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href="/cpp/compiling-and-building.html" class="sidebar-link">Compiling and Building</a></li><li><a href="/cpp/common-compile-linker-errors-gcc.html" class="sidebar-link">Common compile/linker errors (GCC)</a></li><li><a href="/cpp/more-undefined-behaviors-in-c.html" class="sidebar-link">More undefined behaviors in C++</a></li><li><a href="/cpp/unit-testing-in-c.html" class="sidebar-link">Unit Testing in C++</a></li><li><a href="/cpp/c-debugging-and-debug-prevention-tools-techniques.html" class="sidebar-link">C++ Debugging and Debug-prevention Tools & Techniques</a></li><li><a href="/cpp/optimization-in-c.html" class="sidebar-link">Optimization in C++</a></li><li><a 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"><a href="#pointers" class="header-anchor">#</a> Pointers</h1> <p>A pointer is an address that refers to a location in memory. They're commonly used to allow functions or data structures to know of and modify memory without having to copy the memory referred to. Pointers are usable with both primitive (built-in) or user-defined types.</p> <p>Pointers make use of the "dereference" <code>*</code> , "address of" <code>&</code> , and "arrow" <code>-></code> operators. The '*' and '->' operators are used to access the memory being pointed at, and the <code>&</code> operator is used to get an address in memory.</p> <h2 id="pointer-basics"><a href="#pointer-basics" class="header-anchor">#</a> Pointer basics</h2> <p><strong>Note:</strong> in all the following, the existence of the C++11 constant <code>nullptr</code> is assumed. For earlier versions, replace <code>nullptr</code> with <code>NULL</code>, the constant that used to play a similar role.</p> <h3 id="creating-a-pointer-variable"><a href="#creating-a-pointer-variable" class="header-anchor">#</a> Creating a pointer variable</h3> <p>A pointer variable can be created using the specific <code>*</code> syntax, e.g. <code>int *pointer_to_int;</code>.<br>
When a variable is of a <strong>pointer type</strong> (<code>int *</code>), it just contains a memory address. The memory address is the location at which data of the <strong>underlying type</strong> (<code>int</code>) is stored.</p> <p>The difference is clear when comparing the size of a variable with the size of a pointer to the same type:</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token comment">// Declare a struct type `big_struct` that contains</span>
<span class="token comment">// three long long ints.</span>
<span class="token keyword">typedef</span> <span class="token keyword">struct</span> <span class="token punctuation">{</span>
<span class="token keyword">long</span> <span class="token keyword">long</span> <span class="token keyword">int</span> foo1<span class="token punctuation">;</span>
<span class="token keyword">long</span> <span class="token keyword">long</span> <span class="token keyword">int</span> foo2<span class="token punctuation">;</span>
<span class="token keyword">long</span> <span class="token keyword">long</span> <span class="token keyword">int</span> foo3<span class="token punctuation">;</span>
<span class="token punctuation">}</span> big_struct<span class="token punctuation">;</span>
<span class="token comment">// Create a variable `bar` of type `big_struct`</span>
big_struct bar<span class="token punctuation">;</span>
<span class="token comment">// Create a variable `p_bar` of type `pointer to big_struct`.</span>
<span class="token comment">// Initialize it to `nullptr` (a null pointer).</span>
big_struct <span class="token operator">*</span>p_bar0 <span class="token operator">=</span> <span class="token keyword">nullptr</span><span class="token punctuation">;</span>
<span class="token comment">// Print the size of `bar`</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> <span class="token string">"sizeof(bar) = "</span> <span class="token operator"><<</span> <span class="token keyword">sizeof</span><span class="token punctuation">(</span>bar<span class="token punctuation">)</span> <span class="token operator"><<</span> std<span class="token double-colon punctuation">::</span>endl<span class="token punctuation">;</span>
<span class="token comment">// Print the size of `p_bar`.</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> <span class="token string">"sizeof(p_bar0) = "</span> <span class="token operator"><<</span> <span class="token keyword">sizeof</span><span class="token punctuation">(</span>p_bar0<span class="token punctuation">)</span> <span class="token operator"><<</span> std<span class="token double-colon punctuation">::</span>endl<span class="token punctuation">;</span>
<span class="token comment">/* Produces:
sizeof(bar) = 24
sizeof(p_bar0) = 8
*/</span>
</code></pre></div><h3 id="taking-the-address-of-another-variable"><a href="#taking-the-address-of-another-variable" class="header-anchor">#</a> Taking the address of another variable</h3> <p>Pointers can be assigned between each other just as normal variables; in this case, it is the <strong>memory address</strong> that is copied from one pointer to another, <strong>not the actual data</strong> that a pointer points to.<br>
Moreover, they can take the value <code>nullptr</code> which represents a null memory location. A pointer equal to <code>nullptr</code> contains an invalid memory location and hence it does not refer to valid data.</p> <p>You can get the memory address of a variable of a given type by prefixing the variable with the <strong>address of</strong> operator <code>&</code>. The value returned by <code>&</code> is a pointer to the underlying type which contains the memory address of the variable (which is valid data <strong>as long as the variable does not go out of scope</strong>).</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token comment">// Copy `p_bar0` into `p_bar_1`.</span>
