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<Title>Function Objects</Title>
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<H1>Function Objects</H1>
<Table CellPadding=0 CellSpacing=0 width=100%>
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<TD Align=left><Img src = "functors.gif" Alt="" WIDTH = "194" HEIGHT = "38" ></TD>
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<TR>
<TD Align=left VAlign=top><b>Category</b>: functors</TD>
<TD Align=right VAlign=top><b>Component type</b>: overview</TD>
</TR>
</Table>
<h3>Summary</h3>
A <i>Function Object</i>, or <i>Functor</i> (the two terms are synonymous)
is simply any object that can be called as if it is a function.
An ordinary function is a function object, and so is a function pointer;
more generally, so is an object of a class that defines
<tt>operator()</tt>.
<h3>Description</h3>
The basic function object concepts are <A href="Generator.html">Generator</A>,
<A href="UnaryFunction.html">Unary Function</A>, and <A href="BinaryFunction.html">Binary Function</A>: these describe,
respectively, objects that can be called as <tt>f()</tt>, <tt>f(x)</tt>, and
<tt>f(x,y)</tt>. (This list could obviously be extended to <i>ternary function</i>
and beyond, but, in practice, no STL algorithms require function
objects of more than two arguments.) All other function object
concepts defined by the STL are refinements of these three.
<P>
Function objects that return <tt>bool</tt> are
an important special case.
A <A href="UnaryFunction.html">Unary Function</A> whose return type is <tt>bool</tt> is called a
<A href="Predicate.html">Predicate</A>, and a <A href="BinaryFunction.html">Binary Function</A> whose return type is
<tt>bool</tt> is called a <A href="BinaryPredicate.html">Binary Predicate</A>.
<P>
There is an important distinction, but a somewhat subtle one, between
function objects and <i>adaptable function objects</i>. <A href="#1">[1]</A> In general, a
function object has restrictions on the type of its argument. The
type restrictions need not be simple, though: <tt>operator()</tt> may be
overloaded, or may be a member template, or both. Similarly, there
need be no way for a program to determine what those restrictions are.
An adaptable function object, however, does specify what the argument
and return types are, and provides nested <tt>typedef</tt>s so that those
types can be named and used in programs. If a type <tt>F0</tt> is a model of
<A href="AdaptableGenerator.html">Adaptable Generator</A>, then it must define
<tt>F0::result_type</tt>. Similarly, if <tt>F1</tt> is a model of
<A href="AdaptableUnaryFunction.html">Adaptable Unary Function</A> then it must define
<tt>F1::argument_type</tt> and <tt>F1::result_type</tt>, and if <tt>F2</tt> is a model
of <A href="AdaptableBinaryFunction.html">Adaptable Binary Function</A> then it must define
<tt>F2::first_argument_type</tt>, <tt>F2::second_argument_type</tt>, and
<tt>F2::result_type</tt>.
The STL provides base classes <tt><A href="unary_function.html">unary_function</A></tt> and
<tt><A href="binary_function.html">binary_function</A></tt> to simplify the definition of
<A href="AdaptableUnaryFunction.html">Adaptable Unary Functions</A> and <A href="AdaptableBinaryFunction.html">Adaptable Binary Functions</A>. <A href="#2">[2]</A>
<P>
Adaptable function objects are important because they can be used by
<i>function object adaptors</i>: function objects that transform or
manipulate other function objects. The STL provides many different
function object adaptors, including <tt><A href="unary_negate.html">unary_negate</A></tt> (which returns
the logical complement of the value returned by a particular
<A href="AdaptablePredicate.html">AdaptablePredicate</A>), and <tt><A href="unary_compose.html">unary_compose</A></tt> and
<tt><A href="binary_compose.html">binary_compose</A></tt>, which perform composition of function object.
