Initialize base class with data member from derived class [duplicate]

Question

Consider a code

struct X {
    X (int n) {...}
};

struct Base {
    Base(X & x) {...}
};

struct Derived : public Base {
    Derived() : Base(x), x(2) {}
    X x;
};

Code works and i see no issue there but i'm concerned about the look of : Base(x), x(2) statement. First pass the instance to Base and after you initialize the actual object. Is this the the only way to express the intention?

Answer 1

The trick is to derived from one more base class who's function is to own the X.

Note that base class declaration order matters:

struct X {
    X (int n) {}
};

struct Base {
    Base(X & x) {}
};

struct OwnsAnX {
    OwnsAnX(int n) : x_(n) {}

    X& get_x() { return x_; }
private:
    X x_;
};

struct Derived 
: OwnsAnX   // note: order is important
, Base 
{
    Derived() 
    : OwnsAnX(2)
    , Base(get_x()) 
    {}

    // x is accessed through the inherited get_x()
};

but it's error prone if you don't keep the correct order of the classes you're inheriting from

This is a valid concern of the OP. The solution is to enable the compiler warning -Wreorder.

Reversing the order of the base classes then yields:

<source>: In constructor 'Derived::Derived()':
<source>:24:23: warning: base 'OwnsAnX' will be initialized after [-Wreorder]
         , Base(get_x())
                       ^
<source>:24:23: warning:   base 'Base' [-Wreorder]
<source>:22:9: warning:   when initialized here [-Wreorder]
         Derived()
         ^~~~~~~