[英]How to define a Hash class for custom std::basic_string<> specialization class just like std::string?
I have a specialization of std::basic_string
, say, string_t
, and it the same as std::string
except that the third template argument is my custom allocator. 我有
std::basic_string
,例如string_t
,它与 std::string
相同,除了第三个模板参数是我的自定义分配器。
std::basic_string<> std :: basic_string <>
Question: How should I define a hash functor class for this specialization using hash functors already provided in C++ standard library? 问题:我应该如何使用C ++标准库中已经提供的哈希函子来为此专业定义哈希函子类?
I know how to define a Hash functor, but I don't know how to utilize existing std::hash<..>
functors in std
to define my custom one. 我知道如何定义一个散列函数对象,但我不知道如何利用现有
std::hash<..>
的函子std
定义我的自定义一个。 I hesitate to write my own hashing operations, knowing it is reinventing the wheel and is unlikely to be better than std::hash<std::string>
, since the only difference between string_t
and std::string
is just the allocator. 我很犹豫地编写自己的哈希操作,因为它知道它是在重新发明轮子,并且不可能比
std::hash<std::string>
更好,因为string_t
和std::string
之间的唯一区别只是分配器。
cppreference has some examples but they don't help me much - I don't want to construct a temporary std::string
object using my string_t
object's c_str()
method only to feed the temporary object into std::hash<std::string>
to get the hash value, because it entails allocating temporary heap memory. cppreference有一些示例,但它们对我没有太大帮助-我不想使用我的
string_t
对象的c_str()
方法构造一个临时std::string
对象,而只是将临时对象输入std::hash<std::string>
来获取哈希值,因为它需要分配临时堆内存。
I'm using C++14 and I want to stick to standard library. 我正在使用C ++ 14,我想坚持使用标准库。
Question: How should I define a hash functor class for this specialization using hash functors already provided in C++ standard library?
问题:我应该如何使用C ++标准库中已经提供的哈希函子来为此专业定义哈希函子类?
The short and sad answer is that there is no way to do this. 简短而悲伤的答案是,没有办法做到这一点。 The standard library does not offer hash functions for sequences of integral types.
标准库不提供整数类型序列的哈希函数。
boost::hash
is superior in every way to std::hash
. boost::hash
在所有方面都优于std::hash
。 I would suggest you define your std::hash
specialisation in terms of it. 我建议您根据它定义
std::hash
专门化。
Furthermore, if you can, specify boost::hash<>
as the hashing function for all unordered containers. 此外,如果可以的话,请为所有无序容器指定
boost::hash<>
作为哈希函数。 You'll never regret it. 您永远不会后悔。
std::hash
is a half-formed library. std::hash
是半格式的库。
#include <string>
#include <cassert>
#include <unordered_set>
#include <boost/functional/hash.hpp>
struct my_alloc ...
using my_string = std::basic_string<char, std::char_traits<char>, my_alloc>;
std::size_t hash_value(::my_string const& s)
{
return boost::hash_range(s.begin(), s.end());
}
namespace std {
template<> struct hash<::my_string>
{
std::size_t operator()(::my_string const& s) const
{
return hash_value(s);
}
};
}
int main()
{
auto x = my_string("Hello");
using Set1 = std::unordered_set<my_string, boost::hash<my_string>>;
auto set1 = Set1 { x };
auto h = std::hash<my_string>();
auto hh = h(x);
assert(hh == hash_value(x));
return int(hh);
}
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