如何制作指向模板函数的指针？

Question

I have a template sort function that takes pointer to another function as a paramater to define sorting rule.

template<typename T>
void sort(T* arr, int size, function<bool(T, T)>& comparer);

and i want to call this function with lambda expression

sort(arr, size, [](int num1, int num2) { if (num1 > num2) { return true; } else { return false; } });

However i had an error:

Error (active) E0304 no instance of overloaded function "sort" matches the argument list

Is there a way to create pointer to template function? How can i do this?

Answer 1

Template argument deduction will try to deduce T from both the first parameter T* arr and the third one function<bool(T, T)>& comparer .

This will fail, because the lambda doesn't actually have type function<bool(T, T)> for any T . (It can only be converted to one.)

In function templates function objects are usually taken directly by a template parameter, not as std::function :

template<typename T, typename F>
void sort(T* arr, int size, F comparer);

or

template<typename T, typename F>
void sort(T* arr, int size, F&& comparer);

depending on whether you want a copy of the function object or just a reference.

Answer 2

1. You probably want to take a const reference.
2. You have convert to std::function explicitly.

#include <functional>

template <typename T>
void sort(T* arr, int size, std::function<bool(T, T)> const & comparer)
{
        // sort
}

int main()
{
    int const size = 10;
    int arr[size];
    sort(arr, size, std::function{[](int num1, int num2) {
        if (num1 > num2) {
            return true;
        } else {
            return false;
        }
    }});
}

If you can change the signature of sort , see walnut's answer .