[英]Using QString as the key in std::unordered_map
I'm trying to use QString
as the key in a std::unordered_map
, however I get the error:我正在尝试使用
QString
作为std::unordered_map
的键,但是我收到错误消息:
error C2280: 'std::hash<_Kty>::hash(const std::hash<_Kty> &)': attempting to reference a deleted function
错误 C2280:“std::hash<_Kty>::hash(const std::hash<_Kty> &)”:试图引用已删除的函数
I can't switch to QHash
because the value-type of the map is non-copyable.我无法切换到
QHash
因为地图的值类型是不可复制的。 Is there any way to make this work?有什么办法可以使这项工作?
Put the hash
implementation in a header, and make sure that you include that header everywhere the map is used.将
hash
实现放在标题中,并确保在使用地图的任何地方都包含该标题。
A trivial implementation that forwards to qHash
should be sufficient:转发到
qHash
简单实现应该就足够了:
#include <QHash>
#include <QString>
#include <functional>
namespace std {
template<> struct hash<QString> {
std::size_t operator()(const QString& s) const noexcept {
return (size_t) qHash(s);
}
};
}
Even though std::size_t
is larger than unsigned int
on common 64 bit platforms, and thus the hash doesn't change across its full length - this isn't a problem.尽管
std::size_t
在常见的 64 位平台上比unsigned int
大,因此散列不会在其全长范围内改变 - 这不是问题。 The standard places no such requirement on an std::hash
implementation.该标准对
std::hash
实现没有这样的要求。
The problem is that there is no std::hash<QString>()
specialization.问题是没有
std::hash<QString>()
。 It's easy enough to define your own with reasonably good performance based on the dbj2 algorithm:基于dbj2算法定义你自己的具有相当好的性能是很容易的:
#include <QString>
#include <unordered_map>
namespace std
{
template<> struct hash<QString>
{
std::size_t operator()(const QString& s) const noexcept
{
const QChar* str = s.data();
std::size_t hash = 5381;
for (int i = 0; i < s.size(); ++i)
hash = ((hash << 5) + hash) + ((str->row() << 8) | (str++)->cell());
return hash;
}
};
}
include that in files which use a QString
in a std::unordered_map
and the error goes away.在
std::unordered_map
使用QString
文件中包含它,错误就会消失。
It seems that in newer versions of Qt, std::hash is defined for QString so you can use it with std::unordered_map directly.似乎在较新版本的 Qt 中,为 QString 定义了 std::hash,因此您可以直接将其与 std::unordered_map 一起使用。 (It works with Qt 5.14 on my machine.)
(它适用于我机器上的 Qt 5.14。)
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