http:^^www.xraylith.wisc.edu^~khan^software^stl^stl.newbie.html

This data set contains WWW-pages collected from computer science departments of various universities

   char buf[1024];
   strcpy(buf, "THIS_WOULD_CHANGE_MAGICALLY");
   list<char*> list1;
   list1.push_back(buf);
   ostream_iterator<char*> citer(cout, " ");
   copy(list1.begin(), list1.end(), citer);

   // you should see one string, the one in buf above.

   strcpy(buf, "YIKES!");
   copy(list1.begin(), list1.end(), citer);

   // SURPRISE. your list is changed now.
   
</pre>
<p>
In general, do not use <tt>char*</tt> as container objects, rather write a
simple string class (ask me if you need one) and use that instead.

<hr>
<!WA69><!WA69><!WA69><!WA69><!WA69><!WA69><a href=#pointers>Pointers and STL</a> 
<br>
<!WA70><!WA70><!WA70><!WA70><!WA70><!WA70><a href=#index>Back to index</a>
<hr>

<a name=class_ptr_char><h3>More gotchas in storing char*</h3></a>
Here's an example of a set of strings that can cause lots of headache.

<PRE>
<STRONG>#include</STRONG> &lt;<!WA71><!WA71><!WA71><!WA71><!WA71><!WA71><A HREF="#stl.h">stl.h</A>&gt;
<STRONG>#include</STRONG> &lt;iostream.h&gt;

<STRONG>int</STRONG> main<STRONG>(</STRONG><STRONG>int</STRONG><STRONG>,</STRONG> <STRONG>char</STRONG><STRONG>*</STRONG><STRONG>[</STRONG><STRONG>]</STRONG><STRONG>)</STRONG> <STRONG>{</STRONG>
    <STRONG>static</STRONG> <STRONG>char</STRONG><STRONG>*</STRONG> names<STRONG>[</STRONG><STRONG>]</STRONG> <STRONG>=</STRONG> <STRONG>{</STRONG>&quot;<EM>one</EM>&quot;<STRONG>,</STRONG> &quot;<EM>two</EM>&quot;<STRONG>,</STRONG> &quot;<EM>three</EM>&quot;<STRONG>}</STRONG><STRONG>;</STRONG>
    set<STRONG>&lt;</STRONG><STRONG>char</STRONG><STRONG>*</STRONG><STRONG>,</STRONG> less<STRONG>&lt;</STRONG><STRONG>char</STRONG><STRONG>*</STRONG><STRONG>&gt;</STRONG> <STRONG>&gt;</STRONG> name_set<STRONG>;</STRONG>
    name_set<STRONG>.</STRONG>insert<STRONG>(</STRONG>names<STRONG>[</STRONG>0<STRONG>]</STRONG><STRONG>)</STRONG><STRONG>;</STRONG>
    name_set<STRONG>.</STRONG>insert<STRONG>(</STRONG>names<STRONG>[</STRONG>1<STRONG>]</STRONG><STRONG>)</STRONG><STRONG>;</STRONG>
    name_set<STRONG>.</STRONG>insert<STRONG>(</STRONG>names<STRONG>[</STRONG>2<STRONG>]</STRONG><STRONG>)</STRONG><STRONG>;</STRONG>

