实例化后模板的专业化？

Question

我的完整代码太长了,但这里有一个代码片段,它将反映我的问题的本质:

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 {

        }
};

}

当我编译此代码时,我收到以下错误:

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>’

现在我必须为这些类专门化std :: hash(因为标准的std :: hash定义不包括用户定义的类型).当我在类的定义之前移动这些模板特化时BPCFGParser,我会尝试各种各样的错误,并在某处(http://www.parashift.com/c++-faq-lite/misc-technical-issues) .html)我读到了:

每当您使用类作为模板参数时,该类的声明必须完整,而不是简单地向前声明.

所以我被卡住了.我不能在BPCFGParser定义之后专门化模板,我不能在BPCFGParser定义之前专门化它们,我怎么能让它工作？

---

您需要将专门化移动到BPCFGParser内部的内部类中.这样做符合这两个要求.

非常感谢你的答案:)

hashclass是在命名空间中定义的std.它不允许我hash在非命名空间范围内专门化模板.甚至如下:

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

不工作.但是当我附上专业化namespace std {}时,它会给出奇怪的错误:

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

在velocityreviews中给出的答案中,有人声称无法在类中定义名称空间.所以我仍然被困住了.

Answer 1

您需要将专门化移动到BPCFGParser内部的内部类中.这样做符合这两个要求

1. 专业化是在完整定义之后 ActiveEquivClass
2. 在使用专业化之前

例:

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;

};