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); }

Some other comments in addition to those in the answers: The return type of main must be int. arr[arrSize]; is incorrect because it accesses an element that doesn't exist (arr[arrSize] is one past the end of the array). Since the function doesn't modify arr, if you do choose to take the array by reference, it should be a const reference. — James McNellis
– James McNellis, Commented Apr 20, 2011 at 16:06

James McNellis · Accepted Answer · 2011-04-20 15:57:43Z

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.

Yeah , I got it . So How do I correct it to be able to return the max element.
@Mark: It looks like the OP is trying to implement a divide-and-conquer max_element, which might be a good idea if the algorithm was parallelized, but for a non-parallel implementation doesn't make much sense (and max_element would be the right choice).
I am just trying to practice recursion and template functions.

mrusinak · Accepted Answer · 2011-04-20 16:00:19Z

From a quick look, the two things that jump out at me are:

You're using T in different ways in the template function. You're returning a T object, and taking a reference to a T object as an argument - but when you use it, you're passing an an int array as the argument but expect just an int returned
You don't call your template function with any template (ie, myMax<int>(...)) Edit - as Mark B points out, this isn't required however

Thanks, my work with templates has been very limited so I miss things like that

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