Default-initialized vs. Value-initialized

Question

From this answer, In C++03, a POD type gets default initialized if () is omitted otherwise it is value-initialized.

// POD type
struct foo {
     int x;
};

// value-initialized
new foo();

But if a user-defined constructor is provided, is any of the objects below will be considered as default or value-initialized ?

// non-POD type
struct bar {
     bar(int x = 0):x(x) {}
     int x;
};

new bar();
new bar(42);

In your example, both calls to `bar`'s constructor result in providing a value during initialization of `x`. — Chad, Nov 20 '11 at 18:38

Jason · Answer 1 · 2011-11-20T21:07:36.323

In C++03, a POD type gets default initialized if () is omitted otherwise it is value-initialized.

That is not exactly what happens. According to the C++03 spec, section 8.5/9, if no initializer is specified for a non-static POD-type object, then it and its sub-objects "have an indeterminate initial value". That is not the same thing as default-initialization. Default-initialization is the same thing as value-initialization for a POD-type, which would mean the object is zero-initialized (8.5/5), but that can only happen with the presence of an empty initializer (i.e., empty-parenthesis per 8.5/7). Thus you can only default and/or value-initialize a POD-type with an empty initializer. Default initialization for a non-static POD-type does not happen when no initializer is specified.

In your second example, with the non-POD type that has the user-defined constructor, default-initialization would technically take place if you omitted the value-initializer (parenthesis) symbols. In other words:

bar* ptr_a = new bar; //default initialization
bar* ptr_b = new bar(); //value initialization

Keep in mind though that with both non-POD struct or class-types, if there is a user-defined constructor, default-initialization and value initialization, per 8.5/5, both call the user-defined constructor. So in the end, with the type bar as you've declared it, default and value initialization end up doing the same thing.

Kerrek SB · Answer 2 · 2011-11-20T20:50:52.397

If your class has a user-defined default constructor, then both default and value initialization cause that constructor to be called. Whatever happens next is up to the constructor:

struct UDT
{
  int a;
  int b;
  Foo c;
  Foo d;
  UDT() : a(), c() {}
};

Both default and value initialization of an object of class UDT will cause UDT::a and UDT::c to be value-initialized (so a is zero) because the initializer list says so, while UDT::b and UDT::d are themselves default-initialized (so b is uninitialized, and for d apply the same logic recursively).

For details on initialization, see 8.5, and on initializer lists see 12.6.2 (esp. clause 8).

Default-initialized vs. Value-initialized

2 Answers2