<p>Can I construct an <code>std::map</code> where the key type is a reference type, e.g. <code>Foo &</code> and if not, why not?</p>

<p>According to C++ Standard 23.1.2/7 <code>key_type</code> should be assignable. Reference type is not.</p>

Can a reference type be used as the key type in an STL map

Can I construct an std::map where the key type is a reference type, e.g. Foo & and if not, why not?

Can we use any user defined data type as a key in map in C++?

We can use any of the data types as the data type of the key of the map. Even a user-defined data type can be used as key data type.

Does map insert copy or reference?

Yes -- when you insert an item into an std::map, you pass it by value, so what it contains is a copy of what you passed.

Which data structure is used by map in STL?

List: A list in STL is used to implement the doubly-linked lists. Unlike an array, lists are used to store the data, which is not contiguous.

According to C++ Standard 23.1.2/7 key_type should be assignable. Reference type is not.

No, because many of the functions in std::map takes a reference to the keytype and references to references are illegal in C++.

/A.B.