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78 lines
3.1 KiB
Markdown
78 lines
3.1 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>BETTER_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*` or `const wchar_t*`. In that case, it does lexicographic comparison.
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%% description = Mapping enums to arbitrary types and vice versa.
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