Question

The sample code below shows what I am trying to do:

template<int NX, int NY>
class Array
{
public:
  float v[NX][NY];
};

void main()
{
  Array<10,20> grid;
}

The above code won’t compile, but it shows what I want to do. We have a class that contains an array, and the array class doesn’t know its size until compile time. Is there a simple way to do this?

Edit: I want to write a simple reusable array class. That means I need to find a good way to separate the array size from the class.

I also want the class to be fast (and simple) so it must not be dynamically allocated. That means the size can’t be given during run time.

I also don’t want to use the preprocesser to define the size because that means I will have to go through the hassle of changing a number somewhere. That isn’t convenient enough.

Basically, the class doesn’t know its own size until compile time, because that is when the main function tells the class its size.

Edit: The above code is good.

Answer 1

Other than main not returning an int, this is legal code and should compile. In fact, on some compilers this will compile without main returning an int, such as VC++ but this is non-standard behaviour.

You can also store the size at compile time so that you don’t have to calculate it manually.

#include <iostream>

template<int NX, int NY>
class Array
{
public:
  float v[NX][NY];

  int size() const { return ArraySize; }

private:
  enum { ArraySize = NX * NY }; // You can also store rows/cols individually
};

int main()
{
  Array<10,20> grid;

  std::cout << grid.size();

  return 0;
}