将派生的类构造函数参数传递给受保护的成员

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.

PS 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.