Are extra bytes initialized to 0 in C++?

Question

Suppose I have a struct that contains an int and a char:

struct thing_t {
    int value = 0;
    char other_value = 0;
};

Because of the way things are aligned in the memory, sizeof(thing_t) should be 8, since the size of the largest member is 4 bytes. If I create a default-initialized instance of thing_t, will the extra 3 bytes be initialized to 0?

I recognize that this normally wouldn't come up, but I was writing a hash function and the default behavior of the hash function is to hash a generic type as though it were an array of bytes. If the extra bytes were occasionally not initialized to 0, I realized that my approach could cause problems.

Answer 1

I could not find anything in the C++11 standard about what happens to padding when an object is default initialized.

However, the standard is firm about padding when it comes zero-initialization.

From 8.5 Initializers/5

5.2 — if T is a (possibly cv-qualified) non-union class type, each non-static data member and each base-class subobject is zero-initialized and padding is initialized to zero bits;

5.3 — if T is a (possibly cv-qualified) union type, the object's first non-static named data member is zero-initialized and padding is initialized to zero bits;

I would advise using zero initialization instead of default initialization to force the padding bits to be initialized to zero. That would result in predictable values from your hash function.

Answer 2

If I create a default-initialized instance of thing_t, will the extra 3 bytes be initialized to 0?

They may or may not be. There is no requirement for them to be zeroed.

Unfortunately you can't value-initialize the object to get it zero-initialized, which does zero the padding bits, because the default member initalizers stop it from being trivially constructable.

[dcl.init]/8

To value-initialize an object of type T means:

[...]

• if T is a (possibly cv-qualified) class type without a user-provided or deleted default constructor, then the object is zero-initialized and the semantic constraints for default-initialization are checked, and if T has a non-trivial default constructor, the object is default-initialized;

[class.ctor]/6

A default constructor is trivial if it is not user-provided and if:

[...]

• no non-static data member of its class has a default member initializer (12.2)