仅当派生类知道其大小时如何在基类函数中使用成员变量

Question

I want to implement a function in a baseclass that uses members of its derived classes. So with each DerivedClass, I would have a member of a different value. Here's an example. The BaseClass has member function Foo() which uses variable arrStr . This content of this NUL terminated char array however is only to be found in a derived class. How can I make BaseClass "know" the variable arrStr without knowing its size? Is that even possible?

    class BaseClass
    {
    public:
        BaseClass();
        ~BaseClass();

    protected:

    void Foo()
    {
     prtinf("%s\n", arrStr);
    };

    };

    class DerivedClass : public BaseClass
        {
        public:
            DerivedClass();
            ~DerivedClass();

            protected:

            char arrStr[] = "FooString!";
     };

Answer 1

Add (pure) virtual access functions in your base class which you implement in the derived class.

#include <iostream>
#include <string>

class BaseClass
{
protected:
    void Foo()
    {
        std::cout << GetString() << std::endl;
    }

private:    
    virtual std::string GetString() const = 0;
};

class DerivedClass : public BaseClass
{
private:
    virtual std::string GetString() const override
    {
        return "FooString!";
    }
};

Live demo

Answer 2

How can I make BaseClass "know" the variable arrStr without knowing its size? Is that even possible?

You insert it as a parameter when calling Foo:

class BaseClass
{
public:
    virtual void Process() = 0;
protected:
    void Foo(char* str) // <-- value inserted here as parameter
    {
        prtinf("%s\n", arrStr);
    }
};

class DerivedClass : public BaseClass
{
public:
    void Process() override
    {
        Foo(arrStr); // call in derived class, with derived value
    }
protected:
    char arrStr[] = "FooString!";
};

Client code:

BaseClass * b = new DerivedClass;
b->Process(); // call Foo(arrStr) internally