big_struct <span class="token operator">*</span>p_bar1 <span class="token operator">=</span> p_bar0<span class="token punctuation">;</span>
<span class="token comment">// Take the address of `bar` into `p_bar_2`</span>
big_struct <span class="token operator">*</span>p_bar2 <span class="token operator">=</span> <span class="token operator">&</span>bar<span class="token punctuation">;</span>
<span class="token comment">// p_bar1 is now nullptr, p_bar2 is &bar.</span>
p_bar0 <span class="token operator">=</span> p_bar2<span class="token punctuation">;</span>
<span class="token comment">// p_bar0 is now &bar.</span>
p_bar2 <span class="token operator">=</span> <span class="token keyword">nullptr</span><span class="token punctuation">;</span>
<span class="token comment">// p_bar0 == &bar</span>
<span class="token comment">// p_bar1 == nullptr</span>
<span class="token comment">// p_bar2 == nullptr</span>
</code></pre></div><p>In contrast with references:</p> <ul><li>assigning two pointers does not overwrite the memory that the assigned pointer refers to;</li> <li>pointers can be null.</li> <li>the <strong>address of</strong> operator is required explicitly.</li></ul> <h3 id="accessing-the-content-of-a-pointer"><a href="#accessing-the-content-of-a-pointer" class="header-anchor">#</a> Accessing the content of a pointer</h3> <p>As taking an address requires <code>&</code>, as well accessing the content requires the usage of the <strong>dereference operator</strong> <code>*</code>, as a prefix. When a pointer is dereferenced, it becomes a variable of the underlying type (actually, a reference to it). It can then be read and modified, if not <code>const</code>.</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token punctuation">(</span><span class="token operator">*</span>p_bar0<span class="token punctuation">)</span><span class="token punctuation">.</span>foo1 <span class="token operator">=</span> <span class="token number">5</span><span class="token punctuation">;</span>
<span class="token comment">// `p_bar0` points to `bar`. This prints 5.</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> <span class="token string">"bar.foo1 = "</span> <span class="token operator"><<</span> bar<span class="token punctuation">.</span>foo1 <span class="token operator"><<</span> std<span class="token double-colon punctuation">::</span>endl<span class="token punctuation">;</span>
<span class="token comment">// Assign the value pointed to by `p_bar0` to `baz`.</span>
big_struct baz<span class="token punctuation">;</span>
baz <span class="token operator">=</span> <span class="token operator">*</span>p_bar0<span class="token punctuation">;</span>
<span class="token comment">// Now `baz` contains a copy of the data pointed to by `p_bar0`.</span>
<span class="token comment">// Indeed, it contains a copy of `bar`.</span>
<span class="token comment">// Prints 5 as well</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> <span class="token string">"baz.foo1 = "</span> <span class="token operator"><<</span> baz<span class="token punctuation">.</span>foo1 <span class="token operator"><<</span> std<span class="token double-colon punctuation">::</span>endl<span class="token punctuation">;</span>
</code></pre></div><p>The combination of <code>*</code> and the operator <code>.</code> is abbreviated by <code>-></code>:</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code>std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> <span class="token string">"bar.foo1 = "</span> <span class="token operator"><<</span> <span class="token punctuation">(</span><span class="token operator">*</span>p_bar0<span class="token punctuation">)</span><span class="token punctuation">.</span>foo1 <span class="token operator"><<</span> std<span class="token double-colon punctuation">::</span>endl<span class="token punctuation">;</span> <span class="token comment">// Prints 5</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> <span class="token string">"bar.foo1 = "</span> <span class="token operator"><<</span> p_bar0<span class="token operator">-></span>foo1 <span class="token operator"><<</span> std<span class="token double-colon punctuation">::</span>endl<span class="token punctuation">;</span> <span class="token comment">// Prints 5</span>
</code></pre></div><h3 id="dereferencing-invalid-pointers"><a href="#dereferencing-invalid-pointers" class="header-anchor">#</a> Dereferencing invalid pointers</h3> <p>When dereferencing a pointer, you should make sure it points to valid data. Dereferencing an invalid pointer (or a null pointer) can lead to memory access violation, or to read or write garbage data.</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code>big_struct <span class="token operator">*</span><span class="token function">never_do_this</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">// This is a local variable. Outside `never_do_this` it doesn't exist.</span>
big_struct retval<span class="token punctuation">;</span>
retval<span class="token punctuation">.</span>foo1 <span class="token operator">=</span> <span class="token number">11</span><span class="token punctuation">;</span>
<span class="token comment">// This returns the address of `retval`.</span>
<span class="token keyword">return</span> <span class="token operator">&</span>retval<span class="token punctuation">;</span>
<span class="token comment">// `retval` is destroyed and any code using the value returned</span>
