Question

While trying to answer this question I found that the code int* p = new int[10](); compiles fine with VC9 compiler and initializes the integers to 0. So my questions are:

1. First of all is this valid C++ or is
   it a microsoft extension?
2. Is it guaranteed to initialize all
   the elements of the array?
3. Also, is there any difference if I
   do new int; or new int();? Does
   the latter guarantee to initialize
   the variable?

Answer 1

First of all is this valid C++ or is it a microsoft extension?

It is valid in C++, the relevant part of the standard is 5.3.4, with the first paragraph containing the grammar

Is it guaranteed to initialize all the elements of the array?

Yes. Paragraph 5.3.4/15 states that

A new-expression that creates an object of type T initializes that object as follows:

…

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

where value initialized for POD means zero-initialize.

Also, is there any difference if I do new int; or new int();? Does the latter guarantee to initialize the variable?

Yes they are different. According with the quote above new int() will zero-initialize the integer. In a previous block of the same paragraph:

If the new-initializer is omitted:

• If T is a (possibly cv-qualified) non-POD class type (or array thereof), the object is default-initialized (8.5). If T is a const-qualified type, the underlying class type shall have a user-declared default constructor.

• Otherwise, the object created has indeterminate value. If T is a const-qualified type, or a (possibly cv-qualified) POD class type (or array thereof) containing (directly or indirectly) a member of const-qualified type, the program is ill-formed;

so new int will not initialize the memory.