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class="sidebar-link">Internationalization in C++</a></li><li><a href="/cpp/contributors.html" class="sidebar-link">The Contributors</a></li></ul></section></li></ul> </aside> <main class="page"> <div class="theme-default-content content__default"><h1 id="friend-keyword"><a href="#friend-keyword" class="header-anchor">#</a> Friend keyword</h1> <p>Well-designed classes encapsulate their functionality, hiding their implementation while providing a clean, documented interface. This allows redesign or change so long as the interface is unchanged.</p> <p>In a more complex scenario, multiple classes that rely on each others' implementation details may be required. Friend classes and functions allow these peers access to each others' details, without compromising the encapsulation and information hiding of the documented interface.</p> <h2 id="friend-function"><a href="#friend-function" class="header-anchor">#</a> Friend function</h2> <p>A class or a structure may declare any function it's friend. If a function is a friend of a class, it may access all it's protected and private members:</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token comment">// Forward declaration of functions.</span>
<span class="token keyword">void</span> <span class="token function">friend_function</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">void</span> <span class="token function">non_friend_function</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">class</span> <span class="token class-name">PrivateHolder</span> <span class="token punctuation">{</span>
<span class="token keyword">public</span><span class="token operator">:</span>
<span class="token function">PrivateHolder</span><span class="token punctuation">(</span><span class="token keyword">int</span> val<span class="token punctuation">)</span> <span class="token operator">:</span> <span class="token function">private_value</span><span class="token punctuation">(</span>val<span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
<span class="token keyword">private</span><span class="token operator">:</span>
<span class="token keyword">int</span> private_value<span class="token punctuation">;</span>
<span class="token comment">// Declare one of the function as a friend.</span>
<span class="token keyword">friend</span> <span class="token keyword">void</span> <span class="token function">friend_function</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">void</span> <span class="token function">non_friend_function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
PrivateHolder <span class="token function">ph</span><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// Compilation error: private_value is private.</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> ph<span class="token punctuation">.</span>private_value <span class="token operator"><<</span> std<span class="token double-colon punctuation">::</span>endl<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">void</span> <span class="token function">friend_function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">// OK: friends may access private values.</span>
PrivateHolder <span class="token function">ph</span><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> ph<span class="token punctuation">.</span>private_value <span class="token operator"><<</span> std<span class="token double-colon punctuation">::</span>endl<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><p>Access modifiers do not alter friend semantics. Public, protected and private declarations of a friend are equivalent.</p> <p>Friend declarations are not inherited. For example, if we subclass <code>PrivateHolder</code>:</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token keyword">class</span> <span class="token class-name">PrivateHolderDerived</span> <span class="token operator">:</span> <span class="token base-clause"><span class="token keyword">public</span> <span class="token class-name">PrivateHolder</span></span> <span class="token punctuation">{</span>
<span class="token keyword">public</span><span class="token operator">:</span>
<span class="token function">PrivateHolderDerived</span><span class="token punctuation">(</span><span class="token keyword">int</span> val<span class="token punctuation">)</span> <span class="token operator">:</span> <span class="token function">PrivateHolder</span><span class="token punctuation">(</span>val<span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
<span class="token keyword">private</span><span class="token operator">:</span>
<span class="token keyword">int</span> derived_private_value <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
</code></pre></div><p>and try to access it's members, we'll get the following:</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token keyword">void</span> <span class="token function">friend_function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
PrivateHolderDerived <span class="token function">pd</span><span class="token punctuation">(</span><span class="token number">20</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// OK.</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> pd<span class="token punctuation">.</span>private_value <span class="token operator"><<</span> std<span class="token double-colon punctuation">::</span>endl<span class="token punctuation">;</span>
<span class="token comment">// Compilation error: derived_private_value is private.</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> pd<span class="token punctuation">.</span>derived_private_value <span class="token operator"><<</span> std<span class="token double-colon punctuation">::</span>endl<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><p>Note that <code>PrivateHolderDerived</code> member function cannot access <code>PrivateHolder::private_value</code>, while friend function can do it.</p> <h2 id="friend-method"><a href="#friend-method" class="header-anchor">#</a> Friend method</h2> <p>Methods may declared as friends as well as functions:</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token keyword">class</span> <span class="token class-name">Accesser</span> <span class="token punctuation">{</span>
<span class="token keyword">public</span><span class="token operator">:</span>
<span class="token keyword">void</span> <span class="token function">private_accesser</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">class</span> <span class="token class-name">PrivateHolder</span> <span class="token punctuation">{</span>
<span class="token keyword">public</span><span class="token operator">:</span>
