c++ - What is the purpose of the const overloads of std::begin and std::end?

Question

For std::begin, we have two overloads for containers:

template< class C > 
auto begin( C& c ) -> decltype(c.begin());
template< class C > 
auto begin( const C& c ) -> decltype(c.begin());

But the constness of C can be deduced by the usual template deduction rules, so it seems like the second overload is redundant. What am I missing?

Answer 1

It's reasonable to call begin (and end, for that matter) on an rvalue, provided we don't use the resulting iterator after the container has been destroyed. However, passing an rvalue to a parameter of the form T& will not work, which is where the second overload comes into play.

However, it may well be that we're dealing with a thoughtless transformation of a former range-based for proposal wording:

Add the following to the end of [container.concepts.member]:

template<Container C> concept_map Range<C> {
    typedef C::iterator iterator;
    iterator begin( C& c ) { return Container<C>::begin(c); }
    iterator end( C& c )   { return Container<C>::end(c); } };

template<Container C> concept_map Range<const C> {
    typedef C::const_iterator iterator;
    iterator begin( const C& c ) { return Container<C>::begin(c); }
    iterator end( const C& c )   { return Container<C>::end(c); } };

When it became clear that concepts weren't going to make it into C++11, papers were amended, and all four function temploids were presumably translated into equivalent namespace-scope function templates. This had the (potentially unintended) consequence of rvalues being accepted, while the original code was simply intended to distinguish between differently qualified container types.

Note that a modern implementation of begin/end would use forwarding references instead - e.g.

template <typename T>
constexpr auto begin(T&& t)
  -> decltype(std::forward<T>(t).begin()) {
    return    std::forward<T>(t).begin();
}