When using C++ 20 designated intializers it is not required to specify a value for every field:
struct Foo
{
int i;
void* p;
};
can be initialized like this:
Foo bar = {
.i = 22
};
Are the left out fields zero-intialized? So in this example is p guaranteed to be set to nullptr?
Are the left out fields zero-intialized?
In this case, yes. The rule is (from [dcl.init]/5):
For a non-union aggregate, each element that is not an explicitly initialized element is initialized as follows:
- If the element has a default member initializer ([class.mem]), the element is initialized from that initializer.
- Otherwise, if the element is not a reference, the element is copy-initialized from an empty initializer list ([dcl.init.list]).
- Otherwise, the program is ill-formed.
In this case p is not explicitly initialized. It has no default member initializer, so we fall to the second bullet. It is not a reference, so it is copy-initialized from {}. For a void*, that's zero-initialization.
The fields left out are:
default member initializers where provided, empty list-initialization otherwise
source: https://en.cppreference.com/w/cpp/language/aggregate_initialization