将 std::hash 专门化为派生类 [英] Specializing std::hash to derived classes

问题描述

我有一个抽象基类 Hashable，可以从它派生出可以散列的类.我现在想将 std::hash 扩展到所有从 Hashable 派生的类.

I have an abstract base class Hashable that classes that can be hashed derive from. I would now like to extend std::hash to all classes that derive from Hashable.

下面的代码应该就是这样做的.

The following code is supposed to do exactly that.

#include <functional>
#include <type_traits>
#include <iostream>

class Hashable {
public:
    virtual ~Hashable() {}
    virtual std::size_t Hash() const =0;
};

class Derived : public Hashable {
public:
    std::size_t Hash() const {
        return 0;
    }
};

// Specialization of std::hash to operate on Hashable or any class derived from
// Hashable.
namespace std {
template<class C>
struct hash {
  typename std::enable_if<std::is_base_of<Hashable, C>::value, std::size_t>::type
  operator()(const C& object) const {
    return object.Hash();
  }
};
}

int main(int, char**) {
    std::hash<Derived> hasher;
    Derived d;
    std::cout << hasher(d) << std::endl;

    return 0;
}

上面的代码与 gcc 4.8.1 完全符合预期，但是当我尝试使用 gcc 4.7.2 编译它时，我得到以下内容:

The above code works exactly as expected with gcc 4.8.1, but when I try to compile it with gcc 4.7.2, I get the following:

$ g++ -std=c++11 -o test test_hash.cpp
test_hash.cpp:22:8: error: redefinition of ‘struct std::hash<_Tp>’
In file included from /usr/include/c++/4.7/functional:59:0,
                 from test_hash.cpp:1:
/usr/include/c++/4.7/bits/functional_hash.h:58:12: error: previous definition of ‘struct std::hash<_Tp>’
/usr/include/c++/4.7/bits/functional_hash.h: In instantiation of ‘struct  std::hash<Derived>’:
test_hash.cpp:31:24:   required from here
/usr/include/c++/4.7/bits/functional_hash.h:60:7: error: static assertion failed:  std::hash is not specialized for this type

有人能想出一种方法，使 std::hash 的这种特殊化适用于从 Hashable 派生的任何类和 gcc 4.7.2 吗?

Can anybody think of a way to make this specialization of std::hash work for any class derived from Hashable with gcc 4.7.2?

推荐答案

似乎没有合适的方法来做我想做的事.我决定使用以下宏为每个派生类编写单独的特化:

It seems like there is no proper way to do what I wanted to do. I have decided to just write separate specializations for each derived class, using the following macro:

// macro to conveniently define specialization for a class derived from Hashable
#define DEFINE_STD_HASH_SPECIALIZATION(hashable)                               
namespace std {                                                                
template<>                                                                     
struct hash<hashable> {                                                        
  std::size_t operator()(const hashable& object) const {                       
    return object.Hash();                                                      
  }                                                                            
};                                                                             
}

然后

// specialization of std::hash for Derived
DEFINE_STD_HASH_SPECIALIZATION(Derived);

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