"Rani Sharoni" <rani_sharoni@hotmail.com> writes: [snip]
The fact that the additional temporary is const might, theoretically, encourage the optimizer to generate faster code but AFAIK no C++ optimizer is actually aware about the existence of C++ const and *ignore* it.
So I can't cast away const on the reference, because it might be bound to a const copy of the rvalue (leading to undefined behaviour)? BTW, couldn't the optimizer (after verifying that the const & is never const-casted) perform the same optimizations anyway?
Anyway, we don't really care about this elided coping constraint since if compiler actually makes additional coping then it probably has very good reason: X const& r = true ? X() : X(); // VC and GCC introduce additional r-value using the "r-value" move constructor.
The funny thing about the conditional expression is that neither VC++ 7.1 nor g++ 3.3.1 can compile the case with an rvalue-to-lvalue conversion in the following code: class nc { nc (nc const &) { } public: nc () { } nc const &by_ref () const { return *this; } }; struct nc_derived : nc { }; void bar (nc const &); void foo (bool b) { bar (b ? nc() : nc_derived()); // Ill-formed (8.5.3) bar ((b ? nc() : nc_derived()).by_ref()); // OK AFAIK } Both VC++ and g++ report that the nc copy constructor is inaccessible on both of the bar calls. I don't see how the compiler is allowed to introduce a copy in the second one (with the member function call). Comeau online 4.3.3 diagnoses the error on the ill-formed line and accepts the second one. -- Raoul Gough. export LESS='-X'