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156 lines
6.5 KiB
HTML
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<title>Scope and safety - Better Enums</title>
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<meta name="description" content="Better Enums type safety features and limitations." />
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<meta name="author" content="Anton Bachin" />
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<a class="first" href="https://raw.githubusercontent.com/aantron/better-enums/0.11.3/enum.h"
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download>Download</a>
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<a href="https://github.com/aantron/better-enums">GitHub</a>
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<a href="../index.html">Home</a>
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<a href="../tutorial/HelloWorld.html">Tutorial</a>
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<section>
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<h1><a href="../index.html">Better Enums</a></h1>
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<h2>Reflective compile-time enums for <span class="cpp">C++</span></h2>
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<h3>Open-source under the BSD license</h3>
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</section>
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<section class="notes">
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<p>Version 0.11.3</p>
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<p>To install, just add <code>enum.h</code> to your project.</p>
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<p>
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Visit the GitHub repo for issues, feedback, and the latest development.
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</p>
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</section>
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<section class="buttons">
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<a href="https://raw.githubusercontent.com/aantron/better-enums/0.11.3/enum.h"
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download>Download <code>enum.h</code></a>
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<a href="https://github.com/aantron/better-enums">GitHub</a>
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<div class="main">
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<div class="container">
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<p>
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Welcome to the Better Enums tutorials! The code in this tutorial forms a
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valid program, which you can <a href="https://github.com/aantron/better-enums/blob/0.11.3/example/7-safety.cc">download</a> and play with. The
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program runs as part of the automated test suite.
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</p>
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<h2>Scope and safety</h2>
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<p>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.</p>
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<p>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.</p>
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<p><a id="contents"></a><h3 class="contents">Contents</h3><ul class="contents"><li><a href="#Scope">Scope</a></li><li><a href="#ImplicitConversion">Implicit conversion</a></li><li><a href="#DefaultConstructor">Default constructor</a></li></ul></p>
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<a id="Scope"></a><h3>Scope</h3>
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<p>You have probably noticed by now that Better Enums are scoped: when you declare</p>
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<pre>#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>)</pre><p>you don't get names such as <code>Red</code> in the global namespace. Instead, you get
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<code>Channel</code>, and <code>Red</code> is accessible as <code>Channel::Red</code>. This is no big deal in
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<span class="cpp">C++</span><span class="eleven">11</span>, which has <code>enum class</code>. In <span class="cpp">C++</span><span class="eleven">98</span>, 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</p>
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<pre><em>BETTER_ENUM</em>(<em>Node</em>, <em>char</em>, <em>Red</em>, <em>Black</em>)</pre><p>and everything will work as expected.</p>
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<pre>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());</pre><a id="ImplicitConversion"></a><h3>Implicit conversion</h3>
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<p>A major complaint in <span class="cpp">C++</span><span class="eleven">98</span> is that <code>enums</code> 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.</p>
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<p>Better Enums can be made as safe as <code>enum class</code> in <span class="cpp">C++</span><span class="eleven">11</span>, however. If your
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compiler supports <code>enum class</code> and you define
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<code>BETTER_ENUMS_STRICT_CONVERSION</code> before including <code>enum.h</code>, the following code
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will not compile:</p>
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<pre class="comment"> Channel channel = Channel::Red;
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int n = channel;</pre><p>The reason this is not enabled by default is explained in the reference page on
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<a href="../OptInFeatures.html#StrictConversions">strict conversions</a>.</p>
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<p>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>.</p>
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<a id="DefaultConstructor"></a><h3>Default constructor</h3>
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<p>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:</p>
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<pre class="comment"> Channel channel;</pre><p>If this is too strict for your project, you can relax it as described
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<a href="../OptInFeatures.html#DefaultConstructors">here</a>.</p>
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<hr>
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<pre> return 0;
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}</pre>
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<section class="tutorial-footer">
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<p class="next">
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Continue to the next tutorial: <a href="../tutorial/Representation.html">Representation</a>
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</p>
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<p class="up">
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Or, return to the
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<a href="../index.html#Tutorial">tutorial index</a>.
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</p>
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</section>
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</div>
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</div>
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<footer>
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<div class="container">
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Copyright © 2015-2019 Anton Bachin. Released under the BSD 2-clause
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license. See
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<a href="https://github.com/aantron/better-enums/blob/0.11.3/doc/LICENSE">
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LICENSE</a>.
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<br />
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This page is part of the documentation for Better Enums 0.11.3.
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