Passing the derived class constructor parameter to a protected member - c++

Question

I'm trying to practice some polymorphism and i got into some issues.
Here's my code :
class A{ //the base
public:
A(){}
virtual void Log(){};
virtual ~A(){};
private:
protected:
int __value;
};
class B : public A{ //the derived
public:
B(int value):__value(value){} //here's the problem
void Log() override{
std::cout<<__value<<"\n";
}
~B(){};
};
At that lines the error said : "class 'B' does not have any field named '__value'". It does work if i will do it in this way :
class A{
public:
A(){}
virtual void Log(){};
virtual ~A(){};
private:
protected:
int __value;
};
class B : public A{
public:
B(int value){
__value=value;
}
void Log() override{
std::cout<<__value<<"\n";
}
~B(){};
};
I know what i've tried works while i'm accesing the private members, but I want to know if there is some way to make the first attempt work too.
Thanks!

Answer 1

C++ does not work this way. Only a class's constructor can initialize its members.
Only A's constructor can initialize its class member. That's what a constructor's job is. A derived class cannot initialize its base class's members, only it's own class members. A base class only initializes the base class's members. A derived class's constructor can initialize only its own class's members.
What you need to do is add a constructor to A, perhaps a protected constructor, with a parameter that initializes the class member with the parameter:
class A {
// ...
A(int value) : __value{value} {}
// ...
};
And have the derived class's constructor explicitly invoke this constructor.
B(int value) : A{value}
{
}
In some situations you can also delegate the constructor, as an alternative. This should be covered in the advanced C++ chapters of your C++ book.
P.S. You should use modern C++'s uniform initialization syntax, with {...} instead of (...). If you're using an older C++ book that doesn't cover uniform initialization syntax, you should get a more recent book.