From 2329502722539006882 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: f78e5,eaf633a917dfd8af,start X-Google-Attributes: gidf78e5,public X-Google-ArrivalTime: 2003-11-06 22:38:28 PST Path: archiver1.google.com!news2.google.com!news.maxwell.syr.edu!kibo.news.demon.net!mutlu.news.demon.net!demon!mail2news.demon.co.uk!devnull From: alexvn@bigfoot.com ("Alex Vinokur") Newsgroups: comp.std.c++ Subject: Passing functor-parameters in for_each() and transform() Date: Fri, 7 Nov 2003 06:38:26 +0000 (UTC) Lines: 156 Approved: fjh@cs.mu.oz.au (Fergus Henderson , moderator of comp.std.c++) Message-ID: X-Trace: mail2news.demon.co.uk 1068187106 2441 10.0.0.1 (7 Nov 2003 06:38:26 GMT) X-Complaints-To: abuse@demon.net NNTP-Posting-Date: Fri, 7 Nov 2003 06:38:26 +0000 (UTC) X-Received: from mulga.cs.mu.oz.au ([128.250.1.22]) by news.demon.co.uk with esmtp (Exim 4.12) id 1AI0GG-0000dE-00 for mail2news@news.news.demon.net; Fri, 07 Nov 2003 06:38:25 +0000 X-Received: from localhost (localhost [[UNIX: localhost]]) by mulga.cs.mu.OZ.AU id RAA11858; Fri, 7 Nov 2003 17:38:20 +1100 (EST) X-Authentication-Warning: mulga.cs.mu.OZ.AU: fjh set sender to devnull@stump.algebra.com using -f X-Path: comp-std-cpp-robomod!not-for-mail X-Robomod: STUMP, ichudov@algebra.com (Igor Chudov) X-Delivered-To: std-c++@ucar.edu X-Newsgroups: comp.std.c++ X-Reply-To: "Alex Vinokur" X-Orig-NNTP-Posting-Host: pop03-2-ras5-p17.barak.net.il (212.150.100.17) X-Orig-X-Trace: news.uni-berlin.de 1067963156 43348736 212.150.100.17 (16 [79865]) X-Priority: 3 X-MSMail-Priority: Normal X-Newsreader: Microsoft Outlook Express 6.00.2800.1106 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106 X-Spam-Status: No, hits=-4.3 required=5.0 tests=BAYES_10,PRIORITY_NO_NAME version=2.55 X-Spam-Checker-Version: SpamAssassin 2.55 (1.174.2.19-2003-05-19-exp) Xref: archiver1.google.com comp.std.c++:370 Functor-parameters in the for_each() and transform() algorithms are passed by value. Might it make sense to have also algorithms for_each2() and transform2() which pass Functor-parameters by non-const reference? Here is some program which demonstrates probable necessity in such forms of for_each() and transform(). ====== C++ code : File foo.cpp : BEGIN ====== #include #include #include #include #include using namespace std; #define MAX_VALUE(x,y) ((x) > (y) ? (x) : (y)) class Foo { friend ostream& operator<< (ostream& os, const Foo& ioo); private : int sum_; vector vect_; public : Foo (vector x, vector y) : sum_ (0) { const unsigned max_size = MAX_VALUE (x.size (), y.size ()); x.resize(max_size); y.resize(max_size); vect_.resize(max_size); // ------ transform ------ transform (x.begin(), x.end(), y.begin(), vect_.begin(), *this); cout << "sum_ = " << sum_ << " (after transform)" << endl; cout << (*this) << endl; // It seems that it is impossible to change sum_ through transform because // 1) Foo copy constructor is called; // 2) transform() doesn't return Functor-object // ----------------------- // ------ for-each ------- for_each (vect_.begin(), vect_.end(), *this); cout << "sum_ = " << sum_ << " (after for_each-1)" << endl; cout << (*this) << endl; // sum_ is not changed because // 1) Foo copy constructor is called; // 2) Foo value returned is not used // ----------------------- // ------ for-each ------- *this = for_each (vect_.begin(), vect_.end(), *this); cout << "sum_ = " << sum_ << " (after for_each-2)" << endl; cout << (*this) << endl; // sum_ is changed because // * Foo value returned is used to change *this // ----------------------- // ------ for-each ------- *this = for_each (vect_.begin(), vect_.end(), *this); cout << "sum_ = " << sum_ << " (after for_each-3)" << endl; cout << (*this) << endl; // sum_ is changed because // * Foo value returned is used to change *this // ----------------------- } int operator() (int n1, int n2) { sum_ += n1; sum_ += n2; return (n1 + n2); } void operator() (int n) { sum_ += n; } }; ostream& operator<< (ostream& os, const Foo& ioo) { ostringstream oss; oss << "{ vect_ = "; copy (ioo.vect_.begin(), ioo.vect_.end(), ostream_iterator (oss, " ")); return os << oss.str() << "}" << endl; } int main () { const int a1[] = {1, 2, 3, 4, 5}; const int a2[] = {10, 20, 30, 40, 50, 60, 70}; const vector v1 (a1, a1 + sizeof(a1)/sizeof(*a1)); const vector v2 (a2, a2 + sizeof(a2)/sizeof(*a2)); { Foo (v1, v2);} return 0; } ====== C++ code : File foo.cpp : END ======== ====== Compilation & Run : BEGIN ====== $ g++ -v [---omitted---] gcc version 3.3.1 (cygming special) $ g++ foo.cpp $ a sum_ = 0 (after transform) // sum_ not changed { vect_ = 11 22 33 44 55 60 70 } sum_ = 0 (after for_each-1) // sum_ not changed { vect_ = 11 22 33 44 55 60 70 } sum_ = 295 (after for_each-2) // sum_ changed { vect_ = 11 22 33