为什么我不能用std :: unordered_map替换std :: map [英] Why can't I replace std::map with std::unordered_map
问题描述
这个问题可能有点粗略，因为我没有在家里可用的代码，但我知道这个东西否则会整个周末错误。
This question might be a bit sketchy because I do not have the code available at home, but I know this thing otherwise will bug me the whole weekend.
我试图更新一些代码到C + + 11我开始替换一些 std :: map 与 std :: unordered_map 。代码只使用 std :: map :: find（）来访问地图中的特定元素，所以我想替换应该很容易。返回的迭代器存储在 auto -typed变量（ auto res = map.find（x）但是当使用 res-> second.do_stuff（）访问存储的元素时，我得到一个编译器错误，告诉我， struct std :: pair< char，B>没有成员第二现在这真的让我困惑，但很遗憾，我没有时间进一步调查。
When I tried to update some code to C++11 I began replacing some std::map with std::unordered_map. The code only used std::map::find() to access a specific element in the map, so I figured the replacement should be easy. The returned iterator was stored in an auto-typed variable (auto res = map.find( x ), so the typing should check out fine. However when accessing the stored element using res->second.do_stuff() I got a compiler error, telling me, that struct std::pair<char, B> does not have a member second. Now this really confused me, but unfortunately I did not have time to investigate further.
也许这是足够的信息，所以有人可以给我一个关于这个奇怪的编译器错误的提示，或者是我的理解， std :: map code> std :: unordered_map 应该有相同的接口，除了需要排序的部分，不正确？
Maybe this is enough information, so someone can give me a hint on this weird compiler error. Or is my understanding that std::map and std::unordered_map should have the same interface except for the parts which need an ordering, not correct?
EDIT ：：
EDIT:
正如我在这里对这个问题进行了更多的分析，很可能会让某人更好地帮助我，提示在注释中，这不是真正由我访问地图中的元素，而是由代码的其他部分的点。我发现的原因是，我使用类X中的地图存储指向其他元素的类X（一种树结构）的指针。但这似乎适用于 std :: map ，但不适用于 std :: unordered_map 。下面是一些非常简单的代码，展示了这个问题：
As promised here some more analysis on the problem. Most likely this will allow someone to help me out better now. As I guessed from the hints in the comments, this was not really caused by the point where I accessed the elements in the map, but by some other part of the code. The reason I found was, that I used the map within Class X to store pointers to other elements of Class X (a kind of tree structure). However this seems to work for std::map but not for std::unordered_map. Here is some very simple code that exhibits the problem:
#include <stdint.h>
#include <unordered_map>
#include <map>
class Test {
std::map<uint32_t, Test> m_map1; // Works
std::unordered_map<uint32_t, Test> m_map; // gives error: ‘std::pair<_T1, _T2>::second’ has incomplete type
};
int main() {
return 1;
}
std :: map works std :: unordered_map 不工作。任何想法为什么是这样的情况，或者可以做什么来让它使用 std :: unordered_map ？
std::map works std::unordered_map does not work. Any Ideas why this is the case, or what can be done to get it to work with a std::unordered_map?
推荐答案
我想，因为std :: unordered_map需要rehash，因此复制元素，类型需要是完整的，而一个地图，只有使用指针元素，不会出现这个问题。
I guess that because std::unordered_map needs to rehash, and therefore copy elements, the types need to be complete, whereas a map, only ever working with pointers to elements, will not exhibit that problem.
这里的解决方案是有一个无序映射到指针：
The solution here is to have an unordered map to a pointer:
std::unordered_map<uint32_t, std::shared_ptr<Test> >.
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