Question

I am trying to make a class that will represent two different behaivours at the same time. Like the following:

class IMouse {
public:
    virtual void Walk() const = 0;
};

class TSimpleMouse : public IMouse {
public:
    void Walk() const;
};

class IBat {
public:
    virtual void Fly() const = 0;
};

class TSimpleBat : public IBat {
public:
    void Fly() const;
};

template <class TMouse, class TBat>
class TSuperCreatureImpl {
public:
    void Do() const {
        Walk();
        Fly();
    }
};

typedef TSuperCreatureImpl<TSimpleMouse, TSimpleBat> TSimpleSuperCreature;

This is important for me to have the behaviour by deafult (method Do), because I would like to make different typedefs. It seems to be very simple.

But I also would like methods Fly and Walk to have parameters (velocity, for example). How should I modify the architecture to have an opportunity to use many typedefs for different creatures?

How to change the architecture if there are no default constructors for Mouse and Bat?

Thanks a lot.

Answer 1

template <class TMouse, class TBat>
class TSuperCreature : public TMouse, public TBat
{
  public:
    void do() const {
        this->walk();
        this->fly();
    }
};

But I also would like methods Fly and Walk to have parameters (velocity, for example). How should I modify the architecture to have an opportunity to use many typedefs for different creatures? Thanks a lot.

You could give the functions default parameters…

virtual void Walk(double meters_per_second = 1.1) const = 0;

EDIT

(from comment below) Imagine that we need 2 creatures: first one walks fast and flies slow, second one is the opposite. That means we have to write 2 typedefs. But I have no idea how to use velovity constants here. Which parts of architecture must have parameters?

One option is to do something like this:

template <class TMouse, class TBat, int walk_speed, int fly_speed>
class TSuperCreature : public TMouse, public TBat
{
  public:
    void do() const {
        this->walk(walk_speed);
        this->fly(fly_speed);
    }
};

If you want to use doubles, they’re not allowed as template arguments (at least not in C++03), but as a hack you could accept a pair of numbers and divide them inside the template, or more properly you could provide a policy class….

template <class TMouse, class TBat, class Speeds>
class TSuperCreature : public TMouse, public TBat
{
  public:
    void do() const {
        this->walk(Speeds::walk_speed);
        this->fly(Speeds::fly_speed);
    }
};

struct Speeds__Fly_Fast__Walk_Slow   // yes I know double underscores are reserved - I don't care!
{
    static const double walk_speed = 0.5; // m/s
    static const double fly_speed = 10;
};