David Abrahams wrote:
are you familiar with http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2855.html ?
I've just skimmed part of the discussion, just to see what the problem was and to think that noexcept is a great idea that can also help performance and code size.
Until the committee decides exactly how to handle it and compilers catch up with the feature they add, we need a workaround for that problem, both for C++03 and C++0x. The simplest approach requires a trait called has_nothrow_move<T> which can be used to SFINAE-out pair's move constructor unless its members all have a nonthrowing move constructor. It would be reasonable to say
template <class T> struct has_nothrow_move : has_move_constructor {};
What does has_nothrow_move exactly mean? It means that type has a nothrow T(T &&x) overload or that conversion from rvalue ref to a type does not throw (including copy constructor if the type has not overloaded the constructor with both const & and &&)? In the working draft there are some "move" traits that are confusing to me regarding this issue (MoveConstructible requires RvalueOf<T> && Constructible<T, RvalueOf<T>::type> which I think is true for C++03 types). I think has_move_constructor could be just "is_movable" (in the emulation library "is_movable" means "this type has activated move emulation", the T(rv<T> &) overload). For compilers with rvalue references, I'm not a language expert so I need someone that could write such a trait (can we easily detect if a type has the T(T&&t) constructor?). For compilers with rvalue reference, if all types in the course are going to use the portable syntax (that is, they are going to define BOOST_HAS_MOVE_EMULATION macro) BOOST_HAS_MOVE_EMULATION can be used to put a typedef that will mark those types as having a move constructor. Do you also want me to tweak container/detail/pair.hpp implementation and apply SFINAE for move constructors? I can do some work tomorrow (the trait), and commit it to sandbox. Best, Ion