In this example where is the C++ assignment operator used rather than the copy constructor?

As part of an ongoing process of trying to upgrade my C++ skills, I am trying to break some old habits. My old school C programmer inclination is to write this;

void func( Widget &ref )
{
    Widget w;  // default constructor
    int i;
    for( i=0; i<10; i++ )
    {
        w = ref;  // assignment operator 
        // do stuff that modifies w
    }
}

This works well. But I think the following is closer to best practice开发者_StackOverflow社区;

void func( Widget &ref )
{
    for( int i=0; i<10; i++ )
    {
        Widget w = ref; // ??
        // do stuff that modifies w
    }
}

With my Widget class at least, this works fine. But I don't fully understand why. I have two theories;

1) The copy constructor runs 10 times.

2) The copy constructor runs once then the assignment operator runs 9 times.

Both of these trouble me a little. 2) in particular seems artificial and wrong. Is there a third possibility that I am missing ?

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Your first theory is correct: the copy constructor is invoked ten times. This:

Widget w = ref;

is (almost) the same as:

Widget w(ref);

The first is called copy initialization; the second is called direct initialization. Both invoke the copy constructor. The main difference between the two is that the first is invalid if the copy constructor is declared explicit, while the second is valid so long as there is an accessible copy constructor.

You can verify this by declaring and defining both a copy constructor and a copy assignment operator for Widget, and seeing how many times each is called.

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Of course the copy constructor runs 10 times! When you iterate the for(;;) cycle, at each iteration the variables declared inside the braces will go out of scope. If Widget has a destructor, it will be called 10 times (possible performance hit).