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Falk Tannhäuser
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 Posted: Thu Mar 24, 2005 4:34 am Post subject: std::swap() not specialised for container adaptors (23.2.3)? |
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All the standard containers have a swap() member function, and
for all of them, std::swap() is specialised to call said member
function. However, no equivalent exists for the adaptors described
in § 23.2.3 (std::queue, std::priority_queue, std::stack) - thus,
while std::swap() works for them, it will use copy construction
and assignment (these are not explicitly defined for adaptors
in the Standard either, so the compiler-generated ones are used).
This leads both to less-than-optimal performance and to the
possibility of throwing exceptions.
Was there a deliberate decision not to specialise std::swap()
for adaptors (it would just have to call swap() for the underlying
containers, and in case of std::priority_queue for the comparison
object) or has it just been forgotten?
Falk
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Carl Barron
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| Posted: Sat Apr 02, 2005 5:36 am Post subject: Re: std::swap() not specialised for container adaptors (23.2 |
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Falk Tannhäuser <falk.tannhauser (AT) crf (DOT) canon.fr> wrote:
| Quote: | All the standard containers have a swap() member function, and
for all of them, std::swap() is specialised to call said member
function. However, no equivalent exists for the adaptors described
in § 23.2.3 (std::queue, std::priority_queue, std::stack) - thus,
while std::swap() works for them, it will use copy construction
and assignment (these are not explicitly defined for adaptors
in the Standard either, so the compiler-generated ones are used).
This leads both to less-than-optimal performance and to the
possibility of throwing exceptions.
Was there a deliberate decision not to specialise std::swap()
for adaptors (it would just have to call swap() for the underlying
containers, and in case of std::priority_queue for the comparison
object) or has it just been forgotten?
Falk
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There is a solution since the standard makes the needed parts protected,
called inheritance. [just don't destruct via the std adaptors]
create a templated struct inheriting the desired adaptor, provide just
the constructors and forward the args to the adaptor class, writing a
swap member is a snap since the required items are protected members of
the standard adaptors.
example:
template <class T,class C = std::vector
struct swapable_stack:std::stack<T,C>
{
explicit swapble_stack(const C &cont=C()):std::stack<T,C>(cont)
{}
void swap(swapable_stack &r)
{
c.swap(r.c);
}
};
template <class T,class C>
inline
void swap(swapable_stack<T,C> &x,swapable_stack<T,C> &y)
{ x.swap(y);}
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