<span class="token comment">// by `never_do_this` has a pointer to a memory location that</span>
<span class="token comment">// contains garbage data (or is inaccessible).</span>
<span class="token punctuation">}</span>
</code></pre></div><p>In such scenario, <code>g++</code> and <code>clang++</code> correctly issue the warnings:</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token punctuation">(</span>Clang<span class="token punctuation">)</span> warning<span class="token operator">:</span> address of stack memory associated with local variable <span class="token string">'retval'</span> returned <span class="token punctuation">[</span><span class="token operator">-</span>Wreturn<span class="token operator">-</span>stack<span class="token operator">-</span>address<span class="token punctuation">]</span>
<span class="token punctuation">(</span>Gcc<span class="token punctuation">)</span> warning<span class="token operator">:</span> address of local variable ‘retval’ returned <span class="token punctuation">[</span><span class="token operator">-</span>Wreturn<span class="token operator">-</span>local<span class="token operator">-</span>addr<span class="token punctuation">]</span>
</code></pre></div><p>Hence, care must be taken when pointers are arguments of functions, as they could be null:</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token keyword">void</span> <span class="token function">naive_code</span><span class="token punctuation">(</span>big_struct <span class="token operator">*</span>ptr_big_struct<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">// ... some code which doesn't check if `ptr_big_struct` is valid.</span>
ptr_big_struct<span class="token operator">-></span>foo1 <span class="token operator">=</span> <span class="token number">12</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// Segmentation fault.</span>
<span class="token function">naive_code</span><span class="token punctuation">(</span><span class="token keyword">nullptr</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre></div><h2 id="pointer-operations"><a href="#pointer-operations" class="header-anchor">#</a> Pointer Operations</h2> <p>There are two operators for pointers:
Address-of operator (&): Returns the memory address of its operand.
Contents-of (Dereference) operator(*): Returns the value of the variable located at the address specified by its operator.</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token keyword">int</span> var <span class="token operator">=</span> <span class="token number">20</span><span class="token punctuation">;</span>
<span class="token keyword">int</span> <span class="token operator">*</span>ptr<span class="token punctuation">;</span>
ptr <span class="token operator">=</span> <span class="token operator">&</span>var<span class="token punctuation">;</span>
cout <span class="token operator"><<</span> var <span class="token operator"><<</span> endl<span class="token punctuation">;</span>
<span class="token comment">//Outputs 20 (The value of var)</span>
cout <span class="token operator"><<</span> ptr <span class="token operator"><<</span> endl<span class="token punctuation">;</span>
<span class="token comment">//Outputs 0x234f119 (var's memory location)</span>
cout <span class="token operator"><<</span> <span class="token operator">*</span>ptr <span class="token operator"><<</span> endl<span class="token punctuation">;</span>
<span class="token comment">//Outputs 20(The value of the variable stored in the pointer ptr</span>
</code></pre></div><p>The asterisk (*) is used in declaring a pointer for simple purpose of indicating that it is a pointer. Don't confuse this with the <strong>dereference</strong> operator, which is used to obtain the value located at the specified address. They are simply two different things represented with the same sign.</p> <h2 id="pointer-arithmetic"><a href="#pointer-arithmetic" class="header-anchor">#</a> Pointer Arithmetic</h2> <h3 id="increment-decrement"><a href="#increment-decrement" class="header-anchor">#</a> Increment / Decrement</h3> <p>A pointer can be incremented or decremented (prefix and postfix). Incrementing a pointer advances the pointer value to the element in the array one element past the currently pointed to element. Decrementing a pointer moves it to the previous element in the array.</p> <p>Pointer arithmetic is not permitted if the type that the pointer points to is not complete. <code>void</code> is always an incomplete type.</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token keyword">char</span><span class="token operator">*</span> str <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token keyword">char</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">// str = 0x010</span>
<span class="token operator">++</span>str<span class="token punctuation">;</span> <span class="token comment">// str = 0x011 in this case sizeof(char) = 1 byte</span>
<span class="token keyword">int</span><span class="token operator">*</span> arr <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token keyword">int</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">// arr = 0x00100</span>
<span class="token operator">++</span>arr<span class="token punctuation">;</span> <span class="token comment">// arr = 0x00104 if sizeof(int) = 4 bytes</span>
<span class="token keyword">void</span><span class="token operator">*</span> ptr <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token keyword">void</span><span class="token operator">*</span><span class="token punctuation">)</span><span class="token keyword">new</span> <span class="token keyword">char</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 operator">++</span>ptr<span class="token punctuation">;</span> <span class="token comment">// void is incomplete.</span>
