Class member without a default constructor

Question

Suppose I have a class A without a default constructor, a factory method factoryA that returns an object of type A, and a class B that has A as its member. I know that in th

Answer 1

It is not entirely clear to me why so if someone could explain that to me it would be great.

For classes[*], the _a = factoryA(true); line calls _a.operator=(factoryA(true)). Calling a member function on _a requires _a to already be initialised. So if it weren't a compile-time error, it still wouldn't be valid.

Also, what if the parameter to A's constructor needs to be computed inside of B's constructor, say by querying a database or something of that nature? Is there a way to use the setup below without providing A with a default constructor?

As long as A has a copy or move constructor, you can initialise it with a function return value, and that function can do anything you want, even using different constructors for A depending on the arguments provided.

class B {
private:
  A _a;
  static A getA(int i);
public:
  B(int j) : _a(getA(j)) {
  }
};

A B::getA(int j)
{
  if (j > 0)
    return factoryA(true);
  else
    return factoryA(false);
}

[*] I know, there are exceptions.

Answer 2

In this case it's better to use the pointer to A, i.e. A* _a, and then call A constructor wherever you want.

Answer 3

Member objects are always initialized before entry into the body (the part between the curly braces) of the constructor. If you don't mention a member in the initializer list, it gets default constructed. So mention it!

B::B(int j) : _a(factoryA(0 < j)) { };

This calls the function factoryA with the argument value true if j is greater than 0 and false otherwise, and initializes the member _a with the value returned by that call.