Toon Knapen wrote:
Inside a templated class, I need to provide two different
implementations for
a function based on the compile-time decision
::boost::is_convertible< value_type,double >::value.
How can I do this using MPL ?
1) A simple way:
template< typename T >
class my
{
public:
typedef typename T::value_type value_type;
void foo()
{
typedef boost::is_convertible< value_type,double > c;
do_foo(mpl::bool_c());
}
private:
void do_foo(mpl::true_c) { /*...*/ }
void do_foo(mpl::false_c) { /*...*/ }
};
Andrei has written an article that covers this technique -
http://www.cuj.com/experts/1810/alexandr.htm.
2) A sophisticated one (becomes more relevant when you have several "foo"'s
to handle):
template< typename T, typename Derived >
struct foo_impl
{
void foo()
{
Derived& self = static_cast(*this);
// ...
}
};
template< typename Derived >
struct foo_double_impl
{
void foo()
{
Derived& self = static_cast(*this);
// ...
}
};
template< typename T >
class my
: public mpl::select_if<
boost::is_convertible< typename T::value_type,double >
, foo_impl
, foo_double_impl<T>
>::type
{
// ...
private:
friend class foo_impl;
friend class foo_double_impl<T>;
// ...
};
HTH,
Aleksey
