Specialization of template after instantiation?

Question

Hi,

My full code is too long, but here is a snippet that will reflect the essence of my problem:

class BPCFGParser {
  public:

  ...
  ...

  class Edge {
    ...
    ...
  };


  class ActiveEquivClass {
    ...
    ...
  };

  class PassiveEquivClass {
    ...
    ...
  };

  struct EqActiveEquivClass {
    ...
    ...
  };

  struct EqPassiveEquivClass {
    ...
    ...
  };



  unordered_map<ActiveEquivClass, Edge *, hash<ActiveEquivClass>, EqActiveEquivClass> discovered_active_edges;
  unordered_map<PassiveEquivClass, Edge *, hash<PassiveEquivClass>, EqPassiveEquivClass> discovered_passive_edges;

};

namespace std {


template <>
class hash<BPCFGParser::ActiveEquivClass>
{

    public:
        size_t operator()(const BPCFGParser::ActiveEquivClass & aec) const {

        }
};

template <>
class hash<BPCFGParser::PassiveEquivClass>
{

    public:
        size_t operator()(const BPCFGParser::PassiveEquivClass & pec) const {

        }
};

}

When I compile this code, I get the following errors:

In file included from BPCFGParser.cpp:3,
                 from experiments.cpp:2:
BPCFGParser.h:408: error: specialization of ‘std::hash<BPCFGParser::ActiveEquivClass>’     after instantiation
BPCFGParser.h:408: error: redefinition of ‘class                 std::hash<BPCFGParser::ActiveEquivClass>’
/usr/include/c++/4.3/tr1_impl/functional_hash.h:44: error: previous definition of     ‘class std::hash<BPCFGParser::ActiveEquivClass>’
BPCFGParser.h:445: error: specialization of     ‘std::hash<BPCFGParser::PassiveEquivClass>’ after instantiation
BPCFGParser.h:445: error: redefinition of ‘class std::hash<BPCFGParser::PassiveEquivClass>’
/usr/include/c++/4.3/tr1_impl/functional_hash.h:44: error: previous definition of     ‘class std::hash<BPCFGParser::PassiveEquivClass>’

Now I have to specialize std::hash for these classes (because standard std::hash definition does not include user defined types). When I move these template specializations before the definition of class BPCFGParser, I get a variety of errors for a variety of different things tried, and somewhere (http://www.parashift.com/c++-faq-lite/misc-technical-issues.html) I read that:

Whenever you use a class as a template parameter, the declaration of that class must be complete and not simply forward declared.

So I'm stuck. I cannot specialize the templates after BPCFGParser definition, I cannot specialize them before BPCFGParser definition, how may I get this working?

Thanks in advance,
Onur

Answer 1

You need to move the specialization into an inner class inside of BPCFGParser. Doing so meets both requirements

1. Specialization is after the complete definition of ActiveEquivClass
2. Before the use of the specialization

Example:

class BPCFGParser {

  class ActiveEquivClass {
    ...
  };

  template <>
  class hash<ActiveEquivClass> {
     public:
        size_t operator()(const BPCFGParser::ActiveEquivClass & aec) const {
        }
  };
  ...
  unordered_map<ActiveEquivClass, Edge *, hash<ActiveEquivClass>, EqActiveEquivClass> discovered_active_edges;

};

Answer 2

You need to move the specialization into an inner class inside of BPCFGParser. Doing so meets both requirements.

Thank you very much for the answer :)

hash class is defined within the namespace std. It does not allow me to specialize the templates for hash in a non-namespace scope. Even the following:

template <>
  class std::hash<ActiveEquivClass> {
...

did not work. When I enclose the specializations with namespace std {}, however, it gives the weird error of:

In file included from BPCFGParser.cpp:3,
                 from experiments.cpp:2:
BPCFGParser.h:225: error: expected unqualified-id before ‘namespace’
experiments.cpp:7: error: expected `}' at end of input
BPCFGParser.h:222: error: expected unqualified-id at end of input

In an answer given in velocityreviews, someone claims that namespaces cannot be defined within classes. So I'm still stuck.

Thanks again,
Onur

Answer 3

Try moving the hash<> template specialization code before the BPCFGParser class declaration. The error means that hash is expanded based on std::hash defined at /usr/include/c++/4.3/tr1_impl/functional_hash.h; So your specialization is not used before the instantiation. Ideally, your specialization code should be made available for the compiler before the template is expanded.