Question

I was experimenting with static keyword in c++. The following class has a static variable called size which is initialized as 10.

class staticTest
{
    public:
            static int size;
            int x,y;

            staticTest()
            {
                x=0;
                y=0;
            }
            void display()
            {
                printf("%d %d\n",x,y);
            }
};   
int staticTest::size=10;

Now i have another class which uses this static variable. Here is the other class

class my
{
    public:
            int a[staticTest::size];

            my()
            {
                for(int i=0;i<staticTest::size;i++)
                {
                    a[i]=i;
                }
            }

            void display()
            {
                for(int i=0;i<staticTest::size;i++)
                {
                    printf("%d\n",a[i]);
                }
            }
};   

Now i have main function like this

main()
{
    my ni;
    ni.display();
}

I am unable to compile this program. Error is that array bound is not an integer constant. Why is this so?

Answer 1

Only compile-time constants can be used as array sizes.

Declare your integer as static const int.

Also, the (default) value needs to go in the declaration, not the definition:

// in header
class Foo {
public:
  static const int n = 10;
};

// in implementation
const int Foo::n;

---

Based on your comment, consider this: At the moment, you are effectively using a global variable to communicate some dynamic quantity. This should ring alarm bells:

int size; // global

void some_UI_function(); // modifies `size`?!

void some_computation()
{
  int data[size]; // pseudo-code
  // ...
}

This is terrible design, for a whole range of reasons. Instead, you should make the relevant size part of the class:

class my
{
  std::vector<int> m_data;
public:
  my(std::size_t n) : m_data(n) { }
  // ...
};

Now you can communicate the desired size locally:

int main()
{
  size_t n = get_data_from_user();
  my x(n);
  x.compute(); // etc.
}