Should trivial default constructor respect default member initializer here?

Question

Consider the code:

#include <atomic>
#include <iostream>

struct stru {
  int a{};
  int b{};
};

int main() {
  std::atomic<stru> as;
  auto s = as.load();
  std::cout << s.a << ' ' << s.b << std::endl;
}

Note that although stru has default member initializer, it still qualifies as an aggregate type since C++14. std::atomic has a trivial default constructor. According to the standard, should the members of as be initialized to zero? clang 6.0.0 doesn't do this (see here), while gcc 7.2.0 seems so (see here).

Answer 1

Strictly speaking, I think both compilers are right, in that your program exhibits undefined behavior. To quote n4140 (C++14), [atomics.types.operations.req], emphasis mine:

In the following operation definitions:

• an A refers to one of the atomic types.

[...]

A::A() noexcept = default;

Effects: leaves the atomic object in an uninitialized state. [ Note: These semantics ensure compatibility with C. — end note ]

as is uninitialized before the load. So the usual spiel about undefined behavior must follow.

Answer 2

According to cppreference the std::atomic::atomic() constructor doesn't initialize the obj:

atomic() noexcept = default;

1) The default constructor is trivial: no initialization takes place other than zero initialization of static and thread-local objects. std::atomic_init may be used to complete initialization.