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?

Solution 1

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

Solution 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

Solution 3

UPDATE: I completed my program and that very line I was asking about is causing it to crash sometimes, but at a later stage. The problem is that I'm creating a new object without changing the pointer stored in std::map. What is really needed is either reference or pointer to that pointer.

MapIndex *mi = mapM[s];  // <- question about this line
if (!mi)
    mi = new MapIndex();
mi->add(values);

should be changed to:

MapIndex* &mi = mapM[s];  // <- question about this line
if (!mi)
    mi = new MapIndex();
mi->add(values);

I'm surprised nobody noticed this.

Solution 4

The expression data_type() value-initializes an object. For a class type with a default constructor, it is invoked; if it doesn’t exist (or is defaulted), such as pointers, the object is zero-initialized.

So yes, you can rely on your map creating a NULL pointer.

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