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Null Pointer

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The constant NULL is a special pointer value which encodes the idea of "points to nothing." It turns out to be convenient to have a well defined pointer value which represents the idea that a pointer does not have a pointee. It is a runtime error to dereference a NULL pointer. In drawings, the value NULL is usually drawn as a diagonal line between the corners of the pointer variable's box...

The C language uses the symbol NULL for this purpose. NULL is equal to the integer constant 0, so NULL can play the role of a boolean false. Official C++ no longer uses the NULL symbolic constant — use the integer constant 0 directly. Java uses the symbol null.

Pointer AssignmentThe assignment operation (=) between two pointers makes them point to the same pointee. It's a simple rule for a potentially complex situation, so it is worth repeating: assigning one pointer to another makes them point to the same thing. The example below adds a second pointer, second, assigned with the statement second = numPtr;. The result is that second points to the same pointee as numPtr. In the drawing, this means that the second and numPtr boxes both contain arrows pointing to num. Assignment between pointers does not change or even touch the pointees. It just changes which pointee a pointer refers to.

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