</code></pre></div><p>If a pointer to the end element is incremented, then the pointer points to one element past the end of the array. Such a pointer cannot be dereferenced, but it can be decremented.</p> <p>Incrementing a pointer to the one-past-the-end element in the array, or decrementing a pointer to the first element in an array yields undefined behavior.</p> <p>A pointer to a non-array object can be treated, for the purposes of pointer arithmetic, as though it were an array of size 1.</p> <h3 id="addition-subtraction"><a href="#addition-subtraction" class="header-anchor">#</a> Addition / Subtraction</h3> <p>Integer values can be added to pointers; they act as incrementing, but by a specific number rather than by 1. Integer values can be subtracted from pointers as well, acting as pointer decrementing. As with incrementing/decrementing, the pointer must point to a complete type.</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token keyword">char</span><span class="token operator">*</span> str <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token keyword">char</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">// str = 0x010</span>
str <span class="token operator">+=</span> <span class="token number">2</span><span class="token punctuation">;</span> <span class="token comment">// str = 0x010 + 2 * sizeof(char) = 0x012</span>
<span class="token keyword">int</span><span class="token operator">*</span> arr <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token keyword">int</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">// arr = 0x100</span>
arr <span class="token operator">+=</span> <span class="token number">2</span><span class="token punctuation">;</span> <span class="token comment">// arr = 0x100 + 2 * sizeof(int) = 0x108, assuming sizeof(int) == 4.</span>
</code></pre></div><h3 id="pointer-differencing"><a href="#pointer-differencing" class="header-anchor">#</a> Pointer Differencing</h3> <p>The difference between two pointers to the same type can be computed. The two pointers must be within the same array object; otherwise undefined behavior results.</p> <p>Given two pointers <code>P</code> and <code>Q</code> in the same array, if <code>P</code> is the <code>i</code>th element in the array, and <code>Q</code> is the <code>j</code>th element, then <code>P - Q</code> shall be <code>i - j</code>. The type of the result is <code>std::ptrdiff_t</code>, from <code><cstddef></code>.</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token keyword">char</span><span class="token operator">*</span> start <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token keyword">char</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">// str = 0x010</span>
<span class="token keyword">char</span><span class="token operator">*</span> test <span class="token operator">=</span> <span class="token operator">&</span>start<span class="token punctuation">[</span><span class="token number">5</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
std<span class="token double-colon punctuation">::</span>ptrdiff_t diff <span class="token operator">=</span> test <span class="token operator">-</span> start<span class="token punctuation">;</span> <span class="token comment">//Equal to 5.</span>
std<span class="token double-colon punctuation">::</span>ptrdiff_t diff <span class="token operator">=</span> start <span class="token operator">-</span> test<span class="token punctuation">;</span> <span class="token comment">//Equal to -5; ptrdiff_t is signed.</span>
</code></pre></div><h4 id="syntax"><a href="#syntax" class="header-anchor">#</a> Syntax</h4> <ul><li><Data type=""> *<Variable name="">;
</Variable></Data></li> <li><Data type=""> *<Variable name=""> = &<Variable name="" of="" same="" Data="" type="">;
</Variable></Variable></Data></li> <li><Data type=""> *<Variable name=""> = <Value of="" same="" Data="" type="">;
</Value></Variable></Data></li> <li>int *foo; //A pointer which would points to an integer value</li> <li>int *bar = &myIntVar;</li> <li>long *bar[2];</li> <li>long *bar[] = {&myLongVar1, &myLongVar2}; //Equals to: long *bar[2]</li></ul> <h4 id="remarks"><a href="#remarks" class="header-anchor">#</a> Remarks</h4> <p>Be aware of problems when declaring multiple pointers on the same line.</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token keyword">int</span><span class="token operator">*</span> a<span class="token punctuation">,</span> b<span class="token punctuation">,</span> c<span class="token punctuation">;</span> <span class="token comment">//Only a is a pointer, the others are regular ints.</span>
<span class="token keyword">int</span><span class="token operator">*</span> a<span class="token punctuation">,</span> <span class="token operator">*</span>b<span class="token punctuation">,</span> <span class="token operator">*</span>c<span class="token punctuation">;</span> <span class="token comment">//These are three pointers!</span>
<span class="token keyword">int</span> <span class="token operator">*</span>foo<span class="token punctuation">[</span><span class="token number">2</span><span class="token punctuation">]</span><span class="token punctuation">;</span> <span class="token comment">//Both *foo[0] and *foo[1] are pointers.</span>
</code></pre></div></div> <footer class="page-edit"><div class="edit-link"><a href="https://github.com/devtut/generate/edit/master/docs/cpp/pointers.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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