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List constructors [C++ Reference]
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<h2><a name="list_constructors" id="list_constructors">List constructors</a></h2>
<div class="level2">
<p>
Syntax:
</p>
<pre class="c code c++" style="font-family:monospace;"> <span class="co2">#include <list></span>
list<span class="br0">(</span><span class="br0">)</span><span class="sy0">;</span>
list<span class="br0">(</span> <span class="kw4">const</span> list<span class="sy0">&</span> c <span class="br0">)</span><span class="sy0">;</span>
explicit list<span class="br0">(</span> size_type num<span class="sy0">,</span> <span class="kw4">const</span> TYPE<span class="sy0">&</span> val <span class="sy0">=</span> TYPE<span class="br0">(</span><span class="br0">)</span> <span class="br0">)</span><span class="sy0">;</span>
list<span class="br0">(</span> input_iterator start<span class="sy0">,</span> input_iterator end <span class="br0">)</span><span class="sy0">;</span>
~list<span class="br0">(</span><span class="br0">)</span><span class="sy0">;</span></pre>
<p>
The default list constructor takes no arguments, creates a new instance of that
list.
</p>
<p>
The second constructor is a default copy constructor that can be used to create
a new list that is a copy of the given list c.
</p>
<p>
The third constructor creates a list with space for num objects. If val is
specified, each of those objects will be given that value. For example, the
following code creates a list consisting of five copies of the integer 42:
</p>
<pre class="c code c++" style="font-family:monospace;"> list <span class="sy0"><</span>int<span class="sy0">></span> l1<span class="br0">(</span> 5<span class="sy0">,</span> 42 <span class="br0">)</span><span class="sy0">;</span></pre>
<p>
The last constructor creates a list that is initialized to contain the elements
between start and end. For example:
</p>
<pre class="c code c++" style="font-family:monospace;"> <span class="co1">// create a vector of random integers</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">"original list: "</span><span class="sy0">;</span>
list<span class="sy0"><</span>int<span class="sy0">></span> l<span class="sy0">;</span>
<span class="kw1">for</span><span class="br0">(</span> <span class="kw4">int</span> i <span class="sy0">=</span> <span class="nu0">0</span><span class="sy0">;</span> i <span class="sy0"><</span> <span class="nu0">20</span><span class="sy0">;</span> i<span class="sy0">++</span> <span class="br0">)</span> <span class="br0">{</span>
<span class="kw4">int</span> num <span class="sy0">=</span> <span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> rand<span class="br0">(</span><span class="br0">)</span> <span class="sy0">%</span> <span class="nu0">10</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> num <span class="sy0"><<</span> <span class="st0">" "</span><span class="sy0">;</span>
l.<span class="me1">push_back</span><span class="br0">(</span> num <span class="br0">)</span><span class="sy0">;</span>
<span class="br0">}</span>
<a href="http://www.opengroup.org/onlinepubs/009695399/functions/cout.html"><span class="kw3">cout</span></a> <span class="sy0"><<</span> endl<span class="sy0">;</span>
<span class="co1">// delete 5 & 7</span>
list<span class="sy0"><</span>int<span class="sy0">>::</span><span class="me2">iterator</span> iter1 <span class="sy0">=</span> l.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span><span class="sy0">;</span>
<span class="kw1">while</span><span class="br0">(</span> iter1 <span class="sy0">!=</span> l.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span> <span class="br0">)</span> <span class="br0">{</span>
list<span class="sy0"><</span>int<span class="sy0">>::</span><span class="me2">iterator</span> thisone <span class="sy0">=</span> iter1<span class="sy0">;</span>
<span class="sy0">++</span>iter1<span class="sy0">;</span>
<span class="kw1">if</span> <span class="br0">(</span> <span class="sy0">*</span>thisone <span class="sy0">==</span> 5 <span class="sy0">||</span> <span class="sy0">*</span>thisone <span class="sy0">==</span> 7 <span class="br0">)</span> <span class="br0">{</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">"erase "</span> <span class="sy0"><<</span> <span class="sy0">*</span>thisone <span class="sy0"><<</span> endl<span class="sy0">;</span>
l.<span class="me1">erase</span><span class="br0">(</span> thisone <span class="br0">)</span><span class="sy0">;</span>
<span class="br0">}</span>
<span class="br0">}</span>
<span class="co1">// find the first element of l that is even</span>
list<span class="sy0"><</span>int<span class="sy0">>::</span><span class="me2">iterator</span> iter2 <span class="sy0">=</span> l.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span><span class="sy0">;</span>
<span class="kw1">while</span><span class="br0">(</span> iter2 <span class="sy0">!=</span> l.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span> <span class="sy0">&&</span> <span class="sy0">*</span>iter2 <span class="sy0">%</span> 2 <span class="sy0">!=</span> 0 <span class="br0">)</span> <span class="br0">{</span>
<span class="sy0">++</span>iter2<span class="sy0">;</span>
<span class="br0">}</span>
<span class="co1">// find the last element of l that is even</span>
list<span class="sy0"><</span>int<span class="sy0">>::</span><span class="me2">iterator</span> iter3 <span class="sy0">=</span> l.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span><span class="sy0">;</span>
<span class="kw1">do</span> <span class="br0">{</span>
<span class="sy0">--</span>iter3<span class="sy0">;</span>
<span class="br0">}</span> <span class="kw1">while</span><span class="br0">(</span> iter3 <span class="sy0">!=</span> l.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span> <span class="sy0">&&</span> <span class="sy0">*</span>iter3 <span class="sy0">%</span> 2 <span class="sy0">!=</span> 0 <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">"first even number: "</span> <span class="sy0"><<</span> <span class="sy0">*</span>iter2 <span class="sy0"><<</span> <span class="st0">", last even number: "</span> <span class="sy0"><<</span> <span class="sy0">*</span>iter3 <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">"new list: "</span><span class="sy0">;</span>
list<span class="sy0"><</span>int<span class="sy0">></span> l2<span class="br0">(</span> iter2<span class="sy0">,</span> iter3 <span class="br0">)</span><span class="sy0">;</span>
list<span class="sy0"><</span>int<span class="sy0">>::</span><span class="me2">iterator</span> iter4 <span class="sy0">=</span> l2.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span><span class="sy0">;</span>
<span class="kw1">while</span><span class="br0">(</span> iter4 <span class="sy0">!=</span> l2.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span> <span class="br0">)</span> <span class="br0">{</span>
<a href="http://www.opengroup.org/onlinepubs/009695399/functions/cout.html"><span class="kw3">cout</span></a> <span class="sy0"><<</span> <span class="sy0">*</span>iter4 <span class="sy0"><<</span> <span class="st0">" "</span><span class="sy0">;</span>
<span class="sy0">++</span>iter4<span class="sy0">;</span>
<span class="br0">}</span>
<a href="http://www.opengroup.org/onlinepubs/009695399/functions/cout.html"><span class="kw3">cout</span></a> <span class="sy0"><<</span> endl<span class="sy0">;</span></pre>
<p>
When run, this code displays the following output:
</p>
<pre class="code">
original list: 7 9 3 8 0 2 4 8 3 9 0 5 2 2 7 3 7 9 0 2
erase 7
erase 5
erase 7
erase 7
first even number: 8, last even number: 2
new list: 8 0 2 4 8 3 9 0 2 2 3 9 0</pre>
<p>
All of these constructors run in <a href="../../complexity.html" class="wikilink1" title="complexity">linear time</a> except the first, which runs in
<a href="../../complexity.html" class="wikilink1" title="complexity">constant time</a>.
</p>
<p>
The default destructor calls the destructor for each object in the list with linear complexity.
</p>
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