问题描述
我想在我的 unordered_map 中使用由 int、char、char 组成的元组.我是这样做的:
I want to use tuple consisting of int,char,char in my unordered_map. I am doing like this:
#include <string>
#include <unordered_map>
#include <cstring>
#include <iostream>
#include <tuple>
using namespace std;
tuple <int,char,char> kk;
unordered_map<kk,int> map;
int main()
{
map[1,"c","b"]=23;
return 0;
}
但这给了我以下错误:
map.cpp:9:21: error: type/value mismatch at argument 1 in template parameter list for ‘template<class _Key, class _Tp, class _Hash, class _Pred, class _Alloc> class std::unordered_map’
map.cpp:9:21: error: expected a type, got ‘kk’
map.cpp:9:21: error: template argument 3 is invalid
map.cpp:9:21: error: template argument 4 is invalid
map.cpp:9:21: error: template argument 5 is invalid
map.cpp:9:26: error: invalid type in declaration before ‘;’ token
map.cpp: In function ‘int main()’:
map.cpp:14:16: error: assignment of read-only location ‘"b"[map]’
我做错了什么?
推荐答案
unordered_map 的模板参数如下所示:
The template arguments for an unordered_map looks like this:
template<
class Key,
class T,
class Hash = std::hash<Key>,
class KeyEqual = std::equal_to<Key>,
class Allocator = std::allocator< std::pair<const Key, T> >
> class unordered_map;
std::hash 是 不是专门用于元组的a>(向下滚动到库类型的标准特化).因此，您需要提供自己的，如下所示:
std::hash is not specialized for tuples (scroll down to Standard specializations for library types). Therefore you need to provide your own, something like this:
typedef std::tuple<int, char, char> key_t;
struct key_hash : public std::unary_function<key_t, std::size_t>
{
std::size_t operator()(const key_t& k) const
{
return std::get<0>(k) ^ std::get<1>(k) ^ std::get<2>(k);
}
};
// ..snip..
typedef std::unordered_map<const key_t,data,key_hash,key_equal> map_t;
// ^ this is our custom hash
最后，正如 Benjamin Lindley 已经回答的那样，您需要使用 std::make_tuple:
And finally, as Benjamin Lindley answer already addresses, you need to use std::make_tuple:
// d is data
m[std::make_tuple(1, 'a', 'b')] = d;
auto itr = m.find(std::make_tuple(1, 'a', 'b'));
代码来自 使用 std::tuple 作为键对于 std::unordered_map，这里是 Live Example.
The code was grabbed from Using a std::tuple as key for std::unordered_map and here is the Live Example.
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