Many C++ programmers (myself included) would like to have destructive move semantics. Even with the current proposal we can (unsafely) say almost that:
class RAII_Handle {
public:
// Post-condition: h_ is valid
RAII_Handle(const char* c)
: h_(::open(c))
{
if (h_ == nullptr) {
throw false;
}
}
RAII_Handle(RAII_Handle&& h)
: h_(h.h_)
{
h.h_ = nullptr;
}
// exposition only, not self-assignment safe
RAII_Handle& operator=(RAII_Handle&& h)
{
delete h_;
h_ = h.h_;
h.h_ = nullptr;
}
// if h_ is null, calling ::close( ) may crash, protect against it
~RAII_Handle( )
{
if (h_ != nullptr) {
::close(h_);
}
}
// if h_ is null, calling ::execute( ) may crash
void command(const char* c)
{
::execute(h_, c);
}
private:
RAII_Handle(const RAII_Handle&);
RAII_Handle& operator=(const RAII_Handle&);
Handle* h_;
};
The constructor postcondition is violated after a move. I should confess I thought this was the way it should work before reading the move proposal completely (which states that an object should be usable even after a move).
This unsafe design may be fixed by adding open( ), close( ), and is_open( ) member functions. However, if we want to use this kind of destructing move, the destructor must always protect himself from dereferencing a null pointer.
While a fully destructive move semantic would avoid this null-checking, the best solution I have read (from what other programmers propose) would allow stack-RAII'ed variables to leak resources; this is a -big no-. I fully agree with the current move proposal since its flexible enough anyway and not unsafe (unless the programmer really wants it be). For heap or placement-new'ed variables, however, the story may be different.
In the example showed above a just-moved variable's destructor will have no side effects, in fact it will just check if it must clean itself or not. This is true even for move-safe types; many classes will have to be redesigned (std::string in mind, with at least one character as short-string optimization) so move semantics actually allow the desirable performance boost. The effects of their destructors inmediatly after a move could be ignored in most cases. This document proposes to type-trait this information in the same way other type traits work.
Containers that separate destruction from deallocation can use this information to avoid calls to the destructors if just deallocating will suffice. This concept was already explained by some (see references) and I was quited surprised not to see this as part of the standard. This will give us fully-destructive-like performance without actually implementing it. Implementors of containers are free to use or not this information.
template<class T> struct has_trivial_destructor_after_move;
Derived from true_type if v is an object of type T and v's destructor can be ignored just right after moving v.
Default implementation: derived from true_type if std::has_trivial_destructor<T> is derived from true_type.
template<class T> struct has_trivial_reallocation;
Derived from true_type if v is an object of type T and reallocating v is the same as memcpy'ing its bits to the new location without destroying the source.
Default implementation: derived from true_type if std::is_pod<T> is derived from true_type.
Both are classified under "Type Properties" (20.5.4.3) with the same preconditions as their not-fully-specialized default implementations.
Special thanks to Howard Hinnant for his feedback.