Question

I was going through Josuttis’s “Using Map’s as associative arrays” (from The C++ Standard Library – A Tutorial and Reference, 2nd Edition) and came across Using a std::map as an associative array on Stack Overflow. Now I have more questions on the constructors that are called when inserting into a map.

Here is my sample program (not using best coding practices; please excuse me for that):

class C
{
public:

   string s;

   C() { cout << "default " << endl;}

   C(const string& p) : s(p)
   { cout << "one param" << endl;}

   C(const C& obj)
   {
       if (this != &obj)
       {
         s = obj.s;
       }
       cout << "copy constr" << endl;
   }

   C& operator  = (const C& obj)
   {
       if (this != &obj)
       {
             s = obj.s;
       }
      cout << "copy initializer" << endl;
      return *this;
   }
};

int main()
{
    map<int,C> map1;
    C obj("test");

    cout << "Inserting using index" << endl;
    map1[1] = obj;

    cout << "Inserting using insert / pair" << endl;
    map1.insert(make_pair(2,obj));
}

The output for this program is:

one param
Inserting using index
default
copy constr
copy constr
copy initializer
Inserting using insert / pair
copy constr
copy constr
copy constr
copy constr

I was assuming that initializing the map by index should call the default constructor and followed by the assignment operator.

But executing map1[1] = obj creates following output;

Inserting using index
default
copy constr
copy constr
copy initializer

Can someone help me to understand the initialization better?

Answer 1

If you read the specification for std::map, it says that operator[] is equivalent to (in this case)

(*((this->insert(make_pair(1,C()))).first)).second

So this explains all the constructor calls you see. First it calls the default constructor C(). Then it calls make_pair, which copies the C object. Then it calls insert, which makes a copy of the pair object you just made, calling the C copy constructor again. Finally it calls the assignment operator to set the inserted object to the one you are assigning it to.