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;

Answer 2

In standard C++ you need to make a ctor for Foo.

struct Foo {

  Foo(int const a, std::initializer_list<char> const b, short* c)
    : x(a), y(c) {
    assert(b.size() >= 24, "err");
    std::copy(b.begin(), b.begin() + 24, array);
  }

  ~Foo() { delete y; }

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

class Bar {

  Bar() : x(5), foo(5, {'a', 'b', ..., 'y', 'z'},
    new short(5)) { }

  private:

  int x;
  Foo foo;
};

In C++0x you may use uniform initialization list, but still you need dtor for Foo:

class Bar {

  Bar() : x(5), foo{5, new char[24]{'a', 'b', ..., 'y', 'z'},
    new short(5)} { }
  ~Bar() { delete[] foo.array; delete foo.y;}
  }
  private:

  int x;
  Foo foo;
};

To default initialize foo (as Bar() : foo(), x(8) { }) you need to give Foo a default ctor.