C++ Initializing Array of Pointers

Question

I know with pointers you can do this:

T * myPtr = 0;

which sets the pointer to a NULL value. However, when I try to do this:

T * myPtrArray[2] = {0, 0};

I get the "expected expression" syntax error. Why?

Note, I'm using g++ and the code above occurs in a class constructor:

template <class T>
 RBTree<T>::RBTree() {
 m_Data = T();
 m_Children = {0, 0};
 m_Parent = 0;
 m_Color = BLACK;
 }

where the class members in question are declared as:

T m_Data;
RBTree<T> * m_Children[2];
RBTree<T> * m_Parent;
bool m_Color;

• user703016: What compiler? Works for me with g++, clang and msvc.
• yzn-pku: It should work. Any context?
• Jonathan Potter: Please show the actual error message.
• Jonathan Potter: That's assignment, not initialization. There's a difference. Arrays don't support assignment.
• Woody1193: I thought that because it's in the constructor it would be considered initialization. Is this wrong then?

Answer 1

The form T * myPtrArray[2] = {0, 0}; is called aggregate initialization. It has no counterpart in assignment, so writing

T * myPtrArray[2];
myPtrArray = {0, 0};

is invalid.

In the case of class construction, aggregate initialization is unavailable in c++98/03.

If your compiler supports c++11 standard, you can use uniform initialization syntax. There are two ways to initialize m_Children in your example:

#1

In constructor:

template <class T>
RBTree<T>::RBTree(): m_Children {0, 0} /* other init here */ {
}

#2

During class member declaration:

T m_Data;
RBTree<T> * m_Children[2] {0, 0};
RBTree<T> * m_Parent;
bool m_Color;

• juanchopanza: #1 isn't supported pre-C++11. But m_children() is, and is probably the better option because ti does not depend on the number of elements in the array.
• MSalters: ITYM #2 isn't supported (during class member declaration).

Answer 2

As of C++11, you can use nullptr instead of 0. Using nullptr is preferred, as it is a pointer instead of an integer. Then, you could do:

T * myPtrArray[2] = {nullptr, nullptr};

Anyway, your code works fine on my compiler, you can see an example using both 0 and nullptr that compiles without errors on ideone.