How to declare an array without specific size?

Question

How can I declare an array without specific size as a class member? I want to set the size of this array immediately in the class constructor. Is it possible to do such thin

Answer 1

Class member array have to be declared with exact compile-time size. There's no way around it.

The only way you can declare an array as an immediate member of the class and yet be able to decide its size at run time would be the popular "struct hack" technique inherited from C.

Other than that, you have to declare your array as an indirect member of the class: either declare a member of pointer type and allocate memory later, or use some library implementation of run-time sized array (like std::vector)

Answer 2

C++ requires the size of an automatic storage array to be known at compile time, otherwise the array must be dynamically allocated. So you would need dynamic allocation at some level, but you don't have to concern yourself with doing it directly: just use an std::vector:

#include <vector>

class Foo
{
 public:
  Foo() : v_(5) {}
 private:
  std::vector<int> v_;
};

Here, v_ is a vector holding ints, and is constructed to have size 5. The vector takes care of dynamic allocation for you.

In C++14, you will have the option of using std::dynarray, which is very much like an std::vector, except that its size is fixed at construction. This has a closer match to the plain dynamically allocated array functionality.

Answer 3

Variable length arrays are not allowed by C++ standard. The options you have are:

• Use a std::vector or
• Use a pointer to dynamic memory

Note that Variable length arrays are supported by most compilers as a extension, So if you are not worried of portability and your compiler supports it, You can use it. ofcourse it has its own share of problems but its a option given the constraints you cited.

Answer 4

You can use a vector, by including header file #include<vector> It can grow and shrink in size as required and vectors have built in methods/functions which can make your work easy.