Question

class Foo {};

int main()
{
   Foo *foo[500] = { NULL}; 
}

Regardless of O.S/compiler is it standard that the whole array will be set to NULL?

Answer 1

Yes it is valid and guaranteed by the C++ Standard.

Reference:

C++03 Standard 8.5.1 Aggregates
Para 7:

If there are fewer initializers in the list than there are members in the aggregate, then each member not explicitly initialized shall be value-initialized (8.5).
[Example:

 struct S { int a; char* b; int c; };
 S ss = { 1, "asdf" };

initializes ss.a with 1, ss.b with "asdf", and ss.c with the value of an expression of the form int(), that is,0. ]

Value Initialization is defined under:

C++03 8.5 Initializers
Para 5:

To value-initialize an object of type T means:
— if T is a class type (clause 9) with a user-declared constructor (12.1), then the default constructor for T is called (and the initialization is ill-formed if T has no accessible
default constructor);
— if T is a non-union class type without a user-declared constructor, then every non-static
data member and base-class component of T is value-initialized;
— if T is an array type, then each element is value-initialized;
— otherwise, the object is zero-initialized