The following code works fine :

std::map<int, int>& m = std::map<int, int>();
int i = m[0];

But not the following code :

// error C2678: binary '[' : no operator...
const std::map<int, int>& m = std::map<int, int>();
int i = m[0];

Most of the time, I prefer to make most of my stuff to become immutable, due to reason :

http://www.javapractices.com/topic/TopicAction.do?Id=29

I look at map source code. It has

mapped_type& operator[](const key_type& _Keyval)

Is there any reason, why std::map unable to provide

const mapped_type& operator[](const key_type& _Keyval) const