C ++构造函数，常量，继承和避免重复

Question

so I've been learning C++ for a few weeks now but I'm having a bit of trouble:

Class Tool
{
public:
    Tool(const float maxCarried = 1):maxAmountCarried(maxCarried){}
    virtual void Use() = 0;
    /* ... */
}

Class CuttingTool: public Tool
{
public:
    CuttingTool(const float maxCarried):Tool(maxCarried){}
    virtual void Use(){ /* ... */ }
    /* ... */
}

Class Saw: public CuttingTool
{
public:
    Saw(const float maxCarried):CuttingTool(1){}
    virtual void Use(){ /* ... */ }
    /* ... */
}
Class Scissors: public Fruit
{
public:
    Scissors(const float maxCarried):CuttingTool(2){}
    virtual void Use(){ /* ... */ }
    /* ... */
}

A few things to note:

• I'm trying to make a big database of 'Tools'.
• I never change the value of 'maxAmountCarried' so I've set it to const.
• Memory/performance is important because I have a huge vector of Tools.

The problem lies within the fact that I have to keep writing:

ClassName(const float maxCarried):BaseClass(maxCarried){}

It's really tedious, moreover, I worry that if I were to add a new const value I would have to repeat the process all over again (problem when you have 50 classes inheriting from Food :S).

I feel as though I've designed this poorly. Is there a way to avoid repeating the same line of code over and over again or do I just have to suck it up and deal with it?

Thanks in advance.

Answer 1

If your only concern is the repeating initialization list you could use a macro like this:

#define DEFAULT_CONSTRUCTOR(Child, Parent) Child(float max) : Parent(max) {}

and use it like so:

class Saw : public CuttingTool
{
public:
    DEFAULT_CONSTRUCTOR(Saw, CuttingTool) {}
};

You can extend this idea and do something like that:

#define BEGIN_CLASS(Child, Parent) class Child : public Parent { \
                                      public: \
                                         Child(float max) : Parent(max) {}

#define END_CLASS };

and declare your classes:

BEGIN_CLASS(Scissors, Tool)
   void cut_through_paper() {}   // specific method for scissors
END_CLASS

Note that there is no point of using const float as a parameter since you can't change arguments passed by value anyway. You might however want to use const float& to pass an argument by reference, and that will make sense if size of float is bigger than the size of a pointer in your specific platform.

If you never change you max value, you can make it static and share it between all tool instances:

class Tool
{
protected:
   static const float _max;
public:
   Tool() {}
};
const float Tool::_max = 0;

If you'd like to be able to change max value only once (say at the begining of your program, you can add a static function:

static void globalMax(float max) { Tool::_max = max; }

and use it where appropriate:

int main() {
   Tool::globalMax(5);
   ...
   ...
}

Note that you should remove the const from the _max declaration.

Finally, if performance is an issue, you probably need to rethink your design and maybe go with something else (templates maybe?)

Hope that helps!