错误C2893：无法专门化功能模板

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] (ie, 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, eg,

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 .

Answer 2

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

1. 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
2. You don't call your template function with any template (ie, myMax<int>(...) ) Edit - as Mark B points out, this isn't required however