[英]STL set of map iterators
I have an stl unordered_map and I would like to store references to elements in that map. 我有一个stl unordered_map,我想在该地图中存储对元素的引用。 I would like there to be no duplicate references.
我希望没有重复的引用。 I thought I could create an set of iterators which pointed to elements.
我以为我可以创建一组指向元素的迭代器。 For one I wasn't sure that it would recognise , but even so I got some long template errors which I think boiled down to there being no comparison operator < for iterators.
对于一个我不确定它是否可以识别,但是即使如此,我仍然遇到了一些很长的模板错误,我认为这归结为没有针对迭代器的比较运算符<。
I then tried an unordered set compiling with g++ -std=c++0x but got pages of errors. 然后,我尝试使用g ++ -std = c ++ 0x进行无序编译,但出现错误页面。
How can I achieve this goal? 我怎样才能实现这个目标?
#include <set>
#include <unordered_map>
#include <unordered_set>
int main() {
typedef std::unordered_map<int, float> umap;
umap my_map;
umap::const_iterator itr;
for (int i=0;i<10000000;i++) {
my_map[i] = float(i);
}
umap::iterator my_it1 = my_map.find(43);
umap::iterator my_it2 = my_map.find(44);
umap::iterator my_it3 = my_map.find(44);
std::unordered_set<umap::iterator> my_its;
my_its.insert(my_it1);
my_its.insert(my_it2);
my_its.insert(my_it3);
return 0;
}
Since you are trying to store the iterators in an unordered set, you don't need a comparison operator but rather a hash function. 由于您尝试将迭代器存储在无序集中,因此不需要比较运算符,而需要哈希函数。
I'm guessing, since each distinct iterator will be pointing to a distinct key, you can use the hash value of the key for hashing the iterator. 我猜,由于每个不同的迭代器都指向一个不同的键,因此您可以使用键的哈希值对迭代器进行哈希处理。
struct iterator_hash
{
size_t operator()(std::unordered_map<int, float>::const_iterator it) const
{
return std::hash<int>()(it->first);
}
};
and instantiate the unordered set as: 并将无序集实例化为:
std::unordered_set<umap::iterator, iterator_hash> my_its;
unordered_set
also requires an equality function, but the iterators own operator==
should do fine, as long as all the iterators belong to the same unordered_map
. unordered_set
还需要一个相等函数,但是只要所有迭代器都属于同一个unordered_map
,迭代器自己的operator==
就可以了。
Careful with iterator invalidation. 小心迭代器无效。
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