步骤1：为您的类型重载 operator == ：

  bool operator ==（const Some& x，const Some& y）
 {
 return xa == ya; 

 $ / code> 

第二步：专用 std :: hash
$ b

  namespace std 
 {
 template<> code> 
 struct hash< Some> 
 {
 typedef一些argument_type; 
 typedef size_t result_type; 

 size_t operator（）（const Some& x）const 
 {
 return x.a; 
} 
}; 
} 
 

第3步：简单测试：

  int main（）
 {
 std :: unordered_set< Some>测试; 
 test.insert（一些{42}）; 
} 
 

第四步：获利！

I picked the concepts of hash tables in Java, so I was aware that for a generic "hash set" container to work for custom classes, one must provide definition for a hash function and an corresponding equality function.

In Java, this would mean overriding method

int hashCode()

and

boolean equals (Object o)

.

I was expecting the same logic in c++'s STL, but was having trouble understanding the syntax. Specifically, std::unordered_set<> accepts 5 template arguments (can you believe that?), which looks like a monster and makes my head spin.

So I would be appreciate it if one could give a simple example for the current toy class:

class Some{
public :
int a;
};

Of which the hash function simply returns the value of a, and the equality test functions returns true iff the values of member 'a' are the same.

Thanks

Step 1: Overload operator== for your type:

bool operator==(const Some& x, const Some& y)
{
    return x.a == y.a;
}

Step 2: Specialize std::hash for your type:

namespace std
{
    template<>
    struct hash<Some>
    {
        typedef Some argument_type;
        typedef size_t result_type;

        size_t operator()(const Some& x) const
        {
            return x.a;
        }
    };
}

Step 3: A simple test:

int main()
{
    std::unordered_set<Some> test;
    test.insert(Some{42});
}

Step 4: Profit!