Determine array size in constructor initializer

Question

In the code below I would like array to be defined as an array of size x when the Class constructor is called. How can I do that?

class Class
{
public:
  int array[];
  Class(int x) : ??? { }
}

Answer 1

You can't initialize the size of an array with a non-const dimension that can be calculated at compile time (at least not in current C++ standard, AFAIK).

I recommend using std::vector<int> instead of array. It provides array like syntax for most of the operations.

Answer 2

Instead of using a raw array, why not use a vector instead.

class SomeType {
  vector<int> v;
  SomeType(size_t x): v(x) {}
};

Using a vector will give you automatic leak protection in the face of an exception and many other benefits over a raw array.

Answer 3

Use the new operator:

class Class
{
   int* array;
   Class(int x) : array(new int[x]) {};
};

Answer 4

You can't do it in C++ - use a std::vector instead:

#include <vector>

struct A {
   std::vector <int> vec; 
   A( int size ) : vec( size ) {
   }
};

Answer 5

Declare your array as a pointer. You can initialize it in the initializer list later through through new.

Better to use vector for unknown size.

You might want to look at this question as well on variable length arrays.

Answer 6

I don't think it can be done. At least not the way you want. You can't create a statically sized array (array[]) when the size comes from dynamic information (x).

You'll need to either store a pointer-to-int, and the size, and overload the copy constructor, assignment operator, and destructor to handle it, or use std::vector.

class Class
{
  ::std::vector<int> array;
  Class(int x) : array(x) { }
};

Answer 7

Two options:

Use std::vector. This allows easy re-sizing of the array.
Use std::tr1::array. This has a static size.

Both can be correctly initialized in the constructors initializer list.