How to add element to C++ array?

You don't have to use vectors. If you want to stick with plain arrays, you can do something like this:

int arr[] = new int[15];
unsigned int arr_length = 0;

Now, if you want to add an element to the end of the array, you can do this:

if (arr_length < 15) {
  arr[arr_length++] = <number>;
} else {
  // Handle a full array.
}

It's not as short and graceful as the PHP equivalent, but it accomplishes what you were attempting to do. To allow you to easily change the size of the array in the future, you can use a #define.

#define ARRAY_MAX 15

int arr[] = new int[ARRAY_MAX];
unsigned int arr_length = 0;

if (arr_length < ARRAY_MAX) {
  arr[arr_length++] = <number>;
} else {
  // Handle a full array.
}

This makes it much easier to manage the array in the future. By changing 15 to 100, the array size will be changed properly in the whole program. Note that you will have to set the array to the maximum expected size, as you can't change it once the program is compiled. For example, if you have an array of size 100, you could never insert 101 elements.

If you will be using elements off the end of the array, you can do this:

if (arr_length > 0) {
  int value = arr[arr_length--];
} else {
  // Handle empty array.
}

If you want to be able to delete elements off the beginning, (ie a FIFO), the solution becomes more complicated. You need a beginning and end index as well.

#define ARRAY_MAX 15

int arr[] = new int[ARRAY_MAX];
unsigned int arr_length = 0;
unsigned int arr_start = 0;
unsigned int arr_end = 0;

// Insert number at end.
if (arr_length < ARRAY_MAX) {
  arr[arr_end] = <number>;
  arr_end = (arr_end + 1) % ARRAY_MAX;
  arr_length ++;
} else {
  // Handle a full array.
}

// Read number from beginning.
if (arr_length > 0) {
  int value = arr[arr_start];
  arr_start = (arr_start + 1) % ARRAY_MAX;
  arr_length --;
} else {
  // Handle an empty array.
}

// Read number from end.
if (arr_length > 0) {
  int value = arr[arr_end];
  arr_end = (arr_end + ARRAY_MAX - 1) % ARRAY_MAX;
  arr_length --;
} else {
  // Handle an empty array.
}

Here, we are using the modulus operator (%) to cause the indexes to wrap. For example, (99 + 1) % 100 is 0 (a wrapping increment). And (99 + 99) % 100 is 98 (a wrapping decrement). This allows you to avoid if statements and make the code more efficient.

You can also quickly see how helpful the #define is as your code becomes more complex. Unfortunately, even with this solution, you could never insert over 100 items (or whatever maximum you set) in the array. You are also using 100 bytes of memory even if only 1 item is stored in the array.

This is the primary reason why others have recommended vectors. A vector is managed behind the scenes and new memory is allocated as the structure expands. It is still not as efficient as an array in situations where the data size is already known, but for most purposes the performance differences will not be important. There are trade-offs to each approach and it's best to know both.

Arrays in C++ cannot change size at runtime. For that purpose, you should use vector<int> instead.

vector<int> arr;
arr.push_back(1);
arr.push_back(2);

// arr.size() will be the number of elements in the vector at the moment.

As mentioned in the comments, vector is defined in vector header and std namespace. To use it, you should:

#include <vector>

and also, either use std::vector in your code or add

using std::vector; 

or

using namespace std;

after the #include <vector> line.

There is no way to do what you say in C++ with plain arrays. The C++ solution for that is by using the STL library that gives you the std::vector.

You can use a vector in this way:

#include <vector>

std::vector< int > arr;

arr.push_back(1);
arr.push_back(2);
arr.push_back(3);

Use a vector:

#include <vector>

void foo() {
    std::vector <int> v;
    v.push_back( 1 );       // equivalent to v[0] = 1
}