Default initialization of POD types in C++

If we're only talking about PODs then only local and global statics and external variables because they have to be defined somewhere.

PODs allocated with new are also initialized sometimes - if you make the initialization explicit:

int* x = new int();

will create an int initialized to 0 with x pointing to it, whereas

int* x = new int;

will have x point to an uninitialized int.

Sometimes - POD class members - they can be initialized explicitly (without being in a constructor):

struct X
{
   int x;
};

X x;        //x.x is not initialized
X y = X();  //y.x is 0

Local variables with automatic storage duration are not being initialized automatically. Since using uninitialized variables produces undefined behavior, it is a good practice to explicitly initialize your variables even when it's redundant.

About POD types that are being zero-initialized, C++03 standard 3.6.2 Initialization of non-local objects states:

§1 Objects with static storage duration (3.7.1) shall be zero-initialized (8.5) before any other initialization takes place. Zero-initialization and initialization with a constant expression are collectively called static initialization; all other initialization is dynamic initialization. Objects of POD types (3.9) with static storage duration initialized with constant expressions (5.19) shall be initialized before any dynamic initialization takes place.

So it's guaranteed by standard that POD types with static storage duration (whatever their scope is) will be zero-initialized.

POD members of a class (without explicit initialization in a constructor)

This situation is described in 12.6.2 Initializing bases and members, that states (selected parts):

If a given nonstatic data member or base class is not named by a mem-initializer-id (including the case where there is no mem-initializer-list because the constructor has no ctor-initializer), then:

— If the entity is a nonstatic data member..., and the entity class is a non-POD class, the entity is default-initialized (8.5)...

— Otherwise, the entity is not initialized...

After the call to a constructor for class X has completed, if a member of X is neither specified in the constructor’s mem-initializers, nor default-initialized, nor value-initialized, nor given a value during execution of the body of the constructor, the member has indeterminate value.

Example:

class C
{
public:
    C(int x, int z) : x(x), z(z) { }
    int x, y, z;
};

int main(void)
{
    C* c = new C(1,3);
    std::cout << c->y; // value of y is undetermined !!!
}