can defaulted default constructors throw?

Question

Defaulted default constructors are generated by the C++ compiler, the user has no control over them. Can they throw? Is it ok to specify noexcept when declaring one?

The following code compiles fine with gcc.

struct A
{
  A() = default;
};

struct B
{
  B() noexcept = default;
};

int main()
{
  A a;
  B b;

  return 0;
}

Answer 1

It is allowed to add a noexcept specifier to a defaulted special member function (default constructor, copy-constructor, assignment-operator etc.).

A default declared special member function will have a noexcept specifier depending on the noexcept specifiers of the involved functions (its implicit noexcept specifier). If you explicitly specify noexcept compilation should fail if this conflicts with the implicit noexcept specifier.

Answer 2

can default constructors throw?

Yes, they can. For example, if the class has data member(s) whose default constructor throws.

struct Foo
{
  Foo() { /* throws */} 
};

struct Bar
{
  Bar() = default;
  Foo f;
}