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?

sharptooth · Accepted Answer · 2012-07-18 07:07:21Z

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&); };

@Taylor Matyasz: I included that, but really vector is better.

inkooboo · Accepted Answer · 2012-07-18 07:07:47Z

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; };

ForEveR · Accepted Answer · 2012-07-18 07:08:02Z

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 · Accepted Answer · 2012-07-18 07:17:31Z

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.

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