Can I override constructor member initialization order in c++?

can you try making that function static.

I'm not sure I understand your explanation.

I think you can solve the problem if you will:

1. Add a function Set into CBaseClass and instead of using a constructor with the integer as a parameter, you will call this Set method from CMyClass constructor.

2.  use the aggregation:
class CMyClass
{
       CBaseClass m_Base
}
so in CMyClass constructor you can arrange things as you wish.

Probably, I'd use a template. Maybe as it is the attached code.

template<class T>
class CBaseClass
{
public:
	void Do()
	{
		T* pT = static_cast<T*>(this);
		pT->GetData();
	}
	int GetData() { return 0;};
};
 
class CMyClass : public CBaseClass<CMyClass>
{
	int m_nData;
public:
	CMyClass() : m_nData(256) {}
 
	void Action()
	{
		Do();
	}
	int GetData()
	{
		return m_nData;
	}
};
                                              

1:
2:
3:
4:
5:
6:
7:
8:
9:
10:
11:
12:
13:
14:
15:
16:
17:
18:
19:
20:
21:
22:
23:
24:
25:
26:
27:

instead of
CMyClass( CData* pData )
    : m_pData(pData),
      CBaseClass( ExtractIntFromData() )
  {
  }

how about
CMyClass( CData* pData )
    : m_pData(pData),
      CBaseClass( pData->ExtractIntFromData() )
  {
  }

The base class is constructed before the derived class so trying to call a member of the derived class to initialize the base class will result in undefined behaviour. You are trying to call a member on a class that has not been constructed. This behaviour is immutable. This will only work if the member function is static otherwise all bets are off.

You can, of course, separate construction and initialisation into two steps. Have the base class default construct with sensible values (even though they aren't the ones you want at this stage) and add an initialise member, which can be called by your derived class in your derived classes constructor (where you can call any member function safely except virtual ones), which can be used to initialise your base class.

Thanks for the comments people, some very useful stuff there.

Its worth knowing that at member initialisation time, no assumptions can be made about the integrity of the class being initialised.

I agree with AndyAinscow, usually when you start messing around at the borders of C++ language constraints its a good hint there is a design problem!

Good suggestion to aggregate as opposed to inherit, although in my real world case I need the polymorphism inheritance gives.

In the context of the real code this example was based on, I think ExtractIntFromData() does really belong as a member of CDerivedClass. Therefore the most appropriate solution for my real world problem is to have 2 ExtractIntFromData() functions, one that is static and takes CData as a parameter for use in the constructor.  The other simply calls the first passing m_pData.

evilrix - thanks its useful to confirm that it is safe to use non-virtual member functions within a constructor.

I wrote this code initially because I believed you could use them safely within the constructor.  However my mistake was in thinking the member initializer list counted as 'within' the constructor.