Gripe Of The Day

A simple one, really: there's no const version of std::map::operator[].

Now, I understand the reasoning here. After all:

#include <map>

#include <iostream>

using namespace std;

class A {
  map<int, int> m;
public:

  void foo(int i) const { cout << m[i] << endl; }
};
int main(int argc, char* argv[]) {
  A a;
  a.foo(42); // what happens here?
  return 0;
}

Our mythical const version of operator[] can't return a reference, as there's no object to refer to. It can't return a default value, either; there's no guarantee that the map's value type will have a default constructor (though in this case, the type int defaults to 0).

On the other hand, why not simply do as std::vector::operator[] does, and throw the programmer in the deep end if they access an invalid index? Thoughts? Comments? Heated counterarguments?