Question

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

Answer 1

operator[] will create the entry if it does not exist in the map. This is not possible if the operator is implemented for a const map. This is the explanation given in The C++ Programming Language:

Subscripting a map adds a default
element when the key is not found.
Therefore, there is no version of operator[] for const maps.
Furthermore, subscripting can be
used only if the mapped_type (value
type) has a default value. If the
programmer simply wants to see if a
key is present, the find() operation
(§17.4.1.6) can be used to locate a
key without modifying the map.