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;

That's assignment, not initialization. There's a difference. Arrays don't support assignment. — Jonathan Potter
– Jonathan Potter, Commented Oct 14, 2015 at 5:42
I thought that because it's in the constructor it would be considered initialization. Is this wrong then? — Woody1193
– Woody1193, Commented Oct 14, 2015 at 5:43

yzn-pku · Accepted Answer · 2015-10-14 07:48:40Z

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;

edited Oct 14, 2015 at 7:48

answered Oct 14, 2015 at 6:06

yzn-pku

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2 Comments

juanchopanza Over a year ago

#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 Over a year ago

ITYM #2 isn't supported (during class member declaration).

Hawkings · Accepted Answer · 2015-10-14 05:35:40Z

0

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.

answered Oct 14, 2015 at 5:35

Hawkings

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