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<HTML><!-- -- Copyright (c) 1996-1999 -- Silicon Graphics Computer Systems, Inc. -- -- Permission to use, copy, modify, distribute and sell this software -- and its documentation for any purpose is hereby granted without fee, -- provided that the above copyright notice appears in all copies and -- that both that copyright notice and this permission notice appear -- in supporting documentation. Silicon Graphics makes no -- representations about the suitability of this software for any -- purpose. It is provided "as is" without express or implied warranty. -- -- Copyright (c) 1994 -- Hewlett-Packard Company -- -- Permission to use, copy, modify, distribute and sell this software -- and its documentation for any purpose is hereby granted without fee, -- provided that the above copyright notice appears in all copies and -- that both that copyright notice and this permission notice appear -- in supporting documentation. 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It is provided "as is" without express or implied warranty. -- --><Head><Title>Function Objects</Title><!-- Generated by htmldoc --></HEAD><BODY TEXT="#000000" LINK="#006600" ALINK="#003300" VLINK="#7C7F87" BGCOLOR="#FFFFFF"><A HREF="/"><IMG SRC="/images/common/sgilogo_small.gif" ALT="SGI Logo" WIDTH="80" HEIGHT="72" BORDER="0"></A><P><!--end header--><BR Clear><H1>Function Objects</H1><Table CellPadding=0 CellSpacing=0 width=100%><TR><TD Align=left><Img src = "functors.gif" Alt="" WIDTH = "194" HEIGHT = "38" ></TD><TD Align=right><Img src = "overview.gif" Alt="" WIDTH = "194" HEIGHT = "38" ></TD></TR><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 functionobjects of more than two arguments.) All other function objectconcepts defined by the STL are refinements of these three.<P>Function objects that return <tt>bool</tt> arean 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, betweenfunction objects and <i>adaptable function objects</i>. <A href="#1">[1]</A> In general, afunction object has restrictions on the type of its argument. Thetype restrictions need not be simple, though: <tt>operator()</tt> may beoverloaded, or may be a member template, or both. Similarly, thereneed be no way for a program to determine what those restrictions are.An adaptable function object, however, does specify what the argumentand return types are, and provides nested <tt>typedef</tt>s so that thosetypes 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 modelof <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 ormanipulate other function objects. The STL provides many differentfunction object adaptors, including <tt><A href="unary_negate.html">unary_negate</A></tt> (which returnsthe 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 sophisticatedoperations without actually writing a new function object, simplyby combining predefined function objects and function objectadaptors.<h3>Examples</h3>Fill a <tt><A href="Vector.html">vector</A></tt> with random numbers. In this example, the function objectis 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-definedclass.<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 functionobject 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 andless 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 thatadaptable 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 aresimilar 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 notthe 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><!-- start footer --><!-- Footer Begins --><STYLE TYPE="text/css"><!--TD.footer, TD.footer A{ font-family: Arial, helvetica, sans-serif; font-size: 8pt;}A.home {font-family: Arial, helvetica, sans-serif;}--></STYLE><P><A CLASS="home" HREF="index.html">STL Home</A><P><TABLE WIDTH="600" CELLPADDING="0" CELLPADDING="0" BORDER="0"> <TR> <TD ALIGN="RIGHT" CLASS="footer"><A HREF="/company_info/terms.html" TARGET="_top">terms of use</A> | <A HREF="/company_info/privacy.html" TARGET="_top">privacy policy</A></TD> <TD ALIGN="CENTER" CLASS="footer"> | </TD> <TD ALIGN="LEFT" CLASS="footer"><A HREF="/cgi-bin/feedback/" TARGET="_top">contact us</A></TD> </TR><TR> <TD ALIGN="RIGHT" CLASS="footer">Copyright © 1993-2003 Silicon Graphics, Inc. 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