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comp.lang.c++

Hiding const references bound to temporary

casul

10/14/2008 6:36:00 PM

Hi All,

Given the following code :

char const & identity( char const &c ) { return c; }

char const & f( ) { return identity('A'); }
char const & g( ) { return 'A'; }

Of course, the compiler complains of function 'g' returning a
reference to a temporary. But the compiler (I used gcc 4.2.x) doesn't
say anything regarding the function 'f'. I know that temporaries bound
to constant references have a life span that extends to the expression
in which the function call where the bounding took place is evaluated.
Now given that, is the fact of returning in turn the constant
reference returned by 'identity' considered to be part of the
expression in which 'identity' is evaluated, ie the expression is

return identity('A');

or does the evaluated expression just consist of the function call
'identity', ie:

identity('A');

?

I would have said the first case. But I'm not sure. I guess the main
question is: does a call to 'f' yield undefined behaviour ?

Thanks for your comments,

Olivier.
2 Answers

Victor Bazarov

10/14/2008 7:01:00 PM

0

casul wrote:
> Given the following code :
>
> char const & identity( char const &c ) { return c; }
>
> char const & f( ) { return identity('A'); }
> char const & g( ) { return 'A'; }
>
> Of course, the compiler complains of function 'g' returning a
> reference to a temporary. But the compiler (I used gcc 4.2.x) doesn't
> say anything regarding the function 'f'. I know that temporaries bound
> to constant references have a life span that extends to the expression
> in which the function call where the bounding took place is evaluated.
> Now given that, is the fact of returning in turn the constant
> reference returned by 'identity' considered to be part of the
> expression in which 'identity' is evaluated, ie the expression is
>
> return identity('A');

No, it's not an expression. It's a *control statement*. As part of the
statement the expression 'identity('A')' is evaluated (and it's a full
expression).

> or does the evaluated expression just consist of the function call
> 'identity', ie:
>
> identity('A');
>
> ?

There is no semicolon in an expression. A semicolon is a *statement*
separator. Drop the semicolon and you got yourself an expression.

> I would have said the first case. But I'm not sure. I guess the main
> question is: does a call to 'f' yield undefined behaviour ?

IMHO, yes. The temporary disappears as soon as the full expression is
evaluated. By the time anybody attempts to use the return value of the
'identity' function call with 'A' as the argument in any way, the
reference returned has become invalid. Any use of it (like passing up
to the caller of 'f') would be undefined behaviour.

V
--
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I do not respond to top-posted replies, please don't ask

James Kanze

10/14/2008 10:13:00 PM

0

On Oct 14, 8:35 pm, casul <olivier.gr...@gmail.com> wrote:

> Given the following code :

>    char const & identity( char const &c ) { return c; }

>    char const & f( ) { return identity('A'); }
>    char const & g( ) { return 'A'; }

> Of course, the compiler complains of function 'g' returning a
> reference to a temporary. But the compiler (I used gcc 4.2.x)
> doesn't say anything regarding the function 'f'.

How could it? Suppose that identity() and f() where in
different translation units. How could the compiler know that f
is in fact returning a reference to it's temporary?

> I know that temporaries bound to constant references have a
> life span that extends to the expression in which the function
> call where the bounding took place is evaluated.

Then you know something that is completely wrong. A temporary
has a lifetime until the end of the full expression in which it
appears. *IF* the temporary is used to initialize a reference,
it's lifetime is extended to match that of the reference (except
that a temporary bound to a reference being returned explicitly
doesn't have its lifetime extended). But whether the temporary
is bound to some other reference or not has no effect on its
lifetime.

> Now given that, is the fact of returning in turn the constant
> reference returned by 'identity' considered to be part of the
> expression in which 'identity' is evaluated, ie the expression
> is

>     return identity('A');

> or does the evaluated expression just consist of the function
> call 'identity', ie:

>     identity('A');

> ?

The temporary 'A' is used to initialize the reference which is
an argument of identity. The lifetime of that reference ends on
return from identity, which is *before* the end of the full
expression. So the lifetime of the temporary is at the end of
the full expression. (Actually, there's a slight extension
here; if the temporary were used to directly initialize the
return value, it's lifetime would be extended until the return
value was constructed. That's not the case here, however.)

> I would have said the first case. But I'm not sure. I guess
> the main question is: does a call to 'f' yield undefined
> behaviour ?

Of course.

--
James Kanze (GABI Software) email:james.kanze@gmail.com
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