    <STRONG>char</STRONG> buf<STRONG>[</STRONG>256<STRONG>]</STRONG><STRONG>;</STRONG>
    strcpy<STRONG>(</STRONG>buf<STRONG>,</STRONG> &quot;<EM>one</EM>&quot;<STRONG>)</STRONG><STRONG>;</STRONG>
    <STRONG>const</STRONG> <STRONG>char</STRONG><STRONG>*</STRONG> one <STRONG>=</STRONG> buf<STRONG>;</STRONG>
    set<STRONG>&lt;</STRONG><STRONG>char</STRONG><STRONG>*</STRONG><STRONG>,</STRONG> less<STRONG>&lt;</STRONG><STRONG>char</STRONG><STRONG>*</STRONG><STRONG>&gt;</STRONG> <STRONG>&gt;</STRONG><STRONG>::</STRONG>const_iterator it <STRONG>=</STRONG> name_set<STRONG>.</STRONG>find<STRONG>(</STRONG>one<STRONG>)</STRONG><STRONG>;</STRONG>
    <STRONG>if</STRONG> <STRONG>(</STRONG>it <STRONG>==</STRONG> name_set<STRONG>.</STRONG>end<STRONG>(</STRONG><STRONG>)</STRONG><STRONG>)</STRONG> <STRONG>{</STRONG>
        cerr <STRONG>&lt;&lt;</STRONG> &quot;<EM>No such name `</EM>&quot; <STRONG>&lt;&lt;</STRONG> one <STRONG>&lt;&lt;</STRONG> &quot;<EM>' in set!</EM>&quot; <STRONG>&lt;&lt;</STRONG> endl<STRONG>;</STRONG>
    <STRONG>}</STRONG> <STRONG>else</STRONG> <STRONG>{</STRONG>
        cerr <STRONG>&lt;&lt;</STRONG> &quot;<EM>Found name `</EM>&quot; <STRONG>&lt;&lt;</STRONG> one <STRONG>&lt;&lt;</STRONG> &quot;<EM>' in set.</EM>&quot; <STRONG>&lt;&lt;</STRONG> endl<STRONG>;</STRONG>
    <STRONG>}</STRONG>
    <STRONG>return</STRONG> 0<STRONG>;</STRONG>
<STRONG>}</STRONG>
</PRE>

<hr>
<!WA72><!WA72><!WA72><!WA72><!WA72><!WA72><a href=#pointers>Pointers and STL</a> 
<br>
<!WA73><!WA73><!WA73><!WA73><!WA73><!WA73><a href=#index>Back to index</a>
<hr>

<a name=pointer_wrap><h2>Example of a pointer wrapper for storing in
    STL containers</h2></a>
If you <em>have to</em> store pointers in STL containers, especially the
sorted collections such as <tt>set</tt> and <tt>map</tt>, you might want
to wrap the pointers into a simple class that works as a holder for the
pointer (<em>who cleans up the memory afterwards</em>?) See 
<!WA74><!WA74><!WA74><!WA74><!WA74><!WA74><a href=#class_ptr_destruction> here</a> for an example):

<p>

<hr>

<PRE>
<STRONG>#include</STRONG> &lt;<!WA75><!WA75><!WA75><!WA75><!WA75><!WA75><A HREF="#stl.h">stl.h</A>&gt;
<STRONG>#include</STRONG> &lt;iostream.h&gt;

<EM><STRONG>//</STRONG></EM>
<EM><STRONG>//</STRONG> Let's say you want to put pointers to X into multiple STL containers.</EM>
<EM><STRONG>//</STRONG></EM>
<STRONG>class</STRONG> X <STRONG>{</STRONG>
public:
    X<STRONG>(</STRONG><STRONG>int</STRONG> i<STRONG>)</STRONG> <STRONG>:</STRONG> i_<STRONG>(</STRONG>i<STRONG>)</STRONG> <STRONG>{</STRONG> <STRONG>;</STRONG> <STRONG>}</STRONG>
    X<STRONG>(</STRONG><STRONG>const</STRONG> X<STRONG>&amp;</STRONG> x<STRONG>)</STRONG> <STRONG>:</STRONG> i_<STRONG>(</STRONG>x<STRONG>.</STRONG>i_<STRONG>)</STRONG> <STRONG>{</STRONG> <STRONG>}</STRONG>
    <STRONG>~</STRONG>X<STRONG>(</STRONG><STRONG>)</STRONG> <STRONG>{</STRONG> <STRONG>}</STRONG>
    X<STRONG>&amp;</STRONG> <STRONG>operator</STRONG><STRONG>=</STRONG> <STRONG>(</STRONG><STRONG>const</STRONG> X<STRONG>&amp;</STRONG> x<STRONG>)</STRONG> <STRONG>{</STRONG> i_ <STRONG>=</STRONG> x<STRONG>.</STRONG>i_<STRONG>;</STRONG> <STRONG>}</STRONG>

