Size of unknown container

Question

I have always been reading that using this code is not so good:

std::vector<T> my_vector;
...
std::sort(my_vector.begin(), my_vector.end());

It is better to write it as:

std::vector<T> my_vector;
...
std::sort(std::begin(my_vector), std::end(my_vector));

Because std::begin will work with all containers including normal arrays.

What is the alternative to my_vector.size() that will work for all containers including normal arrays?

Answer 1

Actually using std::begin(my_vector) is not the Right Thing! If you want to pick up the customization point you'd rather use

using std::begin;
using std::end;
std::sort(begin(cont), end(cont));

This approach tries to find begin(cont) using ADL and, when failing to locate a suitable versio, fallback to use std::begin.

Sadly, there is no std::size being the default for a customization point like std::begin. It would sort of work to use std::distance():

std::distance(begin(cont), end(cont));

However, for typical node-based containers or, more generally, for non-random access iterators this approach would walk the elements rather then obtaining the size from a stored value. Thus, I'd think you'd want to call cont.size(). It would be relatively straight forward to define a suitable customization point:

namespace util {
    template <typename C>
    typename C::difference_type size(C const& c) {
        return c.size();
    }
    template <typename T, std::size_t N>
    std::size_t size(T const(&)[N]) {
        return N;
    }
}

As was pointed out in comments, a non-member size() function was added to the working paper for C++17 (see the bottom of the synopsis in 24.3 [iterator.synoposis]). N4280 is the paper which proposed the change. This paper also proposes functions empty() and data() which were also added. All of these functions are declared in <iterator>.

The version added to C++17 uses a decltype() on the size() member directly in the return type. In addition it declares the function to be constexpr:

template <typename C>
constexpr auto size(C const& c) -> decltype(c.size()) {
    return c.size();
}