Why do designated initializers zero-initialize the data members?

Question

Below is from cppref of Designated initializers:

struct A { int x; int y; int z; };
A b{.x = 1, .z = 2}; // ok, b.y initialized to 0

By default, all fundamental types are default-initialized rather than zero-initialized in C++.

Why do designated initializers zero-initialize the data members?

Answer 1

b.y will be initialized from an empty initializer list, as the effect, zero-initialized to 0.

For a non-union aggregate, elements for which a designated initializer is not provided are initialized the same as described above for when the number of initializer clauses is less than the number of members (default member initializers where provided, empty list-initialization otherwise):

struct A {
  string str;
  int n = 42;
  int m = -1;
};
A{.m=21}  // Initializes str with {}, which calls the default constructor
          // then initializes n with = 42
          // then initializes m with = 21

From the standard, [dcl.init.aggr]/5:

For a non-union aggregate, each element that is not an explicitly initialized element is initialized as follows:

• (5.1) If the element has a default member initializer ([class.mem]), the element is initialized from that initializer.

• (5.2) Otherwise, if the element is not a reference, the element is copy-initialized from an empty initializer list ([dcl.init.list]).

• (5.3) Otherwise, the program is ill-formed.