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<span class="heading-boost">Boost</span>
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<q>...one of the most highly
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regarded and expertly designed C++ library projects in the
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world.</q> <span class="heading-attribution">— <a href=
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"http://www.gotw.ca/" class="external">Herb Sutter</a> and <a href=
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<div class="titlepage"><div><div><h1 style="clear: both">Two phase construction</h1></div></div></div>
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<p>The first thing to do is to break your object’s construction into two phases:</p>
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<ol>
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<li><p>Place the object into a state where it can be legally destructed
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without doing any initialisation which could throw an exception (i.e. everything
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done in phase 1 is <code>constexpr</code>). This phase usually involves initialising member
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variables to various default values, most often using default member initialisers.
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Most standard C++ library objects
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and containers have <code>constexpr</code> constructors, and thus can be initialised
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during phase 1. If you need to initialise a member variable without
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a <code>constexpr</code> constructor, <code>std::optional<T></code> is the usual workaround.</p></li>
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<li><p>Do the remainder of the construction, the parts which could fail.
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Because phase 1 placed the object into a legally destructible state,
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it is usually the case that one can bail out during phase 2 and the
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destructor will clean things up correctly.</p></li>
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</ol>
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<p>The phase 1 construction will be placed into a <em>private</em> <code>constexpr</code>
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constructor so nobody external can call it. The phase 2 construction will be placed into a static
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member initialisation function which returns a <code>result</code> with either
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the successfully constructed object, or the cause of any failure to
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construct the object.</p>
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<p>Finally, as a phase 3,
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some simple metaprogramming will implement a <code>construct<T>{Args...}()</code>
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free function which will construct any object <code>T</code> by calling its
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static initialisation function with <code>Args...</code> and returning the
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<code>result</code> returned. This isn’t as nice as calling <code>T(Args...)</code> directly,
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but it’s not too bad in practice. And more importantly, it lets you
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write generic code which can construct any unknown object which
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fails via returning <code>result</code>, completely avoiding C++ exception throws.</p>
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</div><p><small>Last revised: December 12, 2017 at 10:02:44 UTC</small></p>
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