[英]C++: Error outputting a custom hash key value from unordered_map to std::cout
I am trying to build an std::unordered_map
with a custom type as a key. 我正在尝试使用自定义类型作为键来构建
std::unordered_map
。 The custom type is a simple std::vector<double>
. 自定义类型是简单的
std::vector<double>
。 The idea is that it will function as a convenient container for 2D points on a grid. 这个想法是,它将充当网格上2D点的便捷容器。 Everything is working correctly except for outputting the hashed key.
除了输出散列键之外,其他所有东西都正常工作。 Here is a sample I put together to illustrate the idea:
我整理了一个示例来说明这个想法:
#include <iostream>
#include <vector>
#include <unordered_map>
#include <boost/functional/hash.hpp>
#include <chrono>
namespace std
{
template<typename Container>
struct hash {
std::size_t operator()(Container const& v) const
{
return boost::hash_range(v.begin(), v.end());
}
};
}
int main()
{
std::unordered_map<std::vector<double>, double> test;
unsigned long t = (unsigned long) std::chrono::system_clock::now().time_since_epoch().count();
std::srand(t);
for (uint i = 0; i < 100 ; ++i)
{
double d1 = i/200.0;
double d2 = i/200.0;
std::vector<double> v({d1, d2});
test[v] = d1;
}
std::cout << "Size:" << test.size() << std::endl;
for (const auto& it : test )
{
std::cout << it.first << ":" << it.second << std::endl;
}
return 0;
}
The hash specialisation template is courtesy of another SO thread. 哈希专门化模板由另一个SO线程提供。 The trouble is that g++ spits out the following error when I try to compile the above:
问题是当我尝试编译以上代码时,g ++会吐出以下错误:
cannot bind 'std::ostream {aka std::basic_ostream<char>}' lvalue to 'std::basic_ostream<char>&&'
std::cout << it.first << ":" << it.second << std::endl;
^
It is obvious that it stumbles on it.first
. 很明显,它
it.first
。 The code compiles and runs correctly if I remove it.first
. 如果我删除
it.first
,代码将编译并正确运行。 I understand that the output will not be a vector of doubles. 我知道输出不会是双精度的向量。 I did look around SO for quite a while, but I couldn't find a definitive answer on how to
std::cout
the hash value from an unordered map with a custom key type. 我确实四处逛逛,但是我找不到关于如何从具有自定义键类型的无序映射中
std::cout
哈希值的确切答案。 Any feedback will be highly appreciated. 任何反馈将不胜感激。
Thank you in advance! 先感谢您!
Edit: 编辑:
Thank you everyone for your input. 谢谢大家的投入。 This was my first encounter with non-primitive types as hashed keys, so I had the wrong idea about how the key/value pairs were stored (I assumed that the hashed value is the key, whereas in fact it is the actual custom type).
这是我第一次遇到非原始类型作为散列键的情况,因此我对键/值对的存储方式有错误的认识(我假设散列值是键,而实际上是实际的自定义类型) 。
The value_type
of an unordered_map<K,V>
is pair<const K, V>
. unordered_map<K,V>
的value_type
是pair<const K, V>
。 That's what you get when you iterate over it with a range-for. 那就是当您对范围进行迭代时所得到的。 There's no
operator<<
overload for vector
s, causing the error you see. vector
s没有operator<<
重载,从而导致您看到的错误。
namespace std
{
template<typename Container>
struct hash {
std::size_t operator()(Container const& v) const
{
return boost::hash_range(v.begin(), v.end());
}
};
}
This isn't a specialization of std::hash
. 这不是
std::hash
的专业化。 It's a redefinition of the primary template, which in your case only compiled by pure happenstance. 它是对主模板的重新定义,在您的情况下,该模板仅由纯偶然性编译。 (The implementation will have to leave the primary
std::hash
template undefined, and will have to actually declare hash
in the std
namespace and not an inlined namespace. Your code breaks up completely on libc++ , for example.) (该实现将必须保留未定义的主要
std::hash
模板,并且必须在std
名称空间而不是内联名称空间中实际声明hash
。例如,您的代码在libc ++上完全崩溃 。)
A specialization would look like 一个专业化看起来像
namespace std
{
// full specialization
template<>
struct hash<Foo> {
// ^^^^^
std::size_t operator()(Foo const& v) const
{
// ...
}
};
// partial specialization
template<typename T>
struct hash<Bar<T>>{
// ^^^^^^^^
std::size_t operator()(Bar<T> const& v) const
{
// ...
}
};
}
Note the explicit template argument list following hash
. 注意
hash
的显式模板参数列表。 That's the indication that this is a specialization. 这表明这是专业化。
It is illegal to specialize std::hash
for std::vector<double>
anyway, because it doesn't depend on a user-defined type. 无论如何,将
std::hash
专门用于std::vector<double>
是非法的,因为它不依赖于用户定义的类型。 Writing your own hasher is easy: 编写自己的哈希器很容易:
struct container_hasher {
template<typename Container>
std::size_t operator()(Container const& v) const
{
using std::begin;
using std::end;
return boost::hash_range(begin(v), end(v));
}
};
Note that I templated the operator()
instead of the type itself - this makes writing the hasher type easier. 请注意,我对
operator()
而不是类型本身进行了模板化-这使编写哈希器类型更加容易。 The using
followed by an unqualified call enables ADL for begin
and end
. using
后跟不合格的调用使ADL可以begin
和end
。
And then the definition of test
becomes 然后
test
的定义变成
std::unordered_map<std::vector<double>, double, container_hasher> test;
As far as I am aware, there is no standard interface for exposing the hash value (as opposed to the hashing function) from std::unordered_map
. 据我所知,没有标准接口可用于从
std::unordered_map
公开哈希值(与哈希函数相对)。
As you have seen, dereferencing a std::unordered_map<Key,V>::iterator
yields something convertible to std::unordered_map<Key,V>::value_type
, which in turn is a std::pair<const Key,V>
representing a (key,value) pair, rather than a (hashed key,value) pair. 如您所见,取消引用
std::unordered_map<Key,V>::iterator
产生可转换为std::unordered_map<Key,V>::value_type
,而后者又是std::pair<const Key,V>
代表(键,值)对,而不是(散列的键,值)对。
Correspondingly, it.first
is giving you a std::vector<double>
rather than a std::size_t
. 相应地,
it.first
首先给您一个std::vector<double>
而不是std::size_t
。
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