Moving inserted container element if possible
- by doublep
I'm trying to achieve the following optimization in my container library:
when inserting an lvalue-referenced element, copy it to internal storage;
but when inserting rvalue-referenced element, move it if supported.
The optimization is supposed to be useful e.g. if contained element type is something like std::vector, where moving if possible would give substantial speedup.
However, so far I was unable to devise any working scheme for this. My container is quite complicated, so I can't just duplicate insert() code several times: it is large. I want to keep all "real" code in some inner helper, say do_insert() (may be templated) and various insert()-like functions would just call that with different arguments.
My best bet code for this (a prototype, of course, without doing anything real):
#include <iostream>
#include <utility>
struct element
{
element () { };
element (element&&) { std::cerr << "moving\n"; }
};
struct container
{
void insert (const element& value)
{ do_insert (value); }
void insert (element&& value)
{ do_insert (std::move (value)); }
private:
template <typename Arg>
void do_insert (Arg arg)
{ element x (arg); }
};
int
main ()
{
{
// Shouldn't move.
container c;
element x;
c.insert (x);
}
{
// Should move.
container c;
c.insert (element ());
}
}
However, this doesn't work at least with GCC 4.4 and 4.5: it never prints "moving" on stderr. Or is what I want impossible to achieve and that's why emplace()-like functions exist in the first place?
