Why does a move constructor require a default constructor for its members?

Question

I was trying to implement a move constructor for a class without a copy constructor. I got an error that the default constructor for a member of the class was missing.

Answer 1

Use the constructor's initializer list to initialize the A member. As written, the move constructor uses, as the compiler says, the default constructor for A.

B(B&& b) : a(std::move(b.a)) {}