Heap implementation OII

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

/**
    Implementation of an heap
**/
vector <int> heap;

void insert_key ( int element );
int father ( int i );
int left_children ( int i );
void decrease_key( int i, int val );
int right_children ( int i );
void print_heap ( void );
void heapify ( int i );

int main ( void )
{
    insert_key ( 10 );
    print_heap();
    insert_key ( 20 );
    print_heap();
    insert_key ( 40 );
    print_heap();
    insert_key ( 30 );
    print_heap();
    insert_key ( 15 );
    print_heap();
    insert_key ( 50 );
    print_heap();
    insert_key ( 100 );
    print_heap();

    return 0;
}

void heapify ( int i ) {
    int highest = i;
    int left = left_children( i );
    int right = right_children( i );

    if ( left < heap.size() && heap [ left ] > heap [ highest ] )
        highest = left;

    if ( right < heap.size() && heap [ right ] > heap [ highest ] )
        highest = right;

    /// Current local max = heap [ highest ]
    if ( highest == i )
        return;

    swap ( heap [ i ], heap [ highest ] );
    heapify( highest );
}

void decrease_key( int i, int val ) {
    /// Only decrease allowed
    if ( val >= heap [ i ] ) return;

    heap [ i ] = val;
    heapify ( i );
}

void insert_key ( int element ) {
    int element_position = heap.size();
    heap.push_back( element );

    while ( element_position != 0 && ( heap [ element_position ] > heap [ father ( element_position ) ] ) ) {
        swap ( heap [ element_position ],  heap [ father ( element_position ) ] );
        element_position = father ( element_position );
    }
}

int father ( int i ) { return i / 2; }
int left_children ( int i ) { return 2 * i; }
int right_children ( int i ) { return 2 * i + 1; }

void print_heap ( void ) {
    cout << endl;
    for ( x : heap ) cout << x << " ";
    cout << endl;
}