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GCC 5.2 is a "bugfix" release over GCC 5.1 - not sure why the GCC team updated the minor version number. Package managers seem to have dropped GCC 5.1 and replaced it with 5.2, and I'm guessing usage of 5.1 is discouraged. The testing code has been updated accordingly. Travis is still distributing GCC 5.1 as of the time of this writing.
78 lines
3.0 KiB
Markdown
78 lines
3.0 KiB
Markdown
## Maps
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It is possible to create `constexpr` bidirectional maps between Better Enums and
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any type. This is currently an experimental feature. Feedback is very much
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wanted, but please don't build any mission-critical code on top of this :)
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The way it works is you give Better Enums a function — say,
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`const char* describe(Channel)`. The library enumerates it to make a map.
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The reason for using a function is that a `switch` statement is, I believe, the
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only place where a compiler will check for exhaustiveness. If you forget to
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create a case for one of the enum's constants, the compiler can let you know.
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Obviously, a `switch` statement is not data, and needs to be inside a function.
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It can only be inside a `constexpr` function in $cxx14, so this feature is most
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natural in $cxx14. When you pass the function to Better Enums, the library can
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build up a lookup data structure at compile time.
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Actually, right now, Better Enums doesn't quite do that — it enumerates
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the function *every* time you want to convert to an enum (but not *from* an
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enum). It simply does a linear scan every time. This is because I haven't yet
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found a data structure whose compile-time generation is fast enough for
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practical use.
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---
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#include <iostream>
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<em>#include</em> <<em>enum.h</em>>
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<em>ENUM</em>(<em>Channel</em>, <em>int</em>, <em>Red</em>, <em>Green</em>, <em>Blue</em>)
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We will create a map from this function:
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constexpr <em>const char* describe</em>(<em>Channel channel</em>)
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{
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switch(<em>channel</em>) {
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case <em>Channel::Red</em>: return <em>"the red channel"</em>;
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case <em>Channel::Green</em>: return <em>"the green channel"</em>;
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case <em>Channel::Blue</em>: return <em>"the blue channel"</em>;
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}
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return "needed for gcc 5";
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}
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Here is the map. The actual type is `better_enums::map<Channel, const char*>`.
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constexpr auto <em>descriptions</em> = <em>better_enums::make_map</em>(<em>describe</em>);
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And the usage:
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int main()
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{
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std::cout << <em>descriptions[Channel::Red]</em> << std::endl;
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std::cout << <em>descriptions</em>.<em>from_enum</em>(<em>Channel::Red</em>) << std::endl;
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std::cout << <em>descriptions</em>.<em>to_enum</em>(<em>"the green channel"</em>) << std::endl;
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auto <em>not_a_literal</em> = <em>std::string</em>(<em>"the blue channel"</em>);
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std::cout << <em>descriptions</em>.<em>to_enum</em>(<em>not_a_literal</em>.<em>c_str()</em>) << std::endl;
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return 0;
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}
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---
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`make_map` above produces a value of type `better_enums::map<E, T>`. The full
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signature of the template `better_enums::map` is
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~~~comment
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<em>template <typename Enum, typename T, typename Compare = map_compare<T>></em>
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~~~
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`Compare` has to be a class with a static member function
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`bool less(const T&, const T&)`. The default implementation
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`better_enums::map_compare` simply applies `operator <`, except when `T` is
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`const char*`. In that case, it does lexicographic comparison.
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%% description = Mapping enums to arbitrary types and vice versa.
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