Why can't I access elements with operator[] in a const std::map?

Question

I tried to use the operator[] access the element in a const map, but this method failed. I also tried to use at() to do the same thing. It worked this time. However, I could not find any reference about using at() to access element in a const map. Is at() a newly added function in map? Where can I find more info about this? Thank you very much!

An example could be the following:

#include <iostream>
#include <map>

int main()
{
    std::map<int, char> A;
    A[1] = 'b';
    A[3] = 'c';

    const std::map<int, char> B = A;

    std::cout << B.at(3) << std::endl; // it works
    std::cout << B[3] << std::endl;    // it does not work
}

For using B[3], it returned the following errors during compiling:

t01.cpp:14: error: passing ‘const std::map<int, char, std::less, std::allocator<std::pair<const int, char> > >’ as ‘this’ argument of ‘_Tp& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](const _Key&) [with _Key = int, _Tp = char, _Compare = std::less, _Alloc = std::allocator<std::pair<const int, char> >]’ discards qualifiers

The compiler used is g++ 4.2.1

Answer 1

at() is a new method for std::map in C++11.

Rather than insert a new default constructed element as operator[] does if an element with the given key does not exist, it throws a std::out_of_range exception. (This is similar to the behaviour of at() for deque and vector.)

Because of this behaviour it makes sense for there to be a const overload of at(), unlike operator[] which always has the potential to change the map.