Welcome to OStack Knowledge Sharing Community for programmer and developer-Open, Learning and Share
Welcome To Ask or Share your Answers For Others

Categories

0 votes
293 views
in Technique[技术] by (71.8m points)

c++ - 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?

See Question&Answers more detail:os

与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…
Welcome To Ask or Share your Answers For Others

1 Answer

0 votes
by (71.8m points)

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();
}

与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…
Welcome to OStack Knowledge Sharing Community for programmer and developer-Open, Learning and Share
Click Here to Ask a Question

...