Question

It is possible to pass uninitialized object to a parent class like in the following example

class C
{
    public:
        C(int i): 
            m_i(i)
        {};

        int m_i;
}

class T
{
    public:
        T(C & c):
            m_c(c)
        {
        };

        C & m_c;
};


class ST : public T
{
    public:
        ST():
            T(m_ci),
            m_ci(999)
        {
        };

        C m_ci;
};

In class T constructor, c is a reference to uninitialized object. If class T were using c object during construction, this would possibly lead to an error. But since it’s not, this compiles and works fine. My question is – does it brake some kind of paradigm or good design directives? If so, what are the alternatives, because I found it useful to allocate an object required by parent in a subclass.

On a side note, I wonder why it’s not possible to change initialization order, so that base class constructor would be called after initialization of some members.

Answer 1

You can, but you get undefined behavior.

In Boost’s utilities, you’ll find the base-from-member idiom created by R. Samuel Klatchko. Basically, you make a private base in the place of the private member. This base gets initialized first, and you can use it for other bases:

// ...

class C_base
{
public:
    C_base(int i) :
    m_ci(i)
    {}

    C m_ci;
};


class ST :
    private C_base
    public T
{
    public:
        ST() :
            C_base(999),
            T(m_ci),
        {
        };
};

Boost’s utility eliminates repeated code.