模板类的C ++ std :: tr1 :: hash [英] C++ std::tr1::hash for a templated class

问题描述

 模板< typename T>事情{...}; 
  

我想在unordered_set中使用它：

 模板< typename T> class Bozo {
 typedef unordered_set< Thing< T> > things_type; 
 things_type的东西; 
 ... 
}; 
  

现在类Thing拥有除散列函数外所需的所有东西。我想使这个泛型，所以我尝试类似：

  namespace std {namespace tr1 {
 template< typename T> size_t hash< Thing< T> > :: operator（）（const Thing< T>& t）const {...} 
}} 
  

尝试使用g ++ 4.7进行编译时，它会尖叫

预期初始化器在'<'

关于

  hash< Thing< T> > 
  

声明的一部分。任何线索都有助于挽救我头上留下的少数头发。 > hash :: operator（）（const T&）;只是专注于整个 struct hash 。

  template< typename T> 
 struct Thing {}; 
 
 namespace std {namespace tr1 {
 template< typename T> 
 struct hash< Thing< T>> 
 {
 size_t operator（）（Thing< T>）
 {
 return 42; 
} 
}; 
}} 
  

另一种方法是为<$ c $创建一个哈希器c> Thing ，并将其指定为 unordered_set 的第二个模板参数。

 模板< typename T> 
 struct Thing_hasher 
 {
 size_t operator（）（Thing< T& const）
 {
 return 42; 
} 
}; 
 
 typedef std :: unordered_set< Thing< T>，Thing_hasher< T>> things_type; 
  

I have this templated class:

template <typename T> Thing { ... };

and I would like to use it in an unordered_set:

template <typename T> class Bozo {
  typedef unordered_set<Thing<T> > things_type;
  things_type things;
  ...
};

Now class Thing has everything it needs except a hash function. I would like to make this generic so I try something like:

namespace std { namespace tr1 {
  template <typename T> size_t hash<Thing<T> >::operator()(const Thing<T> &t) const { ... }
}}

Attempts to compile this with g++ 4.7 have it screaming

expected initializer before ‘<’

about the

hash<Thing<T> >

part of the declaration. Any clues will help save the few remaining hairs on my head.

解决方案

You cannot provide a specialization for just hash::operator()(const T&); just specialize the entire struct hash.

template<typename T>
struct Thing {};

namespace std { namespace tr1 {
    template<typename T>
    struct hash<Thing<T>>
    {
        size_t operator()( Thing<T> const& )
        {
            return 42;
        }
    };
}}

Another way to do this is to create a hasher for Thing, and specify this as the second template argument for the unordered_set.

template<typename T>
struct Thing_hasher
{
  size_t operator()( Thing<T>& const )
  {
    return 42;
  }
};

typedef std::unordered_set<Thing<T>, Thing_hasher<T>> things_type;

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