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129 lines
4.1 KiB
Markdown
129 lines
4.1 KiB
Markdown
## Special values
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Suppose your project has a convention where each enum has special *invalid* and
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*default* values. With Better Enums, you can encode that directly at compile
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time, and then access each enum's special values using syntax like
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`Channel c = default_` and `Channel c = invalid`. This can make your code adapt
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automatically to changes in enum definitions, as well as make it easier to read
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and understand its intent.
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---
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#include <iostream>
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#include <stdexcept>
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<em>#include <enum.h></em>
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### Invalid
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Perhaps the invalid value is usually called `Invalid`, but not in all enums. You
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can encode that using a function template for the common case, and a macro that
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creates specializations:
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<em>template</em> <<em>typename Enum</em>>
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constexpr Enum find_invalid() { return Enum::Invalid; }
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<em>#define OVERRIDE_INVALID</em>(<em>Enum</em>, <em>Value</em>) \
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template<> \
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constexpr Enum <em>find_invalid<Enum></em>() { return <em>Enum::Value</em>; }
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Now, you can declare enums like these:
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ENUM(<em>Channel</em>, int, Red, Green, Blue, <em>Invalid</em>)
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ENUM(<em>Compression</em>, int, <em>Undefined</em>, None, Huffman)
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<em>OVERRIDE_INVALID</em>(<em>Compression</em>, <em>Undefined</em>)
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and use them:
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static_assert(<em>find_invalid</em><<em>Channel</em>>() == <em>+Channel::Invalid</em>, "");
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static_assert(<em>find_invalid</em><<em>Compression</em>>() == <em>+Compression::Undefined</em>, "");
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This even supports enums that don't have an invalid value at all. As long as
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they don't have a constant called `Invalid`, you will get a compile-time error
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if you try to call `invalid()` on them — as you probably should!
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### Default
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To encode the policy on default values, we need to do a compile-time check that
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the first value is not invalid. Otherwise, the technique is the same.
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template <typename Enum>
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constexpr Enum find_default()
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{
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return
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Enum::_size < 2 ?
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throw std::logic_error("enum has no valid constants") :
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Enum::_values()[0] == find_invalid<Enum>() ?
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Enum::_values()[1] :
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Enum::_values()[0];
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}
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#define OVERRIDE_DEFAULT(Enum, Value) \
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static_assert(Enum::Value != Enum::Invalid, \
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#Enum ": default cannot equal invalid"); \
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template<> \
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constexpr Enum find_default<Enum>() { return Enum::Value; }
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Usage:
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static_assert(find_default<Channel>() == +Channel::Red, "");
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static_assert(find_default<Compression>() == +Compression::None, "");
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And, if you do
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ENUM(Answer, int, Yes, No, Invalid)
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// OVERRIDE_DEFAULT(Answer, Invalid)
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you will get a helpful compile-time error saying
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`Answer: default cannot equal invalid`.
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### Making the syntax nicer
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For the final touch, we will make the syntax better by introducing new
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"keywords" called `default_` and `invalid` in such a way that we cause the
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compiler to do type inference:
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template <typename Enum>
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struct assert_enum {
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using check = typename Enum::_enumerated;
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using type = Enum;
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};
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struct invalid_t {
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template <typename To>
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constexpr operator To() const { return find_invalid<To>(); }
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template <typename To>
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constexpr To convert() const { return find_invalid<To>(); }
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};
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struct default_t {
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template <typename To>
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constexpr operator To() const { return find_default<To>(); }
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};
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constexpr invalid_t invalid{};
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constexpr default_t default_{};
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static_assert(+Channel::Invalid == invalid, "");
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static_assert(+Compression::Undefined == invalid, "");
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static_assert(+Channel::Red == default_, "");
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static_assert(+Compression::None == default_, "");
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We can now have nice code such as this:
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int main()
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{
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Channel channel = default_;
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std::cout << channel._to_string() << std::endl;
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return 0;
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}
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---
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There are many possible variations of these policies, but I think most of them
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can be encoded in a reasonable fashion using the tools Better Enums provides.
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Enjoy!
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