CODEWITHSUNDEEP
c++c++11
Desert Fish
Desert Fish

Reputation: 181

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

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?

Upvotes: 16

Views: 728

Answers (1)

Columbo
Columbo

Reputation: 60999

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();
}

Upvotes: 10

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