A recent talk about unordered_map in C++ made me realize that I should use unordered_map for most cases where I used map before, because of the efficiency of lookup ( amortized O(1) vs. O(log n) ). Most times I use a map, I use either int or std::string as the key type; hence, I've got no problems with the definition of the hash function. The more I thought about it, the more I came to realize that I can't find any reason of using a std::map over a std::unordered_map in the case of keys with simple types -- I took a look at the interfaces, and didn't find any significant differences that would impact my code.
因此，问题是:是否有任何真正的理由使用std::map而不是std::unordered_map在简单类型的情况下，如int和std::string?
我是从严格的编程角度提出这个问题的——我知道它并不是完全标准的，而且它可能会给移植带来问题。
另外，我认为正确答案之一可能是“对于较小的数据集更有效”，因为开销更小(是真的吗?)——因此，我希望将这个问题限制在键数量非普通的情况下(>1 024)。
编辑:哦，我忘记了显而易见的(感谢GMan!)——是的，地图当然是有序的——我知道，我正在寻找其他原因。
如果你想比较std::map和std::unordered_map实现的速度，你可以使用谷歌的sparsehash项目，它有一个time_hash_map程序来计时。例如，在x86_64 Linux系统上使用gcc 4.4.2
$ ./time_hash_map
TR1 UNORDERED_MAP (4 byte objects, 10000000 iterations):
map_grow 126.1 ns (27427396 hashes, 40000000 copies) 290.9 MB
map_predict/grow 67.4 ns (10000000 hashes, 40000000 copies) 232.8 MB
map_replace 22.3 ns (37427396 hashes, 40000000 copies)
map_fetch 16.3 ns (37427396 hashes, 40000000 copies)
map_fetch_empty 9.8 ns (10000000 hashes, 0 copies)
map_remove 49.1 ns (37427396 hashes, 40000000 copies)
map_toggle 86.1 ns (20000000 hashes, 40000000 copies)
STANDARD MAP (4 byte objects, 10000000 iterations):
map_grow 225.3 ns ( 0 hashes, 20000000 copies) 462.4 MB
map_predict/grow 225.1 ns ( 0 hashes, 20000000 copies) 462.6 MB
map_replace 151.2 ns ( 0 hashes, 20000000 copies)
map_fetch 156.0 ns ( 0 hashes, 20000000 copies)
map_fetch_empty 1.4 ns ( 0 hashes, 0 copies)
map_remove 141.0 ns ( 0 hashes, 20000000 copies)
map_toggle 67.3 ns ( 0 hashes, 20000000 copies)
不要忘记map保持其元素的顺序。如果你不能放弃它，显然你不能使用unordered_map。
另外需要记住的一点是，unordered_map通常会使用更多的内存。Map只有一些内部指针和每个对象的内存。相反，unordered_map有一个大数组(在某些实现中会变得相当大)，然后为每个对象提供额外的内存。如果需要内存感知，map应该会更好，因为它缺少大数组。
所以，如果你需要纯粹的查找-检索，我认为unordered_map是最好的方法。但总会有权衡，如果你负担不起，那你就不能使用它。
仅凭个人经验，我发现在主实体查找表中使用unordered_map而不是map时，性能有了巨大的改进(当然是度量的)。
另一方面，我发现它在重复插入和删除元素时要慢得多。它非常适合相对静态的元素集合，但如果您正在进行大量的插入和删除，那么哈希+桶似乎就会累加起来。(注意，这需要经过多次迭代。)
总结
假设顺序不重要:
如果你打算一次构建一个大的表，并做很多查询，使用std::unordered_map
如果你要构建一个小的表(可能少于100个元素)并进行大量的查询，请使用std::map。这是因为它的读数是O(log n)
如果你要改变表很多，那么std::map可能是一个不错的选择。
如果你有疑问，请使用std::unordered_map。
历史背景
在大多数语言中，无序映射(又名基于哈希的字典)是默认映射，但在c++中，你得到的是有序映射作为默认映射。这是怎么发生的?有些人错误地认为c++委员会用他们独特的智慧做出了这个决定，但不幸的是，事实比这更丑陋。
It is widely believed that C++ ended up with ordered map as default because there are not too many parameters on how they can be implemented. On the other hand, hash based implementations has tons of things to talk about. So to avoid gridlocks in standardization they just got along with ordered map. Around 2005, many languages already had good implementations of hash based implementation and so it was more easier for the committee to accept new std::unordered_map. In a perfect world, std::map would have been unordered and we would have std::ordered_map as separate type.
性能
下面两张图不言自明:
I was intrigued by the answer from @Jerry Coffin, which suggested that the ordered map would exhibit performance increases on long strings, after some experimentation (which can be downloaded from pastebin), I've found that this seems to hold true only for collections of random strings, when the map is initialised with a sorted dictionary (which contain words with considerable amounts of prefix-overlap), this rule breaks down, presumably because of the increased tree depth necessary to retrieve value. The results are shown below, the 1st number column is insert time, 2nd is fetch time.
g++ -g -O3 --std=c++0x -c -o stdtests.o stdtests.cpp
g++ -o stdtests stdtests.o
gmurphy@interloper:HashTests$ ./stdtests
# 1st number column is insert time, 2nd is fetch time
** Integer Keys **
unordered: 137 15
ordered: 168 81
** Random String Keys **
unordered: 55 50
ordered: 33 31
** Real Words Keys **
unordered: 278 76
ordered: 516 298
