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href="/cpp/optimization.html" class="sidebar-link">Optimization</a></li><li><a href="/cpp/profiling.html" class="sidebar-link">Profiling</a></li><li><a href="/cpp/refactoring-techniques.html" class="sidebar-link">Refactoring Techniques</a></li><li><a href="/cpp/internationalization-in-c.html" 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="pointers-to-members"><a href="#pointers-to-members" class="header-anchor">#</a> Pointers to members</h1> <h2 id="pointers-to-static-member-functions"><a href="#pointers-to-static-member-functions" class="header-anchor">#</a> Pointers to static member functions</h2> <p>A <code>static</code> member function is just like an ordinary C/C++ function, except with scope:</p> <ul><li>It is inside a <code>class</code>, so it needs its name decorated with the class name;</li> <li>It has accessibility, with <code>public</code>, <code>protected</code> or <code>private</code>.</li></ul> <p>So, if you have access to the <code>static</code> member function and decorate it correctly, then you can point to the function like any normal function outside a <code>class</code>:</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token keyword">typedef</span> <span class="token keyword">int</span> <span class="token function">Fn</span><span class="token punctuation">(</span><span class="token keyword">int</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// Fn is a type-of function that accepts an int and returns an int</span>
<span class="token comment">// Note that MyFn() is of type 'Fn'</span>
<span class="token keyword">int</span> <span class="token function">MyFn</span><span class="token punctuation">(</span><span class="token keyword">int</span> i<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> <span class="token number">2</span><span class="token operator">*</span>i<span class="token punctuation">;</span> <span class="token punctuation">}</span>
<span class="token keyword">class</span> <span class="token class-name">Class</span> <span class="token punctuation">{</span>
<span class="token keyword">public</span><span class="token operator">:</span>
<span class="token comment">// Note that Static() is of type 'Fn'</span>
<span class="token keyword">static</span> <span class="token keyword">int</span> <span class="token function">Static</span><span class="token punctuation">(</span><span class="token keyword">int</span> i<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> <span class="token number">3</span><span class="token operator">*</span>i<span class="token punctuation">;</span> <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span> <span class="token comment">// Class</span>
<span class="token keyword">int</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
Fn <span class="token operator">*</span>fn<span class="token punctuation">;</span> <span class="token comment">// fn is a pointer to a type-of Fn</span>
fn <span class="token operator">=</span> <span class="token operator">&</span>MyFn<span class="token punctuation">;</span> <span class="token comment">// Point to one function</span>
<span class="token function">fn</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// Call it</span>
fn <span class="token operator">=</span> <span class="token operator">&</span>Class<span class="token double-colon punctuation">::</span>Static<span class="token punctuation">;</span> <span class="token comment">// Point to the other function</span>
<span class="token function">fn</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// Call it</span>
<span class="token punctuation">}</span> <span class="token comment">// main()</span>
</code></pre></div><h2 id="pointers-to-member-functions"><a href="#pointers-to-member-functions" class="header-anchor">#</a> Pointers to member functions</h2> <p>To access a member function of a class, you need to have a "handle" to the particular instance, as either the instance itself, or a pointer or reference to it. Given a class instance, you can point to various of its members with a pointer-to-member, IF you get the syntax correct! Of course, the pointer has to be declared to be of the same type as what you are pointing to...</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token keyword">typedef</span> <span class="token keyword">int</span> <span class="token function">Fn</span><span class="token punctuation">(</span><span class="token keyword">int</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// Fn is a type-of function that accepts an int and returns an int</span>
<span class="token keyword">class</span> <span class="token class-name">Class</span> <span class="token punctuation">{</span>
<span class="token keyword">public</span><span class="token operator">:</span>
<span class="token comment">// Note that A() is of type 'Fn'</span>
<span class="token keyword">int</span> <span class="token function">A</span><span class="token punctuation">(</span><span class="token keyword">int</span> a<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> <span class="token number">2</span><span class="token operator">*</span>a<span class="token punctuation">;</span> <span class="token punctuation">}</span>
<span class="token comment">// Note that B() is of type 'Fn'</span>
<span class="token keyword">int</span> <span class="token function">B</span><span class="token punctuation">(</span><span class="token keyword">int</span> b<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> <span class="token number">3</span><span class="token operator">*</span>b<span class="token punctuation">;</span> <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span> <span class="token comment">// Class</span>
