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80 lines
2.8 KiB
Markdown
80 lines
2.8 KiB
Markdown
## Scope and safety
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This tutorial shows some of the safety features of Better Enums: scope, how to
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control conversions, and the lack of a default constructor.
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On balance, Better Enums are in one way less type-safe than enum class, and in
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another way more type-safe. The first difference in safety is the presence of
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implicit conversion to integral types. The second difference is the lack of a
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default constructor. Both of these can be toggled, so you can make Better Enums
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strictly safer than enum class, or just as safe.
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$internal_toc
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### Scope
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You have probably noticed by now that Better Enums are scoped: when you declare
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#include <cassert>
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<em>#include <enum.h></em>
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<em>BETTER_ENUM</em>(<em>Channel</em>, <em>int</em>, <em>Red</em> = <em>1</em>, <em>Green</em>, <em>Blue</em>)
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you don't get names such as `Red` in the global namespace. Instead, you get
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`Channel`, and `Red` is accessible as `Channel::Red`. This is no big deal in
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$cxx11, which has `enum class`. In $cxx98, however, this typically requires
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effort. Better Enums brings scope uniformly to both variants. So, despite the
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above declaration, you can safely declare
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<em>BETTER_ENUM</em>(<em>Node</em>, <em>char</em>, <em>Red</em>, <em>Black</em>)
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and everything will work as expected.
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int main()
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{
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assert((+<em>Channel::Red</em>)._to_integral() <em>!=</em> (+<em>Node::Red</em>)._to_integral());
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### Implicit conversion
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A major complaint in $cxx98 is that `enums` are implicitly convertible to
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integers. Unfortunately, that is also true of Better Enums, and I haven't found
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a way to forbid the conversions and still have switch case checking.
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Better Enums can be made as safe as `enum class` in $cxx11, however. If your
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compiler supports `enum class` and you define
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`BETTER_ENUMS_STRICT_CONVERSION` before including `enum.h`, the following code
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will not compile:
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~~~comment
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Channel channel = Channel::Red;
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int n = channel;
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~~~
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The reason this is not enabled by default is explained in the reference page on
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[strict conversions](${prefix}OptInFeatures.html#StrictConversions).
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You can conveniently define the macro on your compiler's command line, or by
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creating a little header file that defines it, and then includes
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<code>enum.h</code>. You can then include this new header file in your project
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everywhere where you would have included <code>enum.h</code>.
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### Default constructor
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Better Enums generate without a default constructor. The purpose is to support
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the convention where if a Better Enum exists, then it has a valid value. So, if
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you uncomment this code, the program won't compile:
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~~~comment
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Channel channel;
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~~~
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If this is too strict for your project, you can relax it as described
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[here](${prefix}OptInFeatures.html#DefaultConstructors).
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---
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return 0;
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}
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%% description = Better Enums type safety features and limitations.
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