[Bug libstdc++/40295] New: [C++0x] std::move and builtin types

[Bug libstdc++/40295] New: [C++0x] std::move and builtin types

[jwakely dot gcc at gmail dot com]

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I'm not sure what's going on here, but with a recent 4.5 snapshot this program
fails:

#include <utility>
#include <cassert>

template<typename T>
struct S
{
    S() : buf() { }

    void set(T&& i) { buf = std::move(i); }

    T&& get() { return std::move(buf); }

private:
    T buf;
};

struct pod { };

template class S<int>;
template class S<double>;
template class S<pod>;

int main()
{
    S<int> s;
    s.set(5);
    assert( s.get() == 5 );
}

Compiling with 4.5.0 20090528 gives these warnings:

tmpret.cc: In member function ‘T&& S<T>::get() [with T = int]’:
tmpret.cc:19:   instantiated from here
tmpret.cc:11: warning: returning reference to temporary
tmpret.cc: In member function ‘T&& S<T>::get() [with T = double]’:
tmpret.cc:20:   instantiated from here
tmpret.cc:11: warning: returning reference to temporary
tmpret.cc: In function ‘int main()’:
tmpret.cc:27: warning: ‘<anonymous>’ is used uninitialized in this function

And the assertion fails at runtime.

Notice that the explicit instantiations of S<int> and S<double> trigger the
warning about returning a reference to a temporary for this line:

    T&& get() { return std::move(buf); }

but the S<pod> instantiation doesn't.  I can't see what's wrong with that line,
it seems that std::move creates a temporary when 'T' is a builtin type.

It looks as though std::move(buf) instantiates std::move<T>(T&&) with T = int&,
so the rvalue-reference binds to an int& not an int.

Am I missing something or is this a bug in either std::move or g++?

N.B. the changes to std::move proposed by n2844 are not in the WP yet, so the
specification of std::move may change.


-- 
           Summary: [C++0x] std::move and builtin types
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: jwakely dot gcc at gmail dot com
 GCC build triplet: x86_64-unknown-linux-gnu
  GCC host triplet: x86_64-unknown-linux-gnu
GCC target triplet: x86_64-unknown-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40295

[Bug c++/40295] [C++0x] rvalue-references and builtin types

[jwakely dot gcc at gmail dot com]

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------- Comment #1 from jwakely dot gcc at gmail dot com  2009-05-29 17:08 -------
This is independent of std::move

#include <cassert>

struct S
{
    S() : buf(5) { }

    int&& get() { return static_cast<int&&>(buf); }

    int buf;
};

int main()
{
    S s;
    int&& r = s.get();
    assert( &r == &s.buf );
}


tmpret.cc: In member function ‘int&& S::get()’:
tmpret.cc:7: warning: returning reference to temporary


tmpret: tmpret.cc:16: int main(): Assertion `&r == &s.buf' failed.


Looks like the cast to int&& creates a temporary, which is bound to the
returned reference, triggering the warning.  According to 8.5.3 [dcl.init.ref]
paragraph 5, the reference should be bound to the object, not to a temporary.


-- 

jwakely dot gcc at gmail dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|libstdc++                   |c++
           Keywords|                            |wrong-code
            Summary|[C++0x] std::move and       |[C++0x] rvalue-references
                   |builtin types               |and builtin types


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40295

[Bug c++/40295] [C++0x] rvalue-references and builtin types

[jwakely dot gcc at gmail dot com]

------- Comment #2 from jwakely dot gcc at gmail dot com  2009-06-02 14:14 -------


*** This bug has been marked as a duplicate of 34022 ***


-- 

jwakely dot gcc at gmail dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |DUPLICATE


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40295