On 08/20/2011 06:09 PM, Julian Gonggrijp wrote:
UNDERLYING_TYPE(T) represents a type which is (conceptually speaking) a POD-type with the same member data (and hence the same alignment) as T.
You mean like boost::aligned_storage<sizeof(T), boost::alignment_of<T>::value> ?
The underlying type can be used to allocate space for a T object and to temporarily store the state of a T object.
Using move_raw and UNDERLYING_TYPE, a swap algorithm can be implemented which works and is efficient for any type:
template<class T> void swap (T& x, T& y) { UNDERLYING_TYPE(T) temp; move_raw (x, temp); move_raw (y, x); move_raw (temp, y); }
This clearly looks like undefined behaviour.
The benefits of an underlying type library go far beyond the swap algorithm. For a start, any algorithm which moves values around without an inherent need to copy them could use the same mechanism as the swap algorithm above. Apart from that, it can help to simulate rvalue references and move semantics in pre-C++11 compilers in an efficient way. Concluding, a library which provides these tools has an immense potential for improving efficiency and convenience anywhere C++ is used.
You cannot use bitwise copy to implement move semantics. That just doesn't work.