How to initialize member-struct in initializer list of C++ class?

Question

I have the following class definitions in c++:

struct Foo {
  int x;
  char array[24];
  short* y;
};

class Bar {
  Bar();

  int x;
  Foo foo;
};

and would like to initialize the "foo" struct (with all its members) to zero in the initializer of the Bar class. Can this be done this way:

Bar::Bar()
  : foo(),
    x(8) {
}

... ?

Or what exactly does the foo(x) do in the initializer list?

Or is the struct even initialized automatically to zero from the compiler?

Answer 1

First of all, you should (must !) read this c++ faq regarding POD and aggregates. In your case, Foo is indeed a POD class and foo() is a value initialization :

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

So yes, foo will be zero-initialized. Note that if you removed this initialization from Bar constructor, foo would only be default-initialized :

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;