Question

I have an abstract class CommandPath, and a number of derived classes as below:

class CommandPath {
    public:
        virtual CommandResponse handleCommand(std::string) = 0;
        virtual CommandResponse execute() = 0;
        virtual ~CommandPath() {}
};

class GetTimeCommandPath : public CommandPath {
    int stage;
    public:
        GetTimeCommandPath() : stage(0) {}
        CommandResponse handleCommand(std::string);
        CommandResponse execute();
};

All of the derived classes have the member variable ‘stage’. I want to build a function into all of them which manipulates ‘stage’ in the same way, so rather than defining it many times I thought I’d build it into the parent class. I moved ‘stage’ from the private sections of all of the derived classes into the protected section of CommandPath, and added the function as follows:

class CommandPath {
    protected:
        int stage;
    public:
        virtual CommandResponse handleCommand(std::string) = 0;
        virtual CommandResponse execute() = 0;
        std::string confirmCommand(std::string, int, int, std::string, std::string);
        virtual ~CommandPath() {}
};

class GetTimeCommandPath : public CommandPath {
    public:
        GetTimeCommandPath() : stage(0) {}
        CommandResponse handleCommand(std::string);
        CommandResponse execute();
};

Now my compiler tells me for the constructor lines that none of the derived classes have a member ‘stage’. I was under the impression that protected members are visible to derived classes?

The constructor is the same in all classes, so I suppose I could move it to the parent class, but I’m more concerned about finding out why the derived classes aren’t able to access the variable.

Also, since previously I’ve only used the parent class for pure virtual functions, I wanted to confirm that this is the way to go about adding a function to be inherited by all derived classes.

Answer 1

Try this:

class CommandPath {
protected:
  int stage;
public:
  CommandPath(int stage_) : stage(stage_) {}
};

class GetTimeCommandPath : public CommandPath {
public:
  GetTimeCommandPath(int stage_) : CommandPath(stage_) {}
};

(Omitted extra code for brevity).

You can’t use the initializer list on a parent class’ members, only the current one’s. If that makes sense.