Initializing an abstract class? - c++

Question

Here is my issue. I want to have something like this:
class A {
protected:
int someInt;
virtual void someFunc() = 0;
};
class B : public A {
protected:
virtual void someFunc() { // uses someInt}
public:
B() {//tell the A inside of B to set someInt to whatever it wants}
};
so basically, someInt can be changed, it's not constant, but I want all the classes that implement A to use a value provided by A for someInt.
Thanks

Answer 1

You can use initializer lists in the constructor to call parent constructors:
class A {
protected:
int someInt;
virtual void someFunc() = 0;
A(int x) : someInt(x) {} // Base-class constructor (initialises someInt)
};
class B : public A {
protected:
virtual void someFunc() {}
public:
B() : A(10) {} // Initialises base class via constructor
};

Answer 2

Is this what you want?
class A
{
protected:
int someInt;
public:
A(int _val) : someInt(_val)
{
}; // eo ctor
}; // eo class A
class B : public A
{
public:
B() : A(5) // initialise someInt with 5
{
}; // eo ctor
}; // eo class B
Note, that as "someInt" is protected, you can just set it in B's constructor anyway.
B()
{
A::someInt = 5;
}; // eo ctor

Answer 3

I am not sure what you want, but if you initialize someInt in the constructor of A, subclasses will see that value.
class A {
public:
A() : someInt(5) {}
protected:
int someInt;
virtual void someFunc() = 0;
};
class B : public A {
protected:
virtual void someFunc() { // uses someInt}
public:
B() {// at this point someInt will already have been initialized to 5}
};
When the subclass is constructed, the baseclass constructors are executed first.

Answer 4

class A {
protected:
int someInt;
virtual void someFunc() = 0;
A(){//set someInt here}
};
class B : public A {
protected:
virtual void someFunc() { // uses someInt}
public:
B():A() {}
};
A protected constructor will achieve this. As above. The solutions using a public constructor are fine too, but as the constructor cannot be called directly on thr interface I think protected is better.