C++ using static array in base class, declaring in derived class

Question

I'm trying to create a few classes which all are derived classes from an abstract class (lets call it) BaseClass. In BaseClass I want to operate on static variables (arrays) declared in derived classes. Is there a clever way in C++ to let the compiler know that static variable will be declared in derivered class? Or should I, for instance, in my derived classes' constructors pass a reference to a static variable to base class construct? This is my idea:

class BaseClass
{
   std::vector<float> &vector;

public:
   BaseClass(std::vector<float> &dVector):vector(dVector){};

   void vectorOperation()
   {
       vector.doSomething();
   }
   ...     
}

class DerivedClass : public BaseClass
{
   static std::vector<float> sVector;
   DerivedClass():BaseClass(sVector){};
   ... 
}

Is my solution correct? Is there any better way to do this?

Sry, just noticed it was static. You could use a CRTP I suppose. — WhozCraig
– WhozCraig, Commented Dec 3, 2014 at 21:52
The naming is certainly not the best choice: std::vector<float> &vector; should be s.th. like std::vector<float> &vector_; at least. — πάντα ῥεῖ
– πάντα ῥεῖ, Commented Dec 3, 2014 at 21:53
Your solutions seems simple, elegant and efficient for the purpose you describe. — Christophe
– Christophe, Commented Dec 3, 2014 at 22:04

R Sahu · Accepted Answer · 2014-12-03 21:53:47Z

3

You could use the CRTP pattern but I am not convinced it is better than what you have.

template <typename Derived>
class BaseClass
{
   std::vector<float> &vector;

public:
   BaseClass(): vector(Derived::getVector()){};

};

class DerivedClass : public BaseClass<DerivedClass>
{
   public:

   static std::vector<float>& getVector()
   {
      static std::vector<float> v;
      return v;
   }

   DerivedClass() {};
};

answered Dec 3, 2014 at 21:53

R Sahu

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2 Comments

Rames Over a year ago

I'm not sure this would work how I want it to, because v is declared in getVector() method, not in class. Would such declaration make v the same vector for every object from DerivedClass?

R Sahu Over a year ago

Yes. Since it is a static member variable, there will be only one of them.

Sergey Kalinichenko · Accepted Answer · 2014-12-03 21:53:23Z

In BaseClass I want to operate on static variables (arrays) declared in derived classes.

A virtual instance member function can delegate the work to an override from a derived class, which could do the access for you. There is no way of doing it in the static context (it does not look like you are trying to do it, though).

should I, for instance, in my derived classes' constructors pass a reference to a static variable to base class constructor?

This can be a solution as well. However, all instances of the derived class would be accessing the same static data region, and need to store a reference to it.

A solution based on a virtual function does not require an additional storage for a reference to the vector:

class BaseClass
{
protected:
   virtual std::vector<float>& vector() = 0;

public:
   void vectorOperation()
   {
       vector().doSomething();
   }
   ...     
};

class DerivedClass : public BaseClass
{
   static std::vector<float> sVector;
protected:
    std::vector<float>& vector() { return sVector; }
   ... 
};

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