C++: Initialization of inherited field

c++ initialization initialization-list
Haspemulator picture Haspemulator · Oct 21, 2010 · Viewed 13.5k times · Source

I've a question about initialization of inherited members in constructor of derived class. Example code:

class A
    {
public:
    int m_int;
    };

class B: public A
    {
public:
    B():m_int(0){}
    };

This code gives me the following output:

In constructor 'B::B()': Line 10: error: class 'B' does not have any field named 'm_int'

(see http://codepad.org/tn1weFFP)

I'm guessing why this happens? m_int should be member of B, and parent class A should already be initialized when initialization of m_int in B happens (because parent constructors run before member initialization of inherited class). Where is a mistake in my reasoning? What is really happens in this code?

EDIT: I'm aware of other possibilities to initialize this member (base constructor or assignment in derived constructor), but I want to understand why is it illegal in the way I try it? Some specific C++ language feature or such? Please point me to a paragraph in C++ standard if possible.

Answer

Ben Jackson picture Ben Jackson · Oct 21, 2010

You need to make a constructor for A (it can be protected so only B can call it) which initializes m_int just as you have, then you invoke :A(0) where you have :m_int(0)

You could also just set m_int = 0 in the body of B's constructor. It is accessible (as you describe) it's just not available in the special constructor syntax.

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