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<h1 class="post-title">Forward declaration in C++</h1>
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<!-- <a href='/'>Tao He</a> -->
<time class="post-date" datetime="2010-11-02">02 Nov 2010</time>
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<p>Why need use forward declaration? when we can use it? I will figure out these problem separately.</p>
<h4 id="why">Why</h4>
<ul>
<li>
<p>Forward-declarations can significantly reduce build times</p>
</li>
<li>
<p>Break cyclic references where two definitions both use each other</p>
</li>
</ul>
<div class="language-c++ highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">File</span> <span class="n">Car</span><span class="p">.</span><span class="n">h</span>
<span class="cp">#include "Wheel.h" // Include Wheel's definition so it can be used in Car.
#include <vector>
</span>
<span class="k">class</span> <span class="nc">Car</span>
<span class="p">{</span>
<span class="n">std</span><span class="o">::</span><span class="n">vector</span><span class="o"><</span><span class="n">Wheel</span><span class="o">></span> <span class="n">wheels</span><span class="p">;</span>
<span class="p">};</span>
<span class="n">File</span> <span class="n">Wheel</span><span class="p">.</span><span class="n">h</span>
<span class="k">class</span> <span class="nc">Car</span><span class="p">;</span> <span class="c1">// forward declaration</span>
<span class="k">class</span> <span class="nc">Wheel</span>
<span class="p">{</span>
<span class="n">Car</span><span class="o">*</span> <span class="n">car</span><span class="p">;</span>
<span class="p">};</span>
</code></pre></div></div>
<h4 id="when">When</h4>
<p>Put yourself in the compiler’s position: when you forward declare a type, all the compiler knows is that this type exists; it knows nothing about its size, members, or methods. This is why it’s called an incomplete type. Therefore, you cannot use the type to declare a member, or a base class, since the compiler would need to know the layout of the type.</p>
<p>Assuming the following forward declaration.</p>
<div class="language-c++ highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">class</span> <span class="nc">X</span><span class="p">;</span>
</code></pre></div></div>
<p>Here’s what you can and cannot do.</p>
<p><strong>What you can do with an incomplete type</strong>:</p>
<ul>
<li>Declare a member to be a pointer or a reference to the incomplete type:</li>
</ul>
<div class="language-c++ highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">class</span> <span class="nc">Foo</span> <span class="p">{</span>
<span class="n">X</span> <span class="o">*</span><span class="n">pt</span><span class="p">;</span>
<span class="n">X</span> <span class="o">&</span><span class="n">pt</span><span class="p">;</span>
<span class="p">};</span>
</code></pre></div></div>
<ul>
<li>Declare functions or methods which accept/return incomplete types:</li>
</ul>
<div class="language-c++ highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kt">void</span> <span class="nf">f1</span><span class="p">(</span><span class="n">X</span><span class="p">);</span>
<span class="n">X</span> <span class="nf">f2</span><span class="p">();</span>
</code></pre></div></div>
<ul>
<li>Define functions or methods which accept/return pointers/references to the incomplete type (but without using its members):</li>
</ul>
<div class="language-c++ highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kt">void</span> <span class="nf">f3</span><span class="p">(</span><span class="n">X</span><span class="o">*</span><span class="p">,</span> <span class="n">X</span><span class="o">&</span><span class="p">)</span> <span class="p">{}</span>
<span class="n">X</span><span class="o">&</span> <span class="n">f4</span><span class="p">()</span> <span class="p">{}</span>
<span class="n">X</span><span class="o">*</span> <span class="nf">f5</span><span class="p">()</span> <span class="p">{}</span>
</code></pre></div></div>
<p><strong>What you cannot do with an incomplete type</strong>:</p>
<ul>
<li>Use it as a base class</li>
</ul>
<div class="language-c++ highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">class</span> <span class="nc">Foo</span> <span class="o">:</span> <span class="n">X</span> <span class="p">{}</span> <span class="c1">// compiler error!</span>
<span class="n">Use</span> <span class="n">it</span> <span class="n">to</span> <span class="n">declare</span> <span class="n">a</span> <span class="n">member</span><span class="o">:</span>
<span class="k">class</span> <span class="nc">Foo</span> <span class="p">{</span>
<span class="n">X</span> <span class="n">m</span><span class="p">;</span> <span class="c1">// compiler error!</span>
<span class="p">};</span>
</code></pre></div></div>
<ul>
<li>Define functions or methods using this type</li>
</ul>
<div class="language-c++ highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kt">void</span> <span class="nf">f1</span><span class="p">(</span><span class="n">X</span> <span class="n">x</span><span class="p">)</span> <span class="p">{}</span> <span class="c1">// compiler error!</span>
<span class="n">X</span> <span class="nf">f2</span><span class="p">()</span> <span class="p">{}</span> <span class="c1">// compiler error!</span>
</code></pre></div></div>
<ul>
<li>Use its methods or fields, in fact trying to dereference a variable with incomplete type</li>
</ul>
<div class="language-c++ highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">class</span> <span class="nc">Foo</span> <span class="p">{</span>
<span class="n">X</span> <span class="o">*</span><span class="n">m</span><span class="p">;</span>
<span class="kt">void</span> <span class="n">method</span><span class="p">()</span>
<span class="p">{</span>
<span class="n">m</span><span class="o">-></span><span class="n">someMethod</span><span class="p">();</span> <span class="c1">// compiler error!</span>
<span class="kt">int</span> <span class="n">i</span> <span class="o">=</span> <span class="n">m</span><span class="o">-></span><span class="n">someField</span><span class="p">;</span> <span class="c1">// compiler error!</span>
<span class="p">}</span>
<span class="p">};</span>
</code></pre></div></div>
<p>When it comes to templates, there is no absolute rule: whether you can use an incomplete type as a template parameter is dependent on the way the type is used in the template.</p>
<p>For instance, <code class="language-plaintext highlighter-rouge">std::vector<T></code> requires its parameter to be a complete type, while <code class="language-plaintext highlighter-rouge">boost::container::vector<T></code> does not. Sometimes, a complete type is required only if you use certain member functions; this is the case for <code class="language-plaintext highlighter-rouge">std::unique_ptr<T></code>, for example.</p>
<p>A well-documented template should indicate in its documentation all the requirements of its parameters, including whether they need to be complete types or not.</p>
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