如果 KEY 是 std::list 或 std::vector 而不是值，则 std::map 的默认行为是什么？

Question

For example, these cases:

using stringlist = std::list<string>;
    
std::map<stringlist, int> orderedMap;
std::unordered_map<stringlist, int> unorderedMap;

How will comparing keys in orderedMap work? Would it compare all items ('sub-keys') in the key one by one in lexical order?

How will computing hash in unorderedMap work?

Answer 1

An ordered map , by default, uses std::less to compare keys, which by default just does lhs < rhs .

The behavior of vector 's operator < is described here:https://en.cppreference.com/w/cpp/container/vector/operator_cmp

And that of list is here: https://en.cppreference.com/w/cpp/container/list/operator_cmp

Yes, they just do a lexicographical comparison, ie they compare their elements one by one.

You can override the behavior by supplying a custom comparison as the third template parameter to map .

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The default behavior of unordered_map is to use std::hash . std::hash does not have specializations for vector and list , so they are not usable as keys. The code should not compile. Try it here: https://godbolt.org/z/kgKmKS

You need to override the behavior by supplying a custom hasher as the third template argument to unordered_map . You can use Boost.Hash, which supports the standard containers: https://www.boost.org/doc/libs/1_73_0/doc/html/hash/reference.html