<span class="token keyword">int</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
Class c<span class="token punctuation">;</span> <span class="token comment">// Need a Class instance to play with</span>
Class <span class="token operator">*</span>p <span class="token operator">=</span> <span class="token operator">&</span>c<span class="token punctuation">;</span> <span class="token comment">// Need a Class pointer to play with</span>
Fn Class<span class="token double-colon punctuation">::</span><span class="token operator">*</span>fn<span class="token punctuation">;</span> <span class="token comment">// fn is a pointer to a type-of Fn within Class</span>
fn <span class="token operator">=</span> <span class="token operator">&</span>Class<span class="token double-colon punctuation">::</span>A<span class="token punctuation">;</span> <span class="token comment">// fn now points to A within any Class</span>
<span class="token punctuation">(</span>c<span class="token punctuation">.</span><span class="token operator">*</span>fn<span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token number">5</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// Pass 5 to c's function A (via fn)</span>
fn <span class="token operator">=</span> <span class="token operator">&</span>Class<span class="token double-colon punctuation">::</span>B<span class="token punctuation">;</span> <span class="token comment">// fn now points to B within any Class</span>
<span class="token punctuation">(</span>p<span class="token operator">-></span><span class="token operator">*</span>fn<span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token number">6</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// Pass 6 to c's (via p) function B (via fn)</span>
<span class="token punctuation">}</span> <span class="token comment">// main()</span>
</code></pre></div><p>Unlike pointers to member variables (in the previous example), the association between the class instance and the member pointer need to be bound tightly together with parentheses, which looks a little strange (as though the <code>.*</code> and <code>->*</code> aren't strange enough!)</p> <h2 id="pointers-to-member-variables"><a href="#pointers-to-member-variables" class="header-anchor">#</a> Pointers to member variables</h2> <p>To access a member of a <code>class</code>, you need to have a "handle" to the particular instance, as either the instance itself, or a pointer or reference to it. Given a <code>class</code> instance, you can point to various of its members with a pointer-to-member, IF you get the syntax correct! Of course, the pointer has to be declared to be of the same type as what you are pointing to...</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token keyword">class</span> <span class="token class-name">Class</span> <span class="token punctuation">{</span>
<span class="token keyword">public</span><span class="token operator">:</span>
<span class="token keyword">int</span> x<span class="token punctuation">,</span> y<span class="token punctuation">,</span> z<span class="token punctuation">;</span>
<span class="token keyword">char</span> m<span class="token punctuation">,</span> n<span class="token punctuation">,</span> o<span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span> <span class="token comment">// Class</span>
<span class="token keyword">int</span> x<span class="token punctuation">;</span> <span class="token comment">// Global variable</span>
<span class="token keyword">int</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
Class c<span class="token punctuation">;</span> <span class="token comment">// Need a Class instance to play with</span>
Class <span class="token operator">*</span>p <span class="token operator">=</span> <span class="token operator">&</span>c<span class="token punctuation">;</span> <span class="token comment">// Need a Class pointer to play with</span>
<span class="token keyword">int</span> <span class="token operator">*</span>p_i<span class="token punctuation">;</span> <span class="token comment">// Pointer to an int</span>
p_i <span class="token operator">=</span> <span class="token operator">&</span>x<span class="token punctuation">;</span> <span class="token comment">// Now pointing to x</span>
p_i <span class="token operator">=</span> <span class="token operator">&</span>c<span class="token punctuation">.</span>x<span class="token punctuation">;</span> <span class="token comment">// Now pointing to c's x</span>
<span class="token keyword">int</span> Class<span class="token double-colon punctuation">::</span><span class="token operator">*</span>p_C_i<span class="token punctuation">;</span> <span class="token comment">// Pointer to an int within Class</span>
p_C_i <span class="token operator">=</span> <span class="token operator">&</span>Class<span class="token double-colon punctuation">::</span>x<span class="token punctuation">;</span> <span class="token comment">// Point to x within any Class</span>
<span class="token keyword">int</span> i <span class="token operator">=</span> c<span class="token punctuation">.</span><span class="token operator">*</span>p_C_i<span class="token punctuation">;</span> <span class="token comment">// Use p_c_i to fetch x from c's instance</span>
p_C_i <span class="token operator">=</span> <span class="token operator">&</span>Class<span class="token double-colon punctuation">::</span>y<span class="token punctuation">;</span> <span class="token comment">// Point to y within any Class</span>