<P>
Finally, the STL includes many different predefined function
objects, including arithmetic operations
(<tt><A href="plus.html">plus</A></tt>, <tt><A href="minus.html">minus</A></tt>, <tt><A href="times.html">multiplies</A></tt>, <tt><A href="divides.html">divides</A></tt>, <tt><A href="modulus.html">modulus</A></tt>,
and <tt><A href="negate.html">negate</A></tt>), comparisons (<tt><A href="equal_to.html">equal_to</A></tt>, <tt><A href="not_equal_to.html">not_equal_to</A></tt>
<tt><A href="greater.html">greater</A></tt>, <tt><A href="less.html">less</A></tt>, <tt><A href="greater_equal.html">greater_equal</A></tt>, and <tt><A href="less_equal.html">less_equal</A></tt>),
and logical operations (<tt><A href="logical_and.html">logical_and</A></tt>, <tt><A href="logical_or.html">logical_or</A></tt>, and
<tt><A href="logical_not.html">logical_not</A></tt>). It is possible to perform very sophisticated
operations without actually writing a new function object, simply
by combining predefined function objects and function object
adaptors.
<h3>Examples</h3>
Fill a <tt><A href="Vector.html">vector</A></tt> with random numbers. In this example, the function object
is simply a function pointer.
<pre>
<A href="Vector.html">vector</A><int> V(100);
<A href="generate.html">generate</A>(V.begin(), V.end(), rand);
</pre>
<P>
Sort a <tt><A href="Vector.html">vector</A></tt> of <tt>double</tt> by magnitude, <i>i.e.</i> ignoring the elements' signs.
In this example, the function object is an object of a user-defined
class.
<pre>
struct less_mag : public <A href="binary_function.html">binary_function</A><double, double, bool> {
bool operator()(double x, double y) { return fabs(x) < fabs(y); }
};
<A href="Vector.html">vector</A><double> V;
...
<A href="sort.html">sort</A>(V.begin(), V.end(), less_mag());
</pre>
<P>
Find the sum of elements in a <tt><A href="Vector.html">vector</A></tt>. In this example, the function
object is of a user-defined class that has local state.
<pre>
struct adder : public <A href="unary_function.html">unary_function</A><double, void>
{
adder() : sum(0) {}
double sum;
void operator()(double x) { sum += x; }
};
<A href="Vector.html">vector</A><double> V;
...
adder result = <A href="for_each.html">for_each</A>(V.begin(), V.end(), adder()); <A href="#3">[3]</A>
cout << "The sum is " << result.sum << endl;
</pre>
<P>
Remove all elements from a <tt><A href="List.html">list</A></tt> that are greater than 100 and
less than 1000.
<pre>
<A href="List.html">list</A><int> L;
...
<A href="List.html">list</A><int>::iterator new_end =
<A href="remove_if.html">remove_if</A>(L.begin(), L.end(),
<A href="binary_compose.html">compose2</A>(<A href="logical_and.html">logical_and</A><bool>(),
<A href="binder2nd.html">bind2nd</A>(<A href="greater.html">greater</A><int>(), 100),
<A href="binder2nd.html">bind2nd</A>(<A href="less.html">less</A><int>(), 1000)));
L.erase(new_end, L.end());
</pre>
<h3>Concepts</h3>
<UL>
<LI>
<A href="Generator.html">Generator</A>
<LI>
<A href="UnaryFunction.html">Unary Function</A>
<LI>
<A href="BinaryFunction.html">Binary Function</A>
</UL>
<UL>
<LI>
<A href="Predicate.html">Predicate</A>
<LI>
<A href="BinaryPredicate.html">Binary Predicate</A>
</UL>
<UL>
<LI>
<A href="AdaptableGenerator.html">Adaptable Generator</A>
<LI>
<A href="AdaptableUnaryFunction.html">Adaptable Unary Function</A>
<LI>
<A href="AdaptableBinaryFunction.html">Adaptable Binary Function</A>
<LI>
