Re: Strange warning from g++ "returning reference to temporary"

"Martin T." <>
Thu, 11 Dec 2008 13:04:52 CST
blargg wrote:

Martin T. wrote:

blargg wrote:

    operator const char* const*() const { return &s; } // OK, right?

I don't think that will compile. You probably meant:
operator const char* const() const { return &s; }

Why not? &s has type 'char* const*', which is convertible to 'const
char* const*' (C++03 section 4.4 paragraph 4).

I'm sorry you're right. I was just not able to parse the syntax :-)


Since the equivalent pointer version is OK, why shouldn't I be able to
take a reference of type 'const char* const&' to an object of type
'char* const'? I cannot change the pointer through the reference (just
as with the equivalent pointer version), so there's no need for a
temporary. Put another way, if there needed to be a temporary, then
the equivalent pointer version would also need a temporary.

You are quite right I think - see below.


If the reference version really is invalid (and the pointer version
not), then this is one glaring difference between references and
pointers where one would expect them to behave the same.

Well. As you a dealing with a [pointer to char] you should compare it
with a [reference to char] and NOT, as you do in the above example,
a [reference to pointer to char].

Let's generalize that, for clarity: "As you a dealing with a [pointer
to T] you should compare it with a [reference to T] and NOT, as you do
in the above example, with a [reference to pointer to T]."

Above, I AM doing the first, where T is 'const pointer to const char':

Yes, I misread what you intended to do.

     struct Foo
        char* s;

        typedef const char* const T;

        operator T* () const { return &s; }
        operator T& () const { return s; }

Doesn't it seem odd that the operator T* above is valid, but operator
T& is not (at least according to a few compilers and posters here)?

I have tried to compile your example with Visual Studio 2005 (language
extensions OFF and warning level 4). It compiles without warnings:

struct Foo
   char* s;

   typedef const char* const T;

    // Both Versions work with MS VC8 (VS 2005) with
    // Language extensions disabled and
    // Warning level 4.
    // (Indeed, the compiler generates exactly the same assembly code for
   operator T* () const { return &s; }
   operator T& () const { return s; }

int main(int, char**)
    Foo f;
    const char*const* xf1 = f; //.operator const char *const *();
    const char*const& xf2 = f; //.operator const char *const &();


    return 0;

I have tried it with comeau online and this gives:
"ComeauTest.c", line 12: warning: returning reference to local temporary
     operator T& () const { return s; }

So it would be quite interesting to see if comeau or gcc actually
produces different code for the two operators ...


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