C++ pointer array initialization help

Question

I had trouble with initializing arrays of pointers. What I found out compiling with gcc c++ (4.6.0) is:

MyClass** a = new MyClass*[100];

Does not always initalize the array of pointers. (most of the time it did give me an array of null pointers which confused me)

MyClass** a = new MyClass*[100]();

DOES initialize all the pointers in the array to 0 (null pointer).

The code I'm writing is meant to be be portable across Windows/Linux/Mac/BSD platforms. Is this a special feature of the gcc c++ compiler? or is it standard C++? Where in the standard does it says so?

Presumably you mean MyClass** a = new MyClass*[100](). Yes, the new initializer is a standard feature. I'm just hunting down a duplicate question. — Lara Bailey
– Lara Bailey, Commented Aug 25, 2011 at 7:02
Not quite a duplicate because it asks why not (which is incorrect) rather than why: stackoverflow.com/questions/6717246/… ... but close enough. Voting to close. — Lara Bailey
– Lara Bailey, Commented Aug 25, 2011 at 7:04
The first version returns uninitialized memory, which of course can be NULL (zero) if it is previously unused. Most OSs clear out memory allocated to a process, for security reasons. — Bo Persson
– Bo Persson, Commented Aug 25, 2011 at 7:05
Duplicate of Do the parentheses after the type name make a difference with new? — James McNellis
– James McNellis, Commented Aug 25, 2011 at 7:13
It not a duplication because this question asks for references to the C++ standard. — Cheers and hth. - Alf
– Cheers and hth. - Alf, Commented Aug 25, 2011 at 7:16

Cheers and hth. - Alf · Accepted Answer · 2011-08-25 07:20:53Z

5

This value-initialization is standard C++.

The relevant standardeese is in C++98 and C++03 §5.3.4/15. In C++98 it was default-initialization, in C++03 and later it's value initialization. For your pointers they both reduce to zero-initialization.

C++03 §5.3.4/15:

– If the new-initializer is of the form (), the item is value-initialized (8.5);

In C++0x that paragraph instead refers to “the initialization rules of 8.5 for direct-initialization”, where in N3290 (the FDIS) you find about the same wording in §8.5/16.

Cheers & hth.,

edited Aug 25, 2011 at 7:20

answered Aug 25, 2011 at 7:11

Cheers and hth. - Alf

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1 Comment

Roderick Taylor Over a year ago

Great answer! Exaclty what I was hoping for. I can now default/value/zero initialize with confidence.

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