| Re: Free Function Detection |
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Group: comp.lang.c++.moderated · Group Profile
Author: Roman.PerepelitsaRoman.Perepelitsa Date: Apr 3, 2008 13:48
On 3 Apr, 17:13, "Nevin :-] Liber" eviloverlord.com> wrote:> Is it possible to detect at compile time if, for a given user defined
> type UDT, that I can make the following function call:
>
> operator<<(std::ostream&, UDT const&)
>
> Basically, I want to call it when I can and do something else when I
> can't.
This implementation is based on boost/detail/is_incrementable.hpp
You can find a lot of comments in this file.
#include
namespace detail
{
struct tag {};
struct any { template any(T const&); };
tag operator<<(std::ostream&, any const&);
tag operator,(tag,int);
char (& check(tag) )[2];
template
char check(T const&);
template
struct impl
{
static T const& x;
static std::ostream& strm;
static const bool value = sizeof(check((strm << x,0))) == 1;
};
}
template
struct is_ostreamable : detail::impl {};
// Test
#include
#include
struct foo {};
struct bar {};
std::ostream & operator<<(std::ostream &, foo const&);
int main()
{
std::cout << is_ostreamable::value << std::endl; // 1
std::cout << is_ostreamable::value << std::endl; // 1
std::cout << is_ostreamable::value << std::endl; // 0
}
HTH,
Roman Perepelitsa.
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