Passing pointer to an array into a function (C++) - c++

Question

I'm trying to pass an array into my function calls for build_max_heap and max_heapify so I can modify the array after each call, but I receive an error saying "candidate function not viable: no known conversion from 'int [9]' to 'int *&'for 1st argument."
#include <iostream>
#include <string>
using namespace std;
void build_max_heap(int*& array, int size);
void max_heapify(int*& array, int size, int index);
void build_max_heap(int*& array, int size)
{
for(int i = size/2; i>=0; i--)
{
max_heapify(array, i);
}
}
void max_heapify(int*& array, int size, int index)
{
int leftChild = 2*index+1;
int rightChild = 2*index+2;
int largest;
int heap_size = size;
if( leftChild <= heap_size && array[leftChild] > array[index])
largest = leftChild;
else
largest = index;
if(rightChild <= heap_size && array[rightChild] > array[largest])
largest = rightChild;
if(largest != index) {
int tempArray = array[index];
array[index] = array[largest];
array[largest] = tempArray;
max_heapify(array, heap_size, largest);
}
}
int main()
{
int array[]={5,3,17,10,84,19,6,22,9};
int size = sizeof(array)/sizeof(array[0]);
build_max_heap(array, size);
return 0;
}

Answer 1

int array[]={5,3,17,10,84,19,6,22,9};
While array can be decayed to a pointer int* to be passed as a function argument, it the pointer cannot be passed as a "non-const reference" int*&, because it is immutable (it is a constant address). You could have passed it as a const reference like this:
void max_heapify(int* const& array, int size, int index)
// ^^^^^^
However, this doesn't make much sense, you can simply pass the pointer by value (a copy of the address of the array), which results in the same: the variable at the caller wont be changed. The usual use case of const& parameters is to pass objects that are expensive to copy, such as std::string. This does not apply to pointers; making a copy of a pointer is as cheap as copying any basic variable.
You should change your functions to take the pointer by value:
void build_max_heap(int* array, int size)
void max_heapify(int* array, int size, int index)
also, correct the call to max_heapify inside build_max_heap, give it the correct number of arguments:
void build_max_heap(int* array, int size)
{
for(int i = size/2; i>=0; i--)
{
max_heapify(array, size, i); // <-- 3 arguments
}
}