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- <h2><a name="capacity" id="capacity">capacity</a></h2>
- <div class="level2">
- <p>
- Syntax:
- </p>
- <pre class="c code c++" style="font-family:monospace;"> <span class="co2">#include <vector></span>
- size_type capacity<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span><span class="sy0">;</span></pre>
- <p>
- The <code>capacity</code> method returns the number of elements that the vector can hold
- before it will need to allocate more space.
- </p>
- <p>
- For example, the following code uses two different methods to set the capacity
- of two vectors. One method passes an argument to the constructor that initializes
- the vector with 10 elements of value 0, and the other method calls the <a href="../../stl/vector/reserve.html" class="wikilink1" title="stl:vector:reserve">reserve</a> method.
- However, the actual size of the vector remains zero.
- </p>
- <pre class="c code c++" style="font-family:monospace;"> vector<span class="sy0"><</span>int<span class="sy0">></span> v1<span class="br0">(</span>10<span class="br0">)</span><span class="sy0">;</span>
- <a href="http://www.opengroup.org/onlinepubs/009695399/functions/cout.html"><span class="kw3">cout</span></a> <span class="sy0"><<</span> <span class="st0">"The capacity of v1 is "</span> <span class="sy0"><<</span> v1.<span class="me1">capacity</span><span class="br0">(</span><span class="br0">)</span> <span class="sy0"><<</span> endl<span class="sy0">;</span>
- <a href="http://www.opengroup.org/onlinepubs/009695399/functions/cout.html"><span class="kw3">cout</span></a> <span class="sy0"><<</span> <span class="st0">"The size of v1 is "</span> <span class="sy0"><<</span> v1.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span> <span class="sy0"><<</span> endl<span class="sy0">;</span>
- vector<span class="sy0"><</span>int<span class="sy0">></span> v2<span class="sy0">;</span>
- v2.<span class="me1">reserve</span><span class="br0">(</span>20<span class="br0">)</span><span class="sy0">;</span>
- <a href="http://www.opengroup.org/onlinepubs/009695399/functions/cout.html"><span class="kw3">cout</span></a> <span class="sy0"><<</span> <span class="st0">"The capacity of v2 is "</span> <span class="sy0"><<</span> v2.<span class="me1">capacity</span><span class="br0">(</span><span class="br0">)</span> <span class="sy0"><<</span> endl<span class="sy0">;</span>
- <a href="http://www.opengroup.org/onlinepubs/009695399/functions/cout.html"><span class="kw3">cout</span></a> <span class="sy0"><<</span> <span class="st0">"The size of v2 is "</span> <span class="sy0"><<</span> v2.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span> <span class="sy0"><<</span> endl<span class="sy0">;</span></pre>
- <p>
- When run, the above code produces the following output:
- </p>
- <pre class="code">
- The capacity of v1 is 10
- The size of v1 is 10
- The capacity of v2 is 20
- The size of v2 is 0</pre>
- <p>
- C++ containers are designed to grow in size dynamically. This frees the
- programmer from having to worry about storing an arbitrary number of elements
- in a container. However, sometimes the programmer can improve the performance
- of their program by giving hints to the compiler about the size of the containers
- that the program will use. These hints come in the form of the <a href="../../stl/vector/reserve.html" class="wikilink1" title="stl:vector:reserve">reserve</a>
- method and the constructor used in the above example, which tell the compiler
- how large the container is expected to get.
- </p>
- <p>
- The <code>capacity</code> method runs in <a href="../../complexity.html" class="wikilink1" title="complexity">constant time</a>.
- </p>
- <p>
- Related Topics: <a href="../../stl/vector/reserve.html" class="wikilink1" title="stl:vector:reserve">reserve</a>, <a href="../../stl/vector/resize.html" class="wikilink1" title="stl:vector:resize">resize</a>, <a href="../../stl/vector/size.html" class="wikilink1" title="stl:vector:size">size</a>
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