Re: [Boost-users] Detecting Valid Expressions for a Type at Compile-Time

22 Jul 2007

      On Sun, July 22, 2007 16:26, Ovanes Markarian wrote:
...
Hi Martin,
if you simply would like to ensure that you compile a correct version, I would suggest the
following solution:
A template function which compares a type instance with itself (makes no sense during the runtime
since is always true, if there is a comparison operator, otherwise a compiler error is issued)
template<class Type_>
  inline void concept_comparison_exists(Type_ const& t)
  {
      static_cast<bool>(t==t);
  }
static_cast<bool>(t==t) is used to notify that compiler that this is an intended code and no
warning should be produced.
Now some tests:
class SomeClassX  //empty class to ensure that out concept works
{};
int main(int argc, char* argv[])
{
int i = 8;
concept_comparison_exists(i);
  concept_comparison_exists(SomeClassX());
return 0;
}
Just in addition to my post, if SomeClassX can be implicitly converted to some type which defines
the operator==, the code will still compile fine. Here an example:

class SomeClassX
{
  public:
     operator int()const
     {
         return 0;
     }
};

this line will produce NO error, since SomeClassX will be implicitly casted to int and int has the
comparison operator.
concept_comparison_exists(SomeClassX());

But in such a case you can't anyway assume what was the intention of the developer. So I would let
it be valid.

With Kind Regards,

Ovanes