Why does std::map not have a const accessor?

Question

The declaration for the [] operator on a std::map is this:

T& operator[] ( const key_type& x ); 

Is there a reason it isn't this?

T& operator[] ( const key_type& x ); const T& operator[] const ( const key_type& x ); 

Because that would be incredibly useful any time you need to access a member map in a const method.

Answer 1

As of C++11 there is std::map::at which offers const and non-const access.

In contrast to operator[] it will throw an std::out_of_range exception if the element is not in the map.

Answer 2

operator[] in a map returns the value at the specified key or creates a new value-initialized element for that key if it's not already present, so it would be impossible.

If operator[] would have a const overload, adding the element wouldn't work.

That answers the question. Alternatives:

For C++03 - you can use iterators (these are const and non-const coupled with find). In C++11 you can use the at method.