    <STRONG>int</STRONG> <STRONG>operator</STRONG><STRONG>(</STRONG><STRONG>)</STRONG><STRONG>(</STRONG><STRONG>)</STRONG> <STRONG>const</STRONG> <STRONG>{</STRONG> <STRONG>return</STRONG> i_<STRONG>;</STRONG> <STRONG>}</STRONG>
private:
    <STRONG>int</STRONG> i_<STRONG>;</STRONG>
<STRONG>}</STRONG><STRONG>;</STRONG>

bool <STRONG>operator</STRONG><STRONG>==</STRONG> <STRONG>(</STRONG><STRONG>const</STRONG> X<STRONG>&amp;</STRONG> x1<STRONG>,</STRONG> <STRONG>const</STRONG> X<STRONG>&amp;</STRONG> x2<STRONG>)</STRONG> <STRONG>{</STRONG>
    <STRONG>return</STRONG> x1<STRONG>(</STRONG><STRONG>)</STRONG> <STRONG>==</STRONG> x2<STRONG>(</STRONG><STRONG>)</STRONG><STRONG>;</STRONG>
<STRONG>}</STRONG>

bool <STRONG>operator</STRONG><STRONG>&lt;</STRONG> <STRONG>(</STRONG><STRONG>const</STRONG> X<STRONG>&amp;</STRONG> x1<STRONG>,</STRONG> <STRONG>const</STRONG> X<STRONG>&amp;</STRONG> x2<STRONG>)</STRONG> <STRONG>{</STRONG>
    <STRONG>return</STRONG> x1<STRONG>(</STRONG><STRONG>)</STRONG> <STRONG>&lt;</STRONG> x2<STRONG>(</STRONG><STRONG>)</STRONG><STRONG>;</STRONG>
<STRONG>}</STRONG>

<EM><STRONG>//</STRONG></EM>
<EM><STRONG>//</STRONG> Define a simple wrapper class to put into STL containers. This one</EM>
<EM><STRONG>//</STRONG> simple wraps X.</EM>
<EM><STRONG>//</STRONG></EM>
<STRONG>class</STRONG> XPtrWrapper <STRONG>{</STRONG>
public:
    XPtrWrapper<STRONG>(</STRONG>X<STRONG>*</STRONG> x <STRONG>=</STRONG> 0<STRONG>)</STRONG> <STRONG>:</STRONG> x_<STRONG>(</STRONG>x<STRONG>)</STRONG> <STRONG>{</STRONG> <STRONG>}</STRONG>
    XPtrWrapper<STRONG>(</STRONG><STRONG>const</STRONG> XPtrWrapper<STRONG>&amp;</STRONG> xw<STRONG>)</STRONG> <STRONG>:</STRONG> x_<STRONG>(</STRONG>xw<STRONG>.</STRONG>x_<STRONG>)</STRONG> <STRONG>{</STRONG> <STRONG>}</STRONG>
    <STRONG>~</STRONG>XPtrWrapper<STRONG>(</STRONG><STRONG>)</STRONG> <STRONG>{</STRONG> <STRONG>}</STRONG>
    XPtrWrapper<STRONG>&amp;</STRONG> <STRONG>operator</STRONG><STRONG>=</STRONG> <STRONG>(</STRONG><STRONG>const</STRONG> XPtrWrapper<STRONG>&amp;</STRONG> xw<STRONG>)</STRONG> <STRONG>{</STRONG> x_ <STRONG>=</STRONG> xw<STRONG>.</STRONG>x_<STRONG>;</STRONG> <STRONG>}</STRONG>

    <STRONG>const</STRONG> X<STRONG>*</STRONG> <STRONG>operator</STRONG><STRONG>(</STRONG><STRONG>)</STRONG> <STRONG>(</STRONG><STRONG>)</STRONG> <STRONG>const</STRONG> <STRONG>{</STRONG> <STRONG>return</STRONG> x_<STRONG>;</STRONG> <STRONG>}</STRONG>
    X<STRONG>*</STRONG> <STRONG>operator</STRONG><STRONG>(</STRONG><STRONG>)</STRONG> <STRONG>(</STRONG><STRONG>)</STRONG> <STRONG>{</STRONG> <STRONG>return</STRONG> x_<STRONG>;</STRONG> <STRONG>}</STRONG>
private:
    X<STRONG>*</STRONG> x_<STRONG>;</STRONG>
<STRONG>}</STRONG><STRONG>;</STRONG>

