating a temporary, and simply swapping out the members of the current object ( this ) with the copy's members. Then the copy dies off with the old data.