🚀 go-pugleaf

damian@molly.cs.monash.edu.au (Damian Conway) writes:

>jason@cygnus.com (Jason Merrill) and other cogniscenti write:

>>> | According to the standard, are the two lines of the main
>>> | equivalent ? My Opinion is yes

>>No.

>Then is it possible to set up a class X wherein:

>	X x(1);
>and
>	X x = 1;

>behave differently?

Yes.  Silly example: Write a message to the terminal from the copy
constructor. If the copy is optimized away, you won't see the message.

>Or is the compiler always allowed to optimize away the temporary and copy 
>constructor, even if that changes the semantics of the program?

In the second example, the compiler is allowed but not required to
optimize away the copy. That is, when you write
	X x = 1;
it can be interpreted as either
	X x(1);
or
	X x(X(1));

This is only one of many situations where you cannot predict whether
a temporary will be generated and the copy constructor used. For
that reason, you should write copy constructors so that program
correctness doesn't depend on whether they are optimized away.
--
Steve Clamage, stephen.clamage@eng.sun.com

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