error C2893: Failed to specialize function template

Question

I am getting this error and another error too ** "IntelliSense: no instance of function template matches the argument list"** when compiling the following code

I know there might be logic mistakes in my function but I need to solve this error first to be able to debug my function .

#include <iostream>
using namespace std;

template<class T>
T myMax (T& arr ,int arrStart ,int arrSize)
{
    if(arrStart==arrSize-1)
        return arr[arrSize];

    int median = (arrStart+arrSize)/2 ;
    T left , right , maximum ;

    left = max(arr,arrStart , median);
    right = max(arr , median+1 , arrSize-1) ;

    if (left>right)
        maximum = left;
    else
        maximum = right ;

    return maximum ;

}

void main()
{

    int arrSize = 5;
    int arr[] = {1,3,4,6,22};

    int x;
    x = myMax(arr,0,arrSize);

}

Answer 1

The argument for parameter arr is of type int[5]. Since you didn't specify a template argument for T when calling myMax, argument deduction happens and T is deduced to be int[5].

Then the compiler attempts to specialize the function template with T = int[5] (i.e., it tries to replace all instances of T with int[5]). This fails because the function returns a T by value but it is not possible to return an array (like int[5]) by value.

It looks like you want T to be the element type. If that is the case, you can explicitly take a reference to the array, e.g.,

template<class T, unsigned N>
T myMax (T (&arr)[N])

Though, a more idiomatic way to write the function would be to have it take a pair of random access iterators and have it return an iterator pointing to the "max" element:

template <typename RandomAccessIt>
RandomAccessIt myMax (RandomAccessIt first, RandomAccessIt last)

first is an iterator to the first element in the range and last is an iterator to one-past-the-end of the range, as is idiomatic for the STL algorithms. Pointers are usable as random access iterators, so this function can be called as

int* pointerToMaxElement = myMax(arr, arr + arrSize);

The advantage of the iterator approach is that it works with any random access range, including an array, std::vector, std::array, and std::deque.