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<Head>
<Title>construct</Title>
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<H1>construct</H1>
<Table CellPadding=0 CellSpacing=0 width=100%>
<TR>
<TD Align=left><Img src = "allocators.gif" Alt="" WIDTH = "194" HEIGHT = "38" ></TD>
<TD Align=right><Img src = "function.gif" Alt="" WIDTH = "194" HEIGHT = "38" ></TD>
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<TR>
<TD Align=left VAlign=top><b>Category</b>: allocators</TD>
<TD Align=right VAlign=top><b>Component type</b>: function</TD>
</TR>
</Table>
<h3>Prototype</h3>
<pre>
template <class T1, class T2> void construct(T1* p, const T2& value);
</pre>
<h3>Description</h3>
In C++, the operator <tt>new</tt> allocates memory for an object and then
creates an object at that location by calling a constructor. Occasionally,
however, it is useful to separate those two operations. <A href="#1">[1]</A> If <tt>p</tt> is
a pointer to memory that has been allocated but not initialized,
then <tt>construct(p, value)</tt> creates an object of type <tt>T1</tt> at the location
pointed to by <tt>p</tt>. The argument <tt>value</tt> is passed as an argument to
<tt>T1</tt>'s constructor.
<h3>Definition</h3>
Defined in the standard header <A href="memory">memory</A>, and in the nonstandard
backward-compatibility header <A href="algo.h">algo.h</A>. The <tt>construct</tt> algorithm
is no longer part of the C++ standard; it was present in early drafts,
and it is retained in this implementation for backward compatibility.
<h3>Requirements on types</h3>
<UL>
<LI>
<tt>T1</tt> must have a constructor that takes a single argument of
type <tt>T2</tt>.
</UL>
<h3>Preconditions</h3>
<UL>
<LI>
<tt>p</tt> is a valid pointer that points to a region of memory whose
size is at least <tt>sizeof(T1)</tt>.
<LI>
The memory pointed to by <tt>p</tt> is uninitialized. That is, no
object has been constructed at the location <tt>p</tt>.
</UL>
<h3>Complexity</h3>
<h3>Example</h3>
<pre>
double* dp = (double*) malloc(sizeof(double));
construct(dp, 3);
assert(*dp == 3);
</pre>
<h3>Notes</h3>
<P><A name="1">[1]</A>
In particular, <tt>construct</tt>, along with other low-level memory
allocation primitives, is used to implement container classes.
<h3>See also</h3>
<A href="Allocators.html">Allocators</A>, <tt><A href="destroy.html">destroy</A></tt>, <tt><A href="uninitialized_copy.html">uninitialized_copy</A></tt>,
<tt><A href="uninitialized_fill.html">uninitialized_fill</A></tt>, <tt><A href="uninitialized_fill_n.html">uninitialized_fill_n</A></tt>,
<tt><A href="raw_storage_iterator.html">raw_storage_iterator</A></tt>
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