C++中如何很好的实现泛型排序

Question

I'm trying to brush up on my C++ templating, and I'm running into the following issue. I'm implementing some basic in-place sorting methods, which I want to work for various types of data containers that can be indexed, and the elements of which can be compared. Specifically, the methods should work for both plain arrays, std::array , std::vector , etc. For some methods this is rather straightforward, like insertion sort:

template<typename T>
void insertion_sort(T& data)
{
    if (std::size(data) < 2)
        return;

    for (int j = 1; j < std::size(data); ++j)
    {
        int i = j;
        while (i > 0 && data[i - 1] > data[i])
        {
            swap_index(data, i, i - 1); // basic helper that swaps data at two indices
            --i;
        }
    }
}

However, for some methods, I also need to know the actual type of the elements stored in data . An example is merge sort: I need to allocate some scratch space to be used for copying stuff around. I tried something like this:

template<typename T>
void mergesort(T& data)
{
    typedef decltype(*std::begin(data)) inner_type;
    inner_type* scratch = new inner_type[std::size(data)];
    ...

but that just gives me errors like "error C2528: 'scratch': pointer to reference is illegal".

In C# I'd just use something like where T: IComparable<T> , and have a parameter of type IList<T> , which seems to be closer to what I want to achieve: have a type T that is comparable, and as parameter some indexable collection of Ts. What is the "proper" way to achieve this with C++ templates? Or should I use something other than templates here? I want to sort the container in place, so I don't think I want to use something like iterators.

Answer 1

The clue is in the error message inner_type is a reference not the value type you are looking for. The following works for me (C++14 is required).

#include <type_traits>

typedef std::remove_reference_t<decltype(*std::begin(data))> inner_type;