What is the default constructor for C++ pointer?

Question

I have code like this:

class MapIndex { private:     typedef std::map<std::string, MapIndex*> Container;     Container mapM;  public:     void add(std::list<std::string>& values)     {         if (values.empty()) // sanity check             return;          std::string s(*(values.begin()));         values.erase(values.begin());         if (values.empty())             return;          MapIndex *&mi = mapM[s];  // <- question about this line         if (!mi)             mi = new MapIndex();         mi->add(values);     } } 

The main concern I have is whether the mapM[s] expression would return reference to NULL pointer if new item is added to the map?

The SGI docs say this: data_type& operator[](const key_type& k) Returns a reference to the object that is associated with a particular key. If the map does not already contain such an object, operator[] inserts the default object data_type().

So, my question is whether the insertion of default object data_type() will create a NULL pointer, or it could create an invalid pointer pointing somewhere in the memory?

Answer 1

It'll create a NULL (0) pointer, which is an invalid pointer anyway :)

Answer 2

Yes it should be a zero (NULL) pointer as stl containers will default initialise objects when they aren't explicitly stored (ie accessing a non-existant key in a map as you are doing or resizing a vector to a larger size).

C++ Standard, 8.5 paragraph 5 states:

To default-initialize an object of type T means:

• If T is a non-POD class type (clause class), the default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor)
• If T is an array type, each element is default-initialized
• Otherwise, the storage for the object iszero-initialized.

You should also note that default initialisation is different to simply ommiting the constructor. When you omit the constructor and simply declare a simple type you will get an indeterminate value.

int a; // not default constructed, will have random data  int b = int(); // will be initialised to zero