Question

In the obj-c, we can create vector objects as follows:

SomeClass* example[100];

or

int count[7000];

But what if we know the size of the vector only at the time init the class?
(Maybe we need example[756] or count[15])

Answer 1

First of all, those aren’t vector objects, they’re compile-time arrays. One of the features of compile time arrays is automatic memory management; that is, you don’t have to worry about allocation and deallocation of these arrays.

If you want to create an array whose size you don’t know until runtime, you’ll need to use new[] and delete[]:

int size = somenumber;
int* arr = new int[size];

// use arr
arr[0] = 4;

// print the first value of arr which is 4
cout << arr[0];

The catch is that after you’re done with this array, you have to deallocate it:

delete[] arr;

If you forget to deallocate something created by new with a corresponding delete¹, you’ll create a memory leak.

You are probably better off using std::vector though because it manages memory for you automatically:

// include the header
#include <vector>

using namespace std; // so we don't have std:: everywhere

vector<int> vec; // create a vector for ints
vec.push_back(4); // add some data
vec.push_back(5);
vec.push_back(6);

// vec now holds 4, 5, and 6

cout << vec[0]; // print the first element of vec which is 4

// we can add as many elements to vec as we want without having to use any
// deallocation functions on it like delete[] or anything
// when vec goes out of scope, it will clean up after itself and you won't have any leaks

¹ Make sure you use delete on pointers that you created with new and delete[] on pointers you make with new[x]. Do not mix and match them. Again, if you use std::vector, you don’t have to worry about this.