Question

Create a UserArray of bit fields which can be declared as follows: The size occupied by our Array will be less then a normal array. Suppose we want an ARRAY of 20 FLAGs (TRUE/FALSE). A bool FLAG[20] will take 20 bytes of memory, while UserArray<bool,bool,0,20> will take 4 bytes of memory.

• Use class Template to create user array.
• Use Bit wise operators to pack the array.

• Equality operation should also be implemented.

  template<class T,int W,int L,int H>//i have used template<class T> 
                                     //but never used such way
  class UserArray{ 
          //....                 
  };        
  typedef UserArray<bool,4,0,20> MyType;
  

where:

• T = type of an array element
• W = width of an array element, 0 < W < 8
• L = low bound of array index (preferably zero)
• H = high bound of array index

A main program:

int main() {
      MyType Display;  //typedef UserArray<T,W,L,H> MyType; defined above

      Display[0] = FALSE; //need to understand that how can we write this?
      Display[1] = TRUE; //need to understand that how can we write this?

      //assert(Display[0]);//commented once, need to understand above code first
      //assert(Display[1]);//commented once..
      //cout << “Size of the Display” << sizeof(Display);//commented once..
}

My doubt is how those parameters i.e T,L,W & H are used in class UserArray and how can we write instance of UserArray as Display[0] & Display[1] what does it represent?

Short & simple example of similar type will be easy for me to understand.

Answer 1

W, L and H are non-type template parameters. You can instantiate a template (at compile-time) with constant values, e.g.:

template <int N>
class MyArray
{
public:
    float data[N];

    void print() { std::cout << "MyArray of size " << N << std::endl; }
};

MyArray<7> foo;
MyArray<8> bar;

foo.print();  // "MyArray of size 7"
bar.print();  // "MyArray of size 8"

In the example above, everywhere that N appears in the template definition, it will be replaced at compile-time by the supplied constant.

Note that MyArray<7> and MyArray<8> are completely different types as far as the compile is concerned.

I have no idea what the solution to your specific problem is. But your code won’t compile, currently, as you have not provided values for the template parameters.