Call base class constructor after member constructor - c++

Question

I have the following class hierarchy, where the Base class depends on its derived class to supply it an argument in its constructor:
class Member
{
public:
Member(int v);
};
class Base
{
public:
Base(const Member& m);
};
class Derived : public Base
{
public:
Derived() : m_(123), Base(m_) // <- here is the problem
{
}
private:
Member m_;
};
The problem is, though, that in Derived's constructor, the Base constructor gets called first, when Derived's member variable m_ which it depends on isn't initialized yet.
Is there a way to force the compiler to call the constructor of m_ first or should I just rework my class hierarchy?

Answer 1

You can simulate initializing your member before the base class by making it it's own base class which you initialize first. You can wrap it in a simple class type and have Derived inherit privately from that type before Base. In the following example, Derived has a Member _m; which is initialized and then used to initialize Base.
class Member
{
public:
Member(int) {}
};
class Base
{
public:
Base(const Member&) {}
};
// The new wrapper
struct member_wrapper
{
member_wrapper(int v) : m_(v) {}
Member m_;
};
class Derived : private member_wrapper, public Base
{
public:
Derived() : member_wrapper(123), Base(m_)
{ }
};
Though in this case, since m_ is already a class type and Derived has no other members with that type, you can just inherit privately from Member directly. If you had a non-class type or multiple members of the same type that needed to be initialized before Base you would need to wrap them.
class Member
{
public:
Member(int) {}
};
class Base
{
public:
Base(const Member&) {}
};
class Derived : private Member, public Base
{
public:
Derived() : Member(123), Base(*this)
{ }
};