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POShould custom containers have free begin/end functions?
 

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    <p>When creating a custom container class that plays by the usual rules (i.e. works with STL algorithms, works with well-behaved generic code, etc.), in C++03 it was sufficient to implement iterator support and member begin/end functions.</p> <p>C++11 introduces two new concepts - range-based for loop and std::begin/end. Range-based for loop understands member begin/end functions, so any C++03 containers support range-based for out of the box. For algorithms the recommended way (according to 'Writing modern C++ code' by Herb Sutter) is to use std::begin instead of member function.</p> <p>However, at this point I have to ask - is the recommended way to call a fully qualified begin() function (i.e. std::begin(c)) or to rely on ADL and call begin(c)?</p> <p>ADL seems useless in this particular case - since std::begin(c) delegates to c.begin() if possible, usual ADL benefits do not seem to apply. And if everybody starts to rely on ADL, all custom containers have to implement extra begin()/end() free functions in their requisite namespaces. However, several sources seem to imply that unqualified calls to begin/end are the recommended way (i.e. <a href="https://svn.boost.org/trac/boost/ticket/6357" rel="noreferrer">https://svn.boost.org/trac/boost/ticket/6357</a>).</p> <p>So what is the C++11 way? Should container library authors write extra begin/end functions for their classes to support unqualified begin/end calls in absence of using namespace std; or using std::begin;?</p>
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    1. CONote the Standard prefers member functions in the range-based for loop; if they cannot be found or the range-init is not of array or class type, the unqualified `begin` and `end` functions are used. The name lookup is explicitly mentioned in [stmt.ranged]/1: "`begin` and `end` are looked up with argument-dependent lookup (3.4.2). For the purposes of this name lookup, namespace `std` is an associated namespace."
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    2. COYes, but if the class does have begin/end member, then the lookup is not performed, right? So - correct me if I'm wrong - from the standard point of view ADL only applies for objects that do not look like standard containers, so it can't be used as a guidance in my case.
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    3. COAddendum: Note that the Standard for the range-based for statement does *not* use unqualified name lookup, but explicitly *argument-dependent* lookup. I tested this with clang++3.2 to support my interpretation: Global `begin`/`end` functions are not found for the range-based for statement if the type of the range-init expression is a class type that's declared in a namespace. I don't know how you could emulate that with your own code.
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