C++ std::map – Why Can’t Operator[] be Used with Const std::map?

c++const-correctnessdictionarystdstdmap

I tried to pass const with vector it works:
Ex:

void damn(const vector <bool> &bb)
{
    for (int i = 0; i < bb.size(); i++)
        cout << bb[i] << endl;
}

But when trying with map, it does not:

void pas(const map <string, float> &mm)
{
    cout << mm["a"];
    cout << mm["b"];
}

I wonder why it doesn't.

Best Answer

map::operator[] is a little odd. It does this:

  1. Look for the key.
  2. If found, return it.
  3. If not, insert it and default-construct its associated value.
  4. Then return a reference to the new value.

Step 3 is incompatible with constness. Rather than have two differently-functioning operator[] overloads, the language forces you to use map::find for const objects.

Alternately, one could argue, what would map::operator[] const do if the argument is not in the map? Throw an exception? Undefined behavior? (After all, that's what vector::operator[] does with an index out of bounds.) In any case, the problem is avoided with only a small inconvenience to us.

my_map.find(key) returns my_map.end() if the key is not found.

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