To help balance out "introductory," "intermediate," and "advanced" content, we're trying an experiment to highlight interesting bite-sized tidbits that are in the first two categories.

Here's today's tidbit from StackOverflow's [c++11] tag:

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.

As the two top answers show, the answer is different in C++98 and C++11, and C++11 is where it's "at" (pardon).

Read answers on StackOverflow...