Re: Strange matching of pointer to member function in a template
On 14.10.2010 06:30, Edward Diener wrote:
Given the program below, using some simple things from Boost to do
compile-time checking:
--------------------------------------------------------------------
#include <boost/mpl/assert.hpp>
#include <boost/type_traits/is_same.hpp>
struct AnotherType
{
int aFunction(int x) { return 1; }
};
template<class T>
struct getClass
{
typedef void type;
};
template<class R,class C>
struct getClass<R C::*>
{
typedef C type;
};
int main()
{
BOOST_MPL_ASSERT((
boost::is_same
<
typename getClass
<
int (AnotherType::*)(int)
>
::type,
AnotherType
>
));
}
--------------------------------------------------------------------
This program compiles without a compiler error from BOOST_MPL_ASSERT
using either gcc or vc++. Yet I do not understand how the signature "int
(AnotherType::*)(int)" matches the getClass specialization.
Can anyone explain why this trick works ?
The compiler is correct - the specialization "pattern"
R C::*
matches *all* pointer to member, including pointer to member
functions. In this case R deduces to int(int) which is a valid
function type and returns the corresponding class type C which is
AnotherType which matches the static assertion.
Just in case you wonder what would happen when you would have
assigned the argument
int (AnotherType::*)(int) const
to your getClass template, check out the open core issue:
http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#547
HTH & Greetings from Bremen
Daniel Kr?gler
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