Default initialization of C++ Member arrays?

Question

This is a simple question, but I can't seem to find a definitive answer.

If we have the following class:

class Test { ...   char testArray[10];  ... };  

When we create an instance of Test, what is the default value of testArray[1]?

If it was a local array, it would be uninitialized.
If it was a static array, it would be initialized to 0.

What does it do when the array is a class member?

Answer 1

From the standard, section 8.5 [dcl.init]:

To default-initialize an object of type T means:

• if T is a (possibly cv-qualified) class type (Clause 9), the default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor);

• if T is an array type, each element is default-initialized;

• otherwise, no initialization is performed.

also section 12.6.2 [class.base.init]:

In a non-delegating constructor, if a given non-static data member or base class is not designated by a mem-initializer-id (including the case where there is no mem-initializer-list because the constructor has no ctor-initializer) and the entity is not a virtual base class of an abstract class (10.4), then

• if the entity is a non-static data member that has a brace-or-equal-initializer, the entity is initialized as specified in 8.5;
• otherwise, if the entity is a variant member (9.5), no initialization is performed;
• otherwise, the entity is default-initialized (8.5).

So because the element type is char, when each element is default-initialized, no initialization is performed. The contents are left with arbitrary values.

Unless, of course, it's a member of an instance of the class, and the instance has static storage duration. Then the whole instance is zero-initialized, array members and all, before execution begins.

Answer 2

It depends on may factors that you forgot to mention.

If your Test has no user-defined constructor or your user-defined constructor makes no efforts to initialize the array, and you declare the object of type Test as

Test test; // no initializer supplied 

then it will behave in exactly the same way as you described above. For an automatic (local) object the contents of the array will remain unpredictable. For a static object the contents is guaranteed to be zero.

If your class has a user-defined constructor, then it will all depend on what constructor does. Again, keep in mind that static objects are always zero-initialized before any constructor has a chance to do anything.

If your class is an aggregate, then the content might depend on the aggregate initializer you supplied in the object declaration. For example

Test test = {}; 

will zero-initialize the array even for an automatic (local) object.