Couple of questions on boost::swap
. Please refer to the code below which is basically a cut-paste from boost/swap.hpp
. I am referring to library version 1.43.0.
namespace boost_swap_impl
{
template<class T>
void swap_impl(T& left, T& right)
{
using namespace std;//use std::swap if argument dependent lookup fails
swap(left,right);
}
template<class T, std::size_t N>
void swap_impl(T (& left)[N], T (& right)[N])
{
for (std::size_t i = 0; i < N; ++i)
{
::boost_swap_impl::swap_impl(left[i], right[i]);
}
}
}
namespace boost
{
template<class T1, class T2>
void swap(T1& left, T2& right)
{
::boost_swap_impl::swap_impl(left, right);
}
}
boost::swap
declared as template <typename T1, typename T2>
when in the rest of the code it is all dealing with the same type? void swap(T&, T&)
I see that it is the global function that gets called from swap_impl(T& left, T& right)
. Is this not a conflict and hence an error condition since swap_impl
also uses namespace std
which has swap defined? std::swap
so you don't get overload ambiguity errors when both std::swap
and boost::swap
are in scope (std::swap
will take precedence).swap
will take precedence over both boost::swap
and std::swap
(as will a namespace-scoped template swap
overloaded for a UDT – think partially-specialized, but not really..). Note that unlike std::swap
, boost::swap
is written explicitly to take advantage of ADL.Here's what the C++03 standard has to say regarding both points – [over.match.best] (§13.3.3/1):
Define ICSi(
F
) as follows:
- if
F
is a static member function, ICS1(F
) is defined such that ICS1(F
) is neither better nor worse than ICS1(G
) for any functionG
, and, symmetrically, ICS1(G
) is neither better nor worse than ICS1(F
); otherwise,- let ICSi(
F
) denote the implicit conversion sequence that converts the i-th argument in the list to the type of the i-th parameter of viable functionF
. 13.3.3.1 defines the implicit conversion sequences and 13.3.3.2 defines what it means for one implicit conversion sequence to be a better conversion sequence or worse conversion sequence than another.Given these definitions, a viable function
F1
is defined to be a better function than another viable functionF2
if for all arguments i, ICSi(F1
) is not a worse conversion sequence than ICSi(F2
), and then
- for some argument j, ICSj(
F1
) is a better conversion sequence than ICSj(F2
), or, if not that,F1
is a non-template function andF2
is a function template specialization, or, if not that,F1
andF2
are function template specializations, and the function template forF1
is more specialized than the template forF2
according to the partial ordering rules described in 14.5.5.2, or, if not that,- the context is an initialization by user-defined conversion (see 8.5, 13.3.1.5, and 13.3.1.6) and the standard conversion sequence from the return type of
F1
to the destination type (i.e., the type of the entity being initialized) is a better conversion sequence than the standard conversion sequence from the return type ofF2
to the destination type.
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With