If we want to copy an array of POD's, we can simply do:
SomePODType src[8] = { ... }, dest[8];
memcpy(&dest,&s rc,sizeof dest);
We can assume that this method is definitely faster than (if not at least
as fast as) the following method:
T *p = dest;
T const *const pover = dest + sizeof dest;
T const *q = src;
do *p++ = *q++;
while (pover != p);
This method won't work for class types, because the constructors and
assignment operators may acquire resources and so forth.
The following is a basic attempt to implement a universal method of copy-
constructing an array:
#include <cstddef>
#include <new>
template<class T,std::size_t len>
void CopyCstr(T const (&src)[len],void *const dest)
{
T *p = (T*)dest;
T const *const pover = p + len;
T const *q = src;
do ::new((void*)p+ +) T(*q++);
while (pover != p);
}
This looks grand, but what happens if we use it to copy-construct an array
of short ints? It will look like as follows:
void CopyCstr(short const (&src)[8],void *const dest)
{
short *p = (short*)dest;
short const *const pover = p + 8;
short const *q = src;
do ::new((void*)p+ +) short(*q++);
while (pover != p);
}
The only problem with this is that it may not be as efficient as it could
be. We'd be better off with simply:
void CopyCstr(short const (&src)[8],void *const dest)
{
memcpy(dest,src ,sizeof src);
}
So I wonder how we can achieve the best of both worlds with the one sole
template function? If we had a way of knowing that the "normal
initialisation" for a particular type was a no-op, then we could take
advantage of it. Something like
template<class T,std::size_t len>
void CopyCstr(T const (&src)[len],void *const dest)
{
if ( NoOp(::new(void *) T) ) memcpy(dest,src ,sizeof src);
else
{
T *p = (T*)dest;
T const *const pover = p + len;
T const *q = src;
do ::new((void*)p+ +) T(*q++);
while (pover != p);
}
Obviously, the "if" conditional would be known at compile-time to be either
true or false, the the opposite command path could be done away with.
Anyway, this was just a thought that went through me head...
--
Frederick Gotham 1 1534
Frederick Gotham wrote:
>
So I wonder how we can achieve the best of both worlds with the one sole
template function? If we had a way of knowing that the "normal
initialisation" for a particular type was a no-op, then we could take
advantage of it.
std::tr1::has_t rivial_copy gives you that information.
// sketch, untested:
template <class Ty, bool>
struct copier
{ /* element by element copy */
static void init(Ty *tgt,
const Ty *src, unsigned count);
};
template <class Ty>
struct copier<Ty, true>
{ /* byte copy */
static void init(Ty *tgt, const Ty *src, unsigned count);
};
void init(Ty *tgt, const Ty *src, unsigned count)
{
copier<Ty, has_trivial_cop y<Ty>::value::
init(tgt, src, count);
}
For a complete example (using assignment, not construction), see listing
8 in the section "Type Traits" in my article at http://www.ddj.com/dept/cpp/184401964 (limited access, unfortunately).
--
-- Pete
Roundhouse Consulting, Ltd. ( www.versatilecoding.com)
Author of "The Standard C++ Library Extensions: a Tutorial and
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