Re: Temporaries

[linguist-gcc at rich-paul dot net]

Sorry, my error.  The code that I'm attempting to use does NOT return
a reference, it returns a Set by value, of course.  I merely mistyped
when I entered the message.

On Sat, 27 May 2000, Joe Buck wrote:

> 
> > I'm attempting to port some code that has the following definitions:
> > --------------------- cut here ---------------
> > class Set {
> > 	public:
> > 	Set &operator |= ( Set & );
> > 	Set &operator | ( Set & );
       ^^^^^^
	    damn
> > };
> > Set &Set::operator | ( Set &rhs ) {
> > 	return Set(rhs)|=lhs;
> > };
> > --------------------- cut here ---------------
> 
> This is not legal C++.
> 
> > Under visual c++, this is fine,
> 
> Actually it may not be fine under C++ ("compiles" != "fine").  That is,
> the code in question is likely to have strange bugs, as you are trying to
> create a reference to a temporary which is on a portion of the stack that
> is popped.
> 
> > but under gcc it is an error (not a warning),
> > because the non-const reference parameter of operator |= is being initialized
> > with an rvalue of type 'String'.
> > 
> > I always thought that an explicit constructor invocation yielded an lvalue.
> 
> No, it produces a temporary.
> 
> > but in some cases, like the one above, it would be
> > quite useful, saving from the longer:
> > {
> > 	Set temp(lhs);
> > 	temp |= rhs;
> > 	return lhs;
> > };
> 
> I write such code all the time, but I make the return type of operator|
> Set, not Set&, and it works fine.  I'll bet that if you do the same you
> can get the code to work.
> 
> > What, if anything, does the standard say on this subject?  I couldn't find
> > anything in Stroustrup (3rd) to clarify.
> 
> The standard bans it.
>