How to initialize the array member with constant member value?

Question

I want to use a constant value to initialize member array.

--Test.h--

class Test {
public:
  static int ARRAY_SIZE;
...
..
private
  int m_array[ARRAY_SIZE];
}

--Test.cpp--

int Test::ARRAY_SIZE = 20;

But, it shows error like this. 'array bound is not an integer constant before ']''

So, I want to know there is a solution for this.

I want fixed size of array. and i don't want any dependency (like additional files for constant value)

What is the best practice for this?

Answer 1

The error is correct. Your array bound is a variable, not a constant.

static int ARRAY_SIZE;

The above declares a variable. The below declares a constant.

static const int ARRAY_SIZE;

For an array's size, there is an additional requirement beyond simply being declared const : the size must be initialized with a constant expression, also known as a compile-time constant (a constant value known by the compiler). If a translation unit sees the declaration of ARRAY_SIZE but does not see its value, then it does not count as a compile-time constant. To remedy this, the initialization needs to be in the header file. Combine initialization with declaration:

static const int ARRAY_SIZE = 20;

Since C++11, there has been a fancy way to express that you have a compile-time constant, which you might want to get into the habit of using when it applies:

static constexpr int ARRAY_SIZE = 20;