On several occasions, I have wanted to write code such as the following,

class fish
{
  template <class T>
  class soup
  {
     // ...
  };

  template <>
  struct std::less<soup>  // Error: explicit specialisation at class scope
  {
    bool operator()(const soup &a, const soup &b) { /* ... */ }
  };

  std::set<soup> octopus;
};


Clearly, it is possible to get around this error.   For example, I could use
a pimpl instead of octopus, or I could declare soup outside fish and perhaps
typedef it into fish, or I could use a differnt functor and specialise
std::less outside of the class.  However all of these solutions have the
feeling (to me at least) of being hacks to overcome a deficiency in the
language.  Can anyone give me a good reason why explicit specialisation (or
for that matter, instantiation) is not permitted at class scope?  Or is it
something that was just not considered necessary?

--
Richard Smith


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