Re: copy constructor and assignment

From:

Francis Glassborow <francis.glassborow@btinternet.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Tue, 15 Dec 2009 10:11:57 CST

Message-ID:

<xt2dnQQllvXYxLrWnZ2dnUVZ8vWdnZ2d@bt.com>

samhas wrote:

Hello,

I have the following class definition:

class test2{
public:
int y;

If you are going to have public data declare the class with the struct
keyword. That can serve as a wakeup call.

  test2(int ui) : y(ui){}
  test2(const test2& t) : y(5){}
  test2& operator= (const test2& t){ y = 6; return *this; }

};

In an expression like

test2 t2(test2(20)); or
test2(t2) = test2(20);

neither the copy-constructor nor the assignment operator gets
executed.
t2 ends up with y .

But in the following case the copy constructor gets called:

test2 t = test2(20);
test2 t2(t); // or test2 t2 = t;

Can someone explain to me what's going on there?

The '=' in a declaration is not an assignment so operator=() would not
be called. However the compiler can in many cases elide the call of a
copy ctor even if doing so suppresses side effects. All that is
happening in is that the compiler is exercising that freedom. It does
not have to justify when it chooses to do so as long as it does whenever
the Standard actually requires a call of the copy ctor. It is required
in the case:

test2 t2(t);
and in the case
test2 t2 = t;

Because in those cases it is the only ctor that can construct t2. But in
the other cases another ctor has to be called first so the compiler can
elide the copy ctor.

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