Question

I’ve been wondering about the language in the C++03 specification surrounding object initialization, specifically section 8.5 paragraph 9 which states,

“If no initializer is specified for an object, and the object is of (possibly cv-qualified) non-POD class type (or
array thereof), the object shall be default-initialized; if the object is of const-qualified type, the underlying
class type shall have a user-declared default constructor. Otherwise, if no initializer is specified for a nonstatic
object, the object and its subobjects, if any, have an indeterminate initial value; if the object or any
of its subobjects are of const-qualified type, the program is ill-formed.”

I want to pay particular attention to the clause, “Otherwise, if no initializer is specified for a nonstatic
object, the object and its subobjects, if any, have an indeterminate initial value”. According to section 8.5 paragraph 5, the definition of a default-initialization falls into three cases:

1. if T is a non-POD class type (clause 9), the default constructor for T is called (and the initialization is
   ill-formed if T has no accessible default constructor)
2. if T is an array type, each element is default-initialized
3. otherwise, the object is zero-initialized.

So as I understand it, paragraph 9 is stating that if we have a non-POD class type that does not have an initializer, then it’s default constructor would be called. What I’m confused by is what happens in the case of POD-class types … it seems from the clause I highlighted that there is no mention of a default constructor call being required for POD-class types. Yet if I created a POD-class type like

struct POD_class
{
    int a;
    int b;
    POD_class() { cout << "Default constructor called" << endl; }
};

int main()
{
    POD_class test;
    return 0;
}

the default constructor of POD_class seems to be called when this code is compiled and run with g++. Therefore, even if POD_class did not have a specific initializer, it seems it was still default-initialized, per case #1 in the definition of default-initialization, because a default constructor for the type was called.

Based on the above scenario, here is my question: For a POD-class, does not default initializing an object as paragraph 9 mentions for non-static POD-classes mean that its default constructor is not called, or that it’s simply not zero-initialized?

Answer 1

Your POD_class is in fact not a POD class. A POD class cannot have a user-declared constructor.