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Blog Archive for / 2019 / 02 /

Begin and End with range-based for loops

Saturday, 23 February 2019

On slack the other day, someone mentioned that lots of companies don't use range-based for loops in their code because they use PascalCase identifiers, and their containers thus have Begin and End member functions rather than the expected begin and end member functions.

Having recently worked in a codebase where this was the case, I thought it would be nice to provide a solution to this problem.

The natural solution would be to provide global overloads of the begin and end functions: these are always checked by range-based for if the member functions begin() and end() are not found. However, when defining global function templates, you need to be sure that they are not too greedy: you don't want them to cause ambiguity in overload resolution or be picked in preference to std::begin or std::end.

My first thought was to jump through metaprogramming hoops checking for Begin() and End() members that return iterators, but then I thought that seemed complicated, so looked for something simpler to start with.

The simplest possible solution is just to declare the functions the same way that std::begin() and std::end() are declared:

template <class C> constexpr auto begin(C &c) -> decltype(c.Begin()) {
    return c.Begin();
}
template <class C> constexpr auto begin(const C &c) -> decltype(c.Begin()) {
    return c.Begin();
}

template <class C> constexpr auto end(C &c) -> decltype(c.End()) {
    return c.End();
}
template <class C> constexpr auto end(const C &c) -> decltype(c.End()) {
    return c.End();
}

Initially I thought that this would be too greedy, and cause problems, but it turns out this is fine.

The use of decltype(c.Begin()) triggers SFINAE, so only types which have a public member named Begin which can be invoked with empty parentheses are considered; for anything else these functions are just discarded and not considered for overload resolution.

The only way this is likely to be a problem is if the user has also defined a begin free function template for a class that has a suitable Begin member, in which case this would potentially introduce overload resolution ambiguity. However, this seems really unlikely in practice: most such function templates will end up being a better match, and any non-template functions are almost certainly a better match.

So there you have it: in this case, the simplest solution really is good enough! Just include this header and you're can freely use range-based for loops with containers that use Begin() and End() instead of begin() and end().

Posted by Anthony Williams
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ACCU 2019 presentation and book signing

Thursday, 21 February 2019

The ACCU 2019 conference is running from 9th-13 April 2019, in Bristol, UK.

This year I will be presenting "Here's my number; call me, maybe. Callbacks in a multithreaded world" on 11th April. The abstract is:

A common pattern in multithreaded applications is the use of callbacks, continuations and task pipelines to divide the processing of data across threads. This has the benefit of ensuring that threads can quickly move on to further processing, and can minimize blocking waits, since tasks are only scheduled when there is work to be done.

The downside is that they can weave a tangled web of connections, and managing object lifetimes can now become complicated.

This presentation will look at ways of managing this complexity and ensuring that your code is as clear as possible, and there is no possibility of dangling references or leaked objects.

I will also be signing copies of the second edition of my book C++ Concurrency In Action now that it is finally in print.

I look forward to seeing you there!

Posted by Anthony Williams
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