i <span class="token operator">=</span> c<span class="token punctuation">.</span><span class="token operator">*</span>p_C_i<span class="token punctuation">;</span> <span class="token comment">// Use p_c_i to fetch y from c's instance</span>
p_C_i <span class="token operator">=</span> <span class="token operator">&</span>Class<span class="token double-colon punctuation">::</span>m<span class="token punctuation">;</span> <span class="token comment">// ERROR! m is a char, not an int!</span>
<span class="token keyword">char</span> Class<span class="token double-colon punctuation">::</span><span class="token operator">*</span>p_C_c <span class="token operator">=</span> <span class="token operator">&</span>Class<span class="token double-colon punctuation">::</span>m<span class="token punctuation">;</span> <span class="token comment">// That's better...</span>
<span class="token punctuation">}</span> <span class="token comment">// main()</span>
</code></pre></div><p>The syntax of pointer-to-member requires some extra syntactic elements:</p> <ul><li>To define the type of the pointer, you need to mention the base type, as well as the fact that it is inside a class: <code>int Class::*ptr;</code>.</li> <li>If you have a class or reference and want to use it with a pointer-to-member, you need to use the <code>.*</code> operator (akin to the <code>.</code> operator).</li> <li>If you have a pointer to a class and want to use it with a pointer-to-member, you need to use the <code>->*</code> operator (akin to the <code>-></code> operator).</li></ul> <h2 id="pointers-to-static-member-variables"><a href="#pointers-to-static-member-variables" class="header-anchor">#</a> Pointers to static member variables</h2> <p>A <code>static</code> member variable is just like an ordinary C/C++ variable, except with scope:</p> <ul><li>It is inside a <code>class</code>, so it needs its name decorated with the class name;</li> <li>It has accessibility, with <code>public</code>, <code>protected</code> or <code>private</code>.</li></ul> <p>So, if you have access to the <code>static</code> member variable and decorate it correctly, then you can point to the variable like any normal variable outside a <code>class</code>:</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token keyword">class</span> <span class="token class-name">Class</span> <span class="token punctuation">{</span>
<span class="token keyword">public</span><span class="token operator">:</span>
<span class="token keyword">static</span> <span class="token keyword">int</span> i<span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span> <span class="token comment">// Class</span>
<span class="token keyword">int</span> Class<span class="token double-colon punctuation">::</span>i <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span> <span class="token comment">// Define the value of i (and where it's stored!)</span>
<span class="token keyword">int</span> j <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">;</span> <span class="token comment">// Just another global variable</span>
<span class="token keyword">int</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">int</span> k <span class="token operator">=</span> <span class="token number">3</span><span class="token punctuation">;</span> <span class="token comment">// Local variable</span>
<span class="token keyword">int</span> <span class="token operator">*</span>p<span class="token punctuation">;</span>
p <span class="token operator">=</span> <span class="token operator">&</span>k<span class="token punctuation">;</span> <span class="token comment">// Point to k</span>
<span class="token operator">*</span>p <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">;</span> <span class="token comment">// Modify it</span>
p <span class="token operator">=</span> <span class="token operator">&</span>j<span class="token punctuation">;</span> <span class="token comment">// Point to j</span>
<span class="token operator">*</span>p <span class="token operator">=</span> <span class="token number">3</span><span class="token punctuation">;</span> <span class="token comment">// Modify it</span>
p <span class="token operator">=</span> <span class="token operator">&</span>Class<span class="token double-colon punctuation">::</span>i<span class="token punctuation">;</span> <span class="token comment">// Point to Class::i</span>
<span class="token operator">*</span>p <span class="token operator">=</span> <span class="token number">4</span><span class="token punctuation">;</span> <span class="token comment">// Modify it</span>
<span class="token punctuation">}</span> <span class="token comment">// main()</span>
</code></pre></div><h4 id="syntax"><a href="#syntax" class="header-anchor">#</a> Syntax</h4> <li>
Assuming a class named Class...
<ul>
- type *ptr = &Class::member; // Point to static members only
- type Class::*ptr = &Class::member; // Point to non-static Class members
<p>For pointers to non-static class members, given the following two definitions:</p> <ul><li>Class instance;</li> <li>Class *p = &instance;</li></ul> <p>Pointers to Class member variables</p> <ul><li>ptr = &Class::i; // Point to variable i within every Class</li> <li>instance.*ptr = 1; // Access instance's i</li> <li>p->*ptr = 1; // Access p's i</li></ul> <p>Pointers to Class member functions</p> <ul><li>ptr = &Class::F; // Point to function 'F' within every Class</li> <li>(instance.*ptr)(5); // Call instance's F</li> <li>(p->*ptr)(6); // Call p's F</li></ul></ul></li></div> <footer class="page-edit"><div class="edit-link"><a href="https://github.com/devtut/generate/edit/master/docs/cpp/pointers-to-members.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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