Olivier ha scritto:
> Hi All,
>
> I was playing around with code (the code did consent) and stumbled
> over this :
>
> #include
>
> char const & hideConstRefToTemporary( char const &c )
> {
> return c;
> }
>
> char const & getChar1( )
> {
> return hideConstRefToTemporary('A');
> }
>
> char const & getChar2( )
> {
> return 'A';
> }
>
> int main( )
> {
> std::cout << getChar() << "\n";
> std::cout << getChar2() << "\n";
> return 0;
> }
>
> If you try and compile this code with gcc (I used version 4.3.2
> 20080827 alpha-testing 1), The compiler will complain of a reference
> to temporary for the function 'getChar2' but not for the function
> 'getChar1'.
>
> Is that a normal behaviour?
The question is ill-formed. Warnings are not a matter treated by the
standard, they are a quality-of-implementation issue. So a compiler can
emit or fail to emit any kind of warning and still that would be
"normal" behaviour.
> Is the code valid?
It depends on your definition of "valid". The code is well-formed, but
the behaviour is undefined, in both cases.
> Is there any word in the C++ standard that talk about this case?
No, because it shouldn't. Even the rule that says that returning a
reference to a temporary is bad is never said explicitly AFAIK, but is
implicit in 3.8.
> I know that temporaries can be bound to a const reference as function
> arguments, but how does that influence the way you use the variable
> representing that temporary ?
Each time a temporary is bound to a reference, the lifetime of the
temporary is extended to be not shorter than the lifetime of the reference.
int f();
void g()
{
const int& i = f(); // #1: i is bound to the temporary
/* ... */
// Lifetime of the temporary extended up to the point where i exits
// from scope. Normally it would have been destroyed at the
// end of line #1
}
However, this rule has no effect on temporaries bound to function
arguments of reference type, because the argument typically has shorter
lifetime:
int f();
void g(const int& i);
void h()
{
g(f()); // lifetime of i ends before lifetime of the temporary
}
HTH,
Ganesh
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Hiding a reference to temporary
