是否可以（以及如何）为C ++模板功能参数添加类型协定检查？

Question

For example, I want to implement my own generic sort function, I want to require the type that would be passed in to be Indexable , and the element inside would be Comparable

template <typename Cont>
    **type_check: Cont is Indexable, Cont::Element is Comparable**
void my_sort(Cont& cont){
    // do sorting work
}

When I do my_sort(vector<int>{1,6,5,4}) would be okay

But when do my_sort(linkedlist<int>{1,6,5,4}) would fail me at Compile/Runtime, because linkedlist is not Indexable .

So is there a way to do such kind of type contract programming?

PS I am in C++ 11 environment, but any Solution in later version of C++ is also welcomed

Answer 1

With SFINAE you can do something like:

template <typename Cont>
auto my_sort(Cont& cont)
-> decltype(cont[42], // Indexable
            void(), // That void to avoid evil overload of operator comma
            std::declval<Cont::Element>() < std::declval<Cont::Element>(), // Comparable
            void()) // That final void for the return type of sort
{
    // do sorting work
}

std::enable_if is an alternative (to the decltype ) if you have the traits ready.

Answer 2

As mentioned in the comments, once the Concepts TS makes its way into the C++ standard, you'll be able to do this with something like:

template <typename T>
concept Sortable = requires(T t) {
    { t[0] < t[0] } -> bool
};

template <Sortable Cont>
my_sort(Cont& cont) {
    // do sorting work
}

Live Demo

An older version of the Concepts TS is implemented by GCC with the -fconcepts flag, but it won't be in the standard until C++20. Until then, you can make do with SFINAE tricks:

template <typename Cont>
std::enable_if_t<std::is_convertible_v<decltype(std::declval<Cont>()[0] < std::declval<Cont>()[0]), bool>>
my_sort(Cont& cont) {
    // ...
}

This will fail to compile if, for a given Cont c , c[0] < c[0] is either not valid or not convertible to bool .
Live Demo