bool <STRONG>operator</STRONG><STRONG>==</STRONG> <STRONG>(</STRONG><STRONG>const</STRONG> XPtrWrapper<STRONG>&amp;</STRONG> xw1<STRONG>,</STRONG> <STRONG>const</STRONG> XPtrWrapper<STRONG>&amp;</STRONG> xw2<STRONG>)</STRONG> <STRONG>{</STRONG>
    <STRONG>return</STRONG> <STRONG>(</STRONG>xw1<STRONG>.</STRONG><STRONG>operator</STRONG><STRONG>(</STRONG><STRONG>)</STRONG><STRONG>(</STRONG><STRONG>)</STRONG> <STRONG>&amp;&amp;</STRONG> xw2<STRONG>.</STRONG><STRONG>operator</STRONG><STRONG>(</STRONG><STRONG>)</STRONG><STRONG>(</STRONG><STRONG>)</STRONG><STRONG>)</STRONG> <STRONG>?</STRONG> <STRONG>*</STRONG>xw1<STRONG>(</STRONG><STRONG>)</STRONG> <STRONG>==</STRONG> <STRONG>*</STRONG>xw2<STRONG>(</STRONG><STRONG>)</STRONG> <STRONG>:</STRONG> false<STRONG>;</STRONG>
<STRONG>}</STRONG>

bool <STRONG>operator</STRONG><STRONG>&lt;</STRONG> <STRONG>(</STRONG><STRONG>const</STRONG> XPtrWrapper<STRONG>&amp;</STRONG> xw1<STRONG>,</STRONG> <STRONG>const</STRONG> XPtrWrapper<STRONG>&amp;</STRONG> xw2<STRONG>)</STRONG> <STRONG>{</STRONG>
    <STRONG>return</STRONG> <STRONG>(</STRONG>xw1<STRONG>(</STRONG><STRONG>)</STRONG> <STRONG>&amp;&amp;</STRONG> xw2<STRONG>(</STRONG><STRONG>)</STRONG><STRONG>)</STRONG> <STRONG>?</STRONG> <STRONG>*</STRONG>xw1<STRONG>(</STRONG><STRONG>)</STRONG> <STRONG>&lt;</STRONG> <STRONG>*</STRONG>xw2<STRONG>(</STRONG><STRONG>)</STRONG> <STRONG>:</STRONG> false<STRONG>;</STRONG>
<STRONG>}</STRONG>

<STRONG>void</STRONG> print<STRONG>(</STRONG><STRONG>const</STRONG> XPtrWrapper<STRONG>&amp;</STRONG> xw<STRONG>)</STRONG> <STRONG>{</STRONG>
    cout <STRONG>&lt;&lt;</STRONG> &quot;<EM> </EM>&quot; <STRONG>&lt;&lt;</STRONG> <STRONG>(</STRONG><STRONG>*</STRONG>xw<STRONG>(</STRONG><STRONG>)</STRONG><STRONG>)</STRONG><STRONG>(</STRONG><STRONG>)</STRONG><STRONG>;</STRONG>
<STRONG>}</STRONG>

<STRONG>int</STRONG> main<STRONG>(</STRONG><STRONG>int</STRONG><STRONG>,</STRONG> <STRONG>char</STRONG><STRONG>*</STRONG><STRONG>[</STRONG><STRONG>]</STRONG><STRONG>)</STRONG> <STRONG>{</STRONG>
    XPtrWrapper bucket<STRONG>[</STRONG>5<STRONG>]</STRONG><STRONG>;</STRONG>
    <STRONG>for</STRONG><STRONG>(</STRONG><STRONG>int</STRONG> i <STRONG>=</STRONG> 0<STRONG>;</STRONG> i <STRONG>&lt;</STRONG> 5<STRONG>;</STRONG> <STRONG>++</STRONG>i<STRONG>)</STRONG> <STRONG>{</STRONG>
        bucket<STRONG>[</STRONG>i<STRONG>]</STRONG> <STRONG>=</STRONG> XPtrWrapper<STRONG>(</STRONG><STRONG>new</STRONG> X<STRONG>(</STRONG>i <STRONG>*</STRONG> i<STRONG>)</STRONG><STRONG>)</STRONG><STRONG>;</STRONG>
    <STRONG>}</STRONG>
    random_shuffle<STRONG>(</STRONG>bucket<STRONG>,</STRONG> bucket <STRONG>+</STRONG> 5<STRONG>)</STRONG><STRONG>;</STRONG>

