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<p>Defines an abstract type which cannot be instantiated, but can be used as a base class.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/abstract_class&action=edit&section=1" title="Edit section: Syntax">edit</a>]</span> <span class="mw-headline" id="Syntax">Syntax</span></h3>
<p><span class="t-spar">pure virtual</span> function is a <a href="virtual.html" title="cpp/language/virtual">virtual function</a> whose <a href="function.html" title="cpp/language/function">declarator</a> has the following syntax:
</p>
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<td class="t-sdsc-nopad"> <span class="t-spar">declarator</span> <span class="t-spar">virt-specifier</span><span class="t-mark">(optional)</span> <code><b>=</b></code> <code><b>0</b></code>
</td>
<td class="t-sdsc-nopad">
</td>
<td class="t-sdsc-nopad">
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<p>Here the sequence <code><b>= 0</b></code> is known as <span class="t-spar">pure-specifier</span>, and appears either immediately after the <span class="t-spar">declarator</span> or after the optional <span class="t-spar">virt-specifier</span> (<a href="override.html" title="cpp/language/override">override</a> or <a href="final.html" title="cpp/language/final">final</a>).
</p><p><span class="t-spar">pure-specifier</span> cannot appear in a member function definition.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> Base <span class="br0">{</span> <span class="kw1">virtual</span> <span class="kw4">int</span> g<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="kw1">virtual</span> ~Base<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> A <span class="sy4">:</span> Base<span class="br0">{</span>
<span class="co1">// ok, declares three member virtual functions, two of them pure</span>
<span class="kw1">virtual</span> <span class="kw4">int</span> f<span class="br0">(</span><span class="br0">)</span> <span class="sy1">=</span> <span class="nu0">0</span>, g<span class="br0">(</span><span class="br0">)</span> override <span class="sy1">=</span> <span class="nu0">0</span>, h<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="co1">// ok, destructor can be pure too</span>
~A<span class="br0">(</span><span class="br0">)</span> <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
<span class="co1">// error: pure-specifier on a function definition</span>
<span class="kw1">virtual</span> <span class="kw4">int</span> b<span class="br0">(</span><span class="br0">)</span><span class="sy1">=</span><span class="nu0">0</span> <span class="br0">{</span><span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<p>An <span class="t-spar">abstract class</span> is a class that either defines or inherits at least one function for which <a href="virtual.html" title="cpp/language/virtual">the final overrider</a> is <span class="t-spar">pure virtual</span>.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/abstract_class&action=edit&section=2" title="Edit section: Explanation">edit</a>]</span> <span class="mw-headline" id="Explanation">Explanation</span></h3>
<p>Abstract classes are used to represent general concepts (for example, Shape), which can be used as base classes for concrete classes (for example, Circle).
</p><p>No objects of an abstract class can be created. Abstract types cannot be used as parameter types, as function return types, or as the type of an explicit conversion. Pointers and references to an abstract class can be declared.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> Abstract <span class="br0">{</span>
<span class="kw1">virtual</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span> <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> <span class="co1">// pure virtual</span>
<span class="br0">}</span><span class="sy4">;</span> <span class="co1">// "Abstract" is abstract</span>
 
<span class="kw1">struct</span> Concrete <span class="sy4">:</span> Abstract <span class="br0">{</span>
<span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span> override <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// non-pure virtual</span>
<span class="kw1">virtual</span> <span class="kw4">void</span> g<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// non-pure virtual</span>
<span class="br0">}</span><span class="sy4">;</span> <span class="co1">// "Concrete" is non-abstract</span>
 
<span class="kw1">struct</span> Abstract2 <span class="sy4">:</span> Concrete <span class="br0">{</span>
<span class="kw4">void</span> g<span class="br0">(</span><span class="br0">)</span> override <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> <span class="co1">// pure virtual overrider</span>
<span class="br0">}</span><span class="sy4">;</span> <span class="co1">// "Abstract2" is abstract</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
<span class="co1">// Abstract a; // Error: abstract class</span>
Concrete b<span class="sy4">;</span> <span class="co1">// OK</span>
Abstract<span class="sy3">&</span> a <span class="sy1">=</span> b<span class="sy4">;</span> <span class="co1">// OK to reference abstract base</span>
a.<span class="me1">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// virtual dispatch to Concrete::f()</span>
<span class="co1">// Abstract2 a2; // Error: abstract class (final overrider of g() is pure)</span>
<span class="br0">}</span></pre></div></div>
<p>The definition of a pure virtual function may be provided (and must be provided if the pure virtual is the <a href="destructor.html" title="cpp/language/destructor">destructor</a>): the member functions of the derived class are free to call the abstract base's pure virtual function using qualified function id. This definition must be provided outside of the class body (the syntax of a function declaration doesn't allow both the pure specifier <code><b>= 0</b></code> and a function body)
</p><p>Making a virtual call to a pure virtual function from a constructor or the destructor of the abstract class is undefined behavior (regardless of whether it has a definition or not)
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> Abstract <span class="br0">{</span>
<span class="kw1">virtual</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span> <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> <span class="co1">// pure virtual</span>
<span class="kw1">virtual</span> <span class="kw4">void</span> g<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// non-pure virtual</span>
~Abstract<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
g<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// okay, calls Abstract::g()</span>
<span class="co1">// f(); // undefined behavior!</span>
Abstract<span class="sy4">::</span><span class="me2">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// okay, non-virtual call</span>
<span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="co1">//definition of the pure virtual function</span>
<span class="kw4">void</span> Abstract<span class="sy4">::</span><span class="me2">f</span><span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <a href="../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"A::f()<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="br0">}</span>
 
<span class="kw1">struct</span> Concrete <span class="sy4">:</span> Abstract <span class="br0">{</span>
<span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span> override <span class="br0">{</span>
Abstract<span class="sy4">::</span><span class="me2">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK: calls pure virtual function</span>
<span class="br0">}</span>
<span class="kw4">void</span> g<span class="br0">(</span><span class="br0">)</span> override <span class="br0">{</span><span class="br0">}</span>
~Concrete<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
g<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// okay, calls Concrete::g()</span>
f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// okay, calls Concrete::f()</span>
<span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/abstract_class&action=edit&section=3" title="Edit section: See also">edit</a>]</span> <span class="mw-headline" id="See_also">See also</span></h3>
<ul><li> <a href="virtual.html" title="cpp/language/virtual">virtual</a>
</li></ul>
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