Why does std::map not have a const accessor?
C++C++11StdC++ Problem Overview
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.
C++ Solutions
Solution 1 - C++
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.
Solution 2 - C++
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.
Solution 3 - C++
These answers are correct in that operator[] has semantics to add a key if it doesn't exist, but I'd like to add another perspective:
Notice how operator[] returns a T&. That is, it returns a reference to the value that is associated with key. But what if there is no key in the map? What should we return? There's no such thing as a "null-reference," and throwing an exception would be annoying.
This would be one good reason for no operator[] const. What would you return to the user if you couldn't add anything to the map (because the operator is const), but they are looking for an item that doesn't exist? A good solution to this problem is to not have the operator[] const.
Solution 4 - C++
The (non-const) operator[] creates the key if it doesn't exist.
The const version of that operator, if it existed, would have to have different semantics, since it wouldn't be able to add a new key.
I am sure you would agree that having const and non-const overloads with significantly different semantics would be a can of worms. Therefore no const version is provided.
There is a const find() member though, so you can use that in your code.
Solution 5 - C++
Because the semantics of operator[] are to add the key if it doesn't exist.