    list<STRONG>&lt;</STRONG>XPtrWrapper<STRONG>&gt;</STRONG> list1<STRONG>;</STRONG>
    copy<STRONG>(</STRONG>bucket<STRONG>,</STRONG> bucket <STRONG>+</STRONG> 5<STRONG>,</STRONG> 
        back_insert_iterator<STRONG>&lt;</STRONG>list<STRONG>&lt;</STRONG>XPtrWrapper<STRONG>&gt;</STRONG> <STRONG>&gt;</STRONG> <STRONG>(</STRONG>list1<STRONG>)</STRONG>
    <STRONG>)</STRONG><STRONG>;</STRONG>

    cout <STRONG>&lt;&lt;</STRONG> &quot;<EM>List of XPtrWrapper: (</EM>&quot;<STRONG>;</STRONG>
    for_each<STRONG>(</STRONG>list1<STRONG>.</STRONG>begin<STRONG>(</STRONG><STRONG>)</STRONG><STRONG>,</STRONG> list1<STRONG>.</STRONG>end<STRONG>(</STRONG><STRONG>)</STRONG><STRONG>,</STRONG> print<STRONG>)</STRONG><STRONG>;</STRONG>
    cout <STRONG>&lt;&lt;</STRONG> &quot;<EM>)</EM>&quot; <STRONG>&lt;&lt;</STRONG> endl<STRONG>;</STRONG>

    <EM><STRONG>//</STRONG></EM>
    <EM><STRONG>//</STRONG> now put these XPtrWrappers into a set. Note that I can use</EM>
    <EM><STRONG>//</STRONG> greater<XPtrWrapper> since I've defined operator> for it.</EM>
    <EM><STRONG>//</STRONG></EM>
    set<STRONG>&lt;</STRONG>XPtrWrapper<STRONG>,</STRONG> greater<STRONG>&lt;</STRONG>XPtrWrapper<STRONG>&gt;</STRONG> <STRONG>&gt;</STRONG> set1<STRONG>;</STRONG>
    copy<STRONG>(</STRONG>list1<STRONG>.</STRONG>begin<STRONG>(</STRONG><STRONG>)</STRONG><STRONG>,</STRONG> list1<STRONG>.</STRONG>end<STRONG>(</STRONG><STRONG>)</STRONG><STRONG>,</STRONG> 
        insert_iterator<STRONG>&lt;</STRONG>set<STRONG>&lt;</STRONG>XPtrWrapper<STRONG>,</STRONG> greater<STRONG>&lt;</STRONG>XPtrWrapper<STRONG>&gt;</STRONG> <STRONG>&gt;</STRONG> <STRONG>&gt;</STRONG> 
            <STRONG>(</STRONG>set1<STRONG>,</STRONG> set1<STRONG>.</STRONG>begin<STRONG>(</STRONG><STRONG>)</STRONG><STRONG>)</STRONG>
    <STRONG>)</STRONG><STRONG>;</STRONG>

    cout <STRONG>&lt;&lt;</STRONG> &quot;<EM>Set of XPtrWrapper : [</EM>&quot;<STRONG>;</STRONG>
    for_each<STRONG>(</STRONG>set1<STRONG>.</STRONG>begin<STRONG>(</STRONG><STRONG>)</STRONG><STRONG>,</STRONG> set1<STRONG>.</STRONG>end<STRONG>(</STRONG><STRONG>)</STRONG><STRONG>,</STRONG> print<STRONG>)</STRONG><STRONG>;</STRONG>
    cout <STRONG>&lt;&lt;</STRONG> &quot;<EM>]</EM>&quot; <STRONG>&lt;&lt;</STRONG> endl<STRONG>;</STRONG>

