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<title>Two phase construction - Boost.Outcome documentation</title>
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  <div class="titlepage"><div><div><h1 style="clear: both">Two phase construction</h1></div></div></div>
  <p>The first thing to do is to break your object&rsquo;s construction into two phases:</p>

<ol>
<li><p>Place the object into a state where it can be legally destructed
without doing any initialisation which could throw an exception (i.e. everything
done in phase 1 is <code>constexpr</code>). This phase usually involves initialising member
variables to various default values, most often using default member initialisers.
Most standard C++ library objects
and containers have <code>constexpr</code> constructors, and thus can be initialised
during phase 1. If you need to initialise a member variable without
a <code>constexpr</code> constructor, 
<a href="https://en.cppreference.com/w/cpp/utility/optional" class="api-reference" target="_blank"><i class="fa fa-book" aria-hidden="true"></i> <code>std::optional&lt;T&gt;</code></a>
 is the usual workaround.</p></li>

<li><p>Do the remainder of the construction, the parts which could fail.
Because phase 1 placed the object into a legally destructible state,
it is usually the case that one can bail out during phase 2 and the
destructor will clean things up correctly.</p></li>
</ol>

<p>The phase 1 construction will be placed into a <em>private</em> <code>constexpr</code>
constructor so nobody external can call it. The phase 2 construction will be placed into a static
member initialisation function which returns a <code>result</code> with either
the successfully constructed object, or the cause of any failure to
construct the object.</p>

<p>Finally, as a phase 3,
some simple metaprogramming will implement a <code>make&lt;T&gt;{Args...}()</code>
free function which will construct any object <code>T</code> by calling its
static initialisation function with <code>Args...</code> and returning the
<code>result</code> returned. This isn&rsquo;t as nice as calling <code>T(Args...)</code> directly,
but it&rsquo;s not too bad in practice. And more importantly, it lets you
write generic code which can construct any unknown object which
fails via returning <code>result</code>, completely avoiding C++ exception throws.</p>


        </div><p><small>Last revised: February 08, 2019 at 22:18:08 UTC</small></p>
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