Question

Coming from a C background, I’ve always assumed the POD types (eg ints) were never automatically zero-initialized in C++, but it seems this was plain wrong!

My understanding is that only ‘naked’ non-static POD values don’t get zero-filled, as shown in the code snippet. Have I got it right, and are there any other important cases that I’ve missed?

static int a;

struct Foo { int a;};

void test()
{
  int b;     
  Foo f;
  int *c = new(int); 
  std::vector<int> d(1);

  // At this point...
  // a is zero
  // f.a is zero
  // *c is zero
  // d[0] is zero
  // ... BUT ... b is undefined     
}  

Answer 1

Assuming you haven’t modified a before calling test(), a has a value of zero, because objects with static storage duration are zero-initialized when the program starts.

d[0] has a value of zero, because the constructor invoked by std::vector<int> d(1) has a second parameter that takes a default argument; that second argument is copied into all of the elements of the vector being constructed. The default argument is T(), so your code is equivalent to:

std::vector<int> d(1, int());

You are correct that b has an indeterminate value.

f.a and *c both have indeterminate values as well. To value initialize them (which for POD types is the same as zero initialization), you can use:

Foo f = Foo();      // You could also use Foo f((Foo()))
int* c = new int(); // Note the parentheses