<A href="AdaptablePredicate.html">Adaptable Predicate</A>
<LI>
<A href="AdaptableBinaryPredicate.html">Adaptable Binary Predicate</A>
</UL>
<h3>Types</h3>
<UL>
<LI>
<tt><A href="plus.html">plus</A></tt>
<LI>
<tt><A href="minus.html">minus</A></tt>
<LI>
<tt><A href="times.html">multiplies</A></tt> (formerly called <tt>times</tt>)
<LI>
<tt><A href="divides.html">divides</A></tt>
<LI>
<tt><A href="modulus.html">modulus</A></tt>,
<LI>
<tt><A href="negate.html">negate</A></tt>
<LI>
<tt><A href="equal_to.html">equal_to</A></tt>
<LI>
<tt><A href="not_equal_to.html">not_equal_to</A></tt>
<LI>
<tt><A href="greater.html">greater</A></tt>
<LI>
<tt><A href="less.html">less</A></tt>
<LI>
<tt><A href="greater_equal.html">greater_equal</A></tt>
<LI>
<tt><A href="less_equal.html">less_equal</A></tt>,
<LI>
<tt><A href="logical_and.html">logical_and</A></tt>
<LI>
<tt><A href="logical_or.html">logical_or</A></tt>
<LI>
<tt><A href="logical_not.html">logical_not</A></tt>
<LI>
<tt><A href="subtractive_rng.html">subtractive_rng</A></tt>
</UL>
<UL>
<LI>
<tt><A href="identity.html">identity</A></tt>
<LI>
<tt><A href="project1st.html">project1st</A></tt>
<LI>
<tt><A href="project2nd.html">project2nd</A></tt>
<LI>
<tt><A href="select1st.html">select1st</A></tt>
<LI>
<tt><A href="select2nd.html">select2nd</A></tt>
</UL>
<UL>
<LI>
<tt><A href="unary_function.html">unary_function</A></tt>
<LI>
<tt><A href="binary_function.html">binary_function</A></tt>
</UL>
<UL>
<LI>
<tt><A href="unary_compose.html">unary_compose</A></tt>
<LI>
<tt><A href="binary_compose.html">binary_compose</A></tt>
<LI>
<tt><A href="unary_negate.html">unary_negate</A></tt>
<LI>
<tt><A href="binary_negate.html">binary_negate</A></tt>
<LI>
<tt><A href="binder1st.html">binder1st</A></tt>
<LI>
<tt><A href="binder2nd.html">binder2nd</A></tt>
<LI>
<tt><A href="pointer_to_unary_function.html">pointer_to_unary_function</A></tt>
<LI>
<tt><A href="pointer_to_binary_function.html">pointer_to_binary_function</A></tt>
</UL>
<h3>Functions</h3>
<UL>
<LI>
<tt><A href="unary_compose.html">compose1</A></tt>
<LI>
<tt><A href="binary_compose.html">compose2</A></tt>
<LI>
<tt><A href="unary_negate.html">not1</A></tt>
<LI>
<tt><A href="binary_negate.html">not2</A></tt>
<LI>
<tt><A href="binder1st.html">bind1st</A></tt>
<LI>
<tt><A href="binder2nd.html">bind2nd</A></tt>
<LI>
<tt><A href="ptr_fun.html">ptr_fun</A></tt>
</UL>
<h3>Notes</h3>
<P><A name="1">[1]</A>
The reason for the name "adaptable function object" is that
adaptable function objects may be used by function object adaptors.
<P><A name="2">[2]</A>
The <tt><A href="unary_function.html">unary_function</A></tt> and <tt><A href="binary_function.html">binary_function</A></tt> bases are
similar to the <tt><A href="input_iterator.html">input_iterator</A></tt>, <tt><A href="output_iterator.html">output_iterator</A></tt>,
<tt><A href="forward_iterator.html">forward_iterator</A></tt>, <tt><A href="bidirectional_iterator.html">bidirectional_iterator</A></tt>, and
<tt><A href="random_access_iterator.html">random_access_iterator</A></tt> bases: they are completely empty,
and serve only to provide type information.
<P><A name="3">[3]</A>
This is an example of how to use function objects; it is not
the recommended way of calculating the sum of elements in a vector.
The <tt><A href="accumulate.html">accumulate</A></tt> algorithm is a better way of calculating a sum.
<h3>See also</h3>
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