Why isn't the [] operator const for STL maps?

Question

Contrived example, for the sake of the question:

void MyClass::MyFunction( int x ) const {   std::cout << m_map[x] << std::endl } 

This won't compile, since the [] operator is non-const.

This is unfortunate, since the [] syntax looks very clean. Instead, I have to do something like this:

void MyClass::MyFunction( int x ) const {   MyMap iter = m_map.find(x);   std::cout << iter->second << std::endl } 

This has always bugged me. Why is the [] operator non-const?

Answer 1

For std::map and std::unordered_map, operator[] will insert the index value into the container if it didn't previously exist. It's a little unintuitive, but that's the way it is.

Since it must be allowed to fail and insert a default value, the operator can't be used on a const instance of the container.

http://en.cppreference.com/w/cpp/container/map/operator_at