我的完整代码太长了,但这里的代码片段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,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 {
}
};
}
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BPCFGParser类的定义之前移动这些模板特化时,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：
Whenever you use a class as a template parameter,the declaration of that class must be complete and not simply forward declared.
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,我怎么能得到这个呢？
You need to move the specialization into an inner class inside of BPCFGParser. Doing so meets both requirements.
非常感谢你的答案:)
哈希类在命名空间std中定义.它不允许我在非命名空间范围内专门化哈希模板.甚至如下：
template <>
class std::hash<ActiveEquivClass> {
...
不工作.但是当我用命名空间std {}括起特化时,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给出的答案中,有人声称命名空间不能在类中定义.所以我仍然被困住了.
解决方法
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
>专业化是在ActiveEquivClass的完整定义之后
>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,EqActiveEquivClass> discovered_active_edges;
};