<span class="token function">PrivateHolder</span><span class="token punctuation">(</span><span class="token keyword">int</span> val<span class="token punctuation">)</span> <span class="token operator">:</span> <span class="token function">private_value</span><span class="token punctuation">(</span>val<span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
<span class="token keyword">friend</span> <span class="token keyword">void</span> <span class="token class-name">Accesser</span><span class="token double-colon punctuation">::</span><span class="token function">private_accesser</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">private</span><span class="token operator">:</span>
<span class="token keyword">int</span> private_value<span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">void</span> <span class="token class-name">Accesser</span><span class="token double-colon punctuation">::</span><span class="token function">private_accesser</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
PrivateHolder <span class="token function">ph</span><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// OK: this method is declares as friend.</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> ph<span class="token punctuation">.</span>private_value <span class="token operator"><<</span> std<span class="token double-colon punctuation">::</span>endl<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><h2 id="friend-class"><a href="#friend-class" class="header-anchor">#</a> Friend class</h2> <p>A whole class may be declared as friend. Friend class declaration means that any member of the friend may access private and protected members of the declaring class:</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token keyword">class</span> <span class="token class-name">Accesser</span> <span class="token punctuation">{</span>
<span class="token keyword">public</span><span class="token operator">:</span>
<span class="token keyword">void</span> <span class="token function">private_accesser1</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">void</span> <span class="token function">private_accesser2</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">class</span> <span class="token class-name">PrivateHolder</span> <span class="token punctuation">{</span>
<span class="token keyword">public</span><span class="token operator">:</span>
<span class="token function">PrivateHolder</span><span class="token punctuation">(</span><span class="token keyword">int</span> val<span class="token punctuation">)</span> <span class="token operator">:</span> <span class="token function">private_value</span><span class="token punctuation">(</span>val<span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
<span class="token keyword">friend</span> <span class="token keyword">class</span> <span class="token class-name">Accesser</span><span class="token punctuation">;</span>
<span class="token keyword">private</span><span class="token operator">:</span>
<span class="token keyword">int</span> private_value<span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">void</span> <span class="token class-name">Accesser</span><span class="token double-colon punctuation">::</span><span class="token function">private_accesser1</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
PrivateHolder <span class="token function">ph</span><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// OK.</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> ph<span class="token punctuation">.</span>private_value <span class="token operator"><<</span> std<span class="token double-colon punctuation">::</span>endl<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">void</span> <span class="token class-name">Accesser</span><span class="token double-colon punctuation">::</span><span class="token function">private_accesser2</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
PrivateHolder <span class="token function">ph</span><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// OK.</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> ph<span class="token punctuation">.</span>private_value <span class="token operator">+</span> <span class="token number">1</span> <span class="token operator"><<</span> std<span class="token double-colon punctuation">::</span>endl<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><p>Friend class declaration is not reflexive. If classes need private access in both directions, both of them need friend declarations.</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token keyword">class</span> <span class="token class-name">Accesser</span> <span class="token punctuation">{</span>
<span class="token keyword">public</span><span class="token operator">:</span>
<span class="token keyword">void</span> <span class="token function">private_accesser1</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">void</span> <span class="token function">private_accesser2</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">private</span><span class="token operator">:</span>
<span class="token keyword">int</span> private_value <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">class</span> <span class="token class-name">PrivateHolder</span> <span class="token punctuation">{</span>
<span class="token keyword">public</span><span class="token operator">:</span>
<span class="token function">PrivateHolder</span><span class="token punctuation">(</span><span class="token keyword">int</span> val<span class="token punctuation">)</span> <span class="token operator">:</span> <span class="token function">private_value</span><span class="token punctuation">(</span>val<span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
<span class="token comment">// Accesser is a friend of PrivateHolder</span>
<span class="token keyword">friend</span> <span class="token keyword">class</span> <span class="token class-name">Accesser</span><span class="token punctuation">;</span>
<span class="token keyword">void</span> <span class="token function">reverse_accesse</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">// but PrivateHolder cannot access Accesser's members.</span>
Accesser a<span class="token punctuation">;</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> a<span class="token punctuation">.</span>private_value<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">private</span><span class="token operator">:</span>
<span class="token keyword">int</span> private_value<span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
</code></pre></div></div> <footer class="page-edit"><div class="edit-link"><a href="https://github.com/devtut/generate/edit/master/docs/cpp/friend-keyword.md" target="_blank" rel="noopener noreferrer">Edit this page on GitHub</a> <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></div> <!----></footer> <div class="page-nav"><p class="inner"><span class="prev">
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