    <EM><STRONG>//</STRONG></EM>
    <EM><STRONG>//</STRONG> now put these integers into a deque. </EM>
    <EM><STRONG>//</STRONG></EM>
    deque<STRONG>&lt;</STRONG>XPtrWrapper<STRONG>&gt;</STRONG> deque1<STRONG>;</STRONG>
    copy<STRONG>(</STRONG>list1<STRONG>.</STRONG>begin<STRONG>(</STRONG><STRONG>)</STRONG><STRONG>,</STRONG> list1<STRONG>.</STRONG>end<STRONG>(</STRONG><STRONG>)</STRONG><STRONG>,</STRONG> 
        back_insert_iterator<STRONG>&lt;</STRONG>deque<STRONG>&lt;</STRONG>XPtrWrapper<STRONG>&gt;</STRONG> <STRONG>&gt;</STRONG> <STRONG>(</STRONG>deque1<STRONG>)</STRONG>
    <STRONG>)</STRONG><STRONG>;</STRONG>

    cout <STRONG>&lt;&lt;</STRONG> &quot;<EM>Deque of XPtrWrapper : (</EM>&quot;<STRONG>;</STRONG>
    for_each<STRONG>(</STRONG>deque1<STRONG>.</STRONG>begin<STRONG>(</STRONG><STRONG>)</STRONG><STRONG>,</STRONG> deque1<STRONG>.</STRONG>end<STRONG>(</STRONG><STRONG>)</STRONG><STRONG>,</STRONG> print<STRONG>)</STRONG><STRONG>;</STRONG>
    cout <STRONG>&lt;&lt;</STRONG> &quot;<EM>)</EM>&quot; <STRONG>&lt;&lt;</STRONG> endl<STRONG>;</STRONG>

    <STRONG>return</STRONG> 0<STRONG>;</STRONG>
<STRONG>}</STRONG>
</PRE>
<hr>

<p>

And the output is:
<hr>

<pre>
List of XPtrWrapper: ( 4 0 16 1 9)
Set of XPtrWrapper : [ 16 9 4 1 0]
Deque of XPtrWrapper : ( 4 0 16 1 9)
</pre>
<p>
<hr>
<!WA76><!WA76><!WA76><!WA76><!WA76><!WA76><a href=#pointers>Pointers and STL</a> 
<br>
<!WA77><!WA77><!WA77><!WA77><!WA77><!WA77><a href=#index>Back to index</a>
<hr>

<a name=pointer_deriv><h2>
    How do I store derived objects in STL containers?</h2>
</a>
Consider a CAD application: you have lots of objects on a screen that all
derive from the same base object. How would you store the derived objects
in an STL container? Let's assume all derived objects have a set of
virtual functions (and use RTTI if you have it). I've done it 3 different
ways:
<p>
<ol>
<li> <!WA78><!WA78><!WA78><!WA78><!WA78><!WA78><a href=#seq_pointer>
    Store the pointer itself in the container</a>. You have to explicitly
    deallocate the memory later on of course. Also, not all STL
    implementations seem to handle storage of pointers uniformly, so I
    would suggest using a wrapper shown below.
<li> <!WA79><!WA79><!WA79><!WA79><!WA79><!WA79><a href=#pointer_deriv_hard>
    Hard-coded wrapper</a> that takes a pointer to the base class
<li> <!WA80><!WA80><!WA80><!WA80><!WA80><!WA80><a href=#pointer_deriv_template>
    Templated pointer wrapper</a> that takes a pointer to the base class
</ol>
<p>

<a name=pointer_deriv_hard><h3>
    Hard-coded wrapper that takes a pointer to the base class</h3></a>

The following example shows 2 classes derived from <tt>Base</tt>, 
<tt>derived1</tt> and <tt>derived2</tt> and a wrapper
<tt>BaseWrapper</tt>. The wrapper class <b>assumes</b> that the base class
provides a virtual <tt>clone</tt> facility and does the memory
management. 
<p>
<em>Note: After the new'd Base derivative is passed to the 
wrapper, it owns it and deletes it in the destructor</em>.
<p>
<hr>

<PRE>
<STRONG>#include</STRONG> &lt;<!WA81><!WA81><!WA81><!WA81><!WA81><!WA81><A HREF="#stl.h">stl.h</A>&gt;
<STRONG>#include</STRONG> &lt;string.h&gt;
<STRONG>#include</STRONG> &lt;iostream.h&gt;

<EM><STRONG>//</STRONG></EM>
<EM><STRONG>//</STRONG> abstract base class</EM>
<EM><STRONG>//</STRONG></EM>
<STRONG>class</STRONG> Base <STRONG>{</STRONG>
public: