How do I dynamically assign the size to an array in C++?

Question

So right now I have a class, called Set, in C++. In it, I have an integer called mCardinality, and an integer array called mElements. I want to be able to accept an integer in the constructor for mCardinality, then use this to assign a size to my integer array, is there any way to do this, or do I absolutely have to have a size for it right off the bat? (I guess I could just make the size the maximum integer, then only use the cardinality for limiting the loops where I deal with the elements, but a neater solution would be preferable).

I did see on here that there may be a way to use std::vector to resize an array, will that work? And...how would that code look? And do vectors work just like arrays in C++ for everything else I want to do with it?

score 3 · Accepted Answer · answered Jul 18 '12 at 07:07

3

Your options are these:

use std::vector or
use plain old array

With vector (recommended):

class YourSet {
public:
   YourSet(int size) : elements(size)
   {
   }
private:
   std::vector<int> elements;
};

With array (not really recommended because of possible exception safety issues):

class YourSet {
public:
   YourSet(int size)
   {
       elements = new int[size];
   }
   ~YourSet()
   {
        delete[] elements;
   }
private:
   int* elements;
   // no implementation to prevent shallow copying
   YourSet( const YourSet&);
   void operator=(const YourSet&);
};

answered Jul 18 '12 at 07:07

sharptooth

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I would prefer to use just the array. How would I do that? – Taylor Matyasz Jul 18 '12 at 07:09
1

@Taylor Matyasz: I included that, but really `vector` is better. – sharptooth Jul 18 '12 at 07:11

score 1 · Answer 2 · answered Jul 18 '12 at 07:07

std::vector will work for your purposes.

#include <vector>

class Set
{
  Set(int len)
   : mElements(len)
  {
  }

  void do_smth()
  {
    size_t vector_len = mElements.size();

    int index = 0;
    int element = mElements[index];
  }


  std::vector<int> mElements;
};

score 1 · Answer 3 · answered Jul 18 '12 at 07:08

Vector holds pointer, and when it`s needed reallocate memory.

T* new_ptr = new T[new_size];
std::copy(ptr, ptr + size, new_ptr);
delete[] ptr;
ptr = new_ptr;
size = new_size;

It is simple example. Vector uses allocator, but at fact something like this.

mathematician1975 · Answer 4 · 2012-07-18T07:17:31.670

GO for a vector. It will resize itself automatically when you add or remove items from it, and you can specify the size of it also if needed.

 vector<myClass> m_vClasses;
 myClass myNewObject;
 m_Classes.push_back(myNewObject); // Vector resized when you add new element

You won't need to worry about memory or other issues (unless myClass is a pointer that points to dynamically allocated memory) assuming that your myClass destructor correctly frees any memory that the class allocates. The vector will deal with the resizing for you automatically.

How do I dynamically assign the size to an array in C++?

4 Answers4