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Possible Duplicate:
Why does stack<const string> not compile in g++?
We know that vector<const int> is not allowed.
But is map<const int, int>, map<int, const int>, or map<const int, const int> allowed?
Why (not)?
Items in a vector must be copy-constructible or (as of C++11) move-constructible.
The key in a map only needs to be destructible, not copy/move constructible, so the requirements are much looser in this respect.
As to why that is, it’s pretty simple: when/if a vector gets resized, the data from the existing buffer must be copied or moved into the new buffer. By contrast, a map normally stores its contents as nodes in a tree. Once a node is created, it will simply exist until it is destroyed. While the tree does need balancing at times, that only requires manipulating pointers between the nodes — the key in the node is never modified after the node is created.