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List:       kde-devel
Subject:    Re: strings and QStrings
From:       Patrick Julien <freak () desource ! net>
Date:       2000-12-10 18:28:53
[Download RAW message or body]

Actually, the object is not being returned by operator=, a reference to that 
object is returned, so no constructor

On December 10, 2000 01:02 pm, David van Hoose wrote:
> Bo Thorsen wrote:
> > On Sun, 10 Dec 2000, David van Hoose wrote:
> > > SNIP
> > >
> > > > This prints out "constructor" twice. It has no effects to change
> > > > optimization levels or not optimize or debug or whatever. So, I
> > > > repeat what I said: "Class c = object" will always call the copy
> > > > constructor and not first call constructor and then operator=.
> > >
> > > I know what it prints. A function that returns the object calls the
> > > copy constructor.
> >
> > Please read the code again, no function is returning an object and
> > thereby calling the copy constructor. The constructors are called only in
> > the creation of the two objects of type B.
> >
> > The discussion was sparked by someone claiming that it's better to do
> > "Object o(someOtherObject)" than "Object o = someOtherObject" which is
> > simply not true since they generate the same code. If you keep saying
> > that it generates different code, I suggest you look up the part in the
> > ISO specs where this specific optimization is mentioned. I'm not going to
> > waste my time doing so.
>
> Listen sir. I do not know how much you know or do not know about C++,
> but I can say for sure that you do not know what you are talking about.
> I am an assembly programmer. I have observed how the compiler does
> implicit
> placements of the constructors. The ONLY reason the code you posted for
> the
> overloaded = operator is because you have the object being returned.
> Change the return to void, and you will NEVER see the constructor.
> Because the = operator has to call the copy constructor for the return,
> if you do not use the return value, the constructor is a better choice
> to use.
>
> > Since you cut out the code being discussed, here it is again:
> >
> > #include <iostream>
> >
> > class A {
> > public:
> >     int i;
> > };
> >
> > class B {
> > public:
> >     B(){}
> >     B(const A &a) { i = a.i; std::cout << "constructor\n"; }
> >     B &operator=(const A &a)
> >         { i = a.i; std::cout << "operator=\n"; return *this; }
>
> // Object returned ------------------------------^^^^^^^^^^^^^
>
> > private:
> >     int i;
> > };
> >
> > int main(int argc, char **argv) {
> >     A a;
> >     B b1 = a;
> >     B b2(a);
> >     return 0;
> > }
 
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