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List:       kde-devel
Subject:    Re: strings and QStrings
From:       David van Hoose <david-vh () home ! com>
Date:       2000-12-10 18:02:54
[Download RAW message or body]

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;
> }

-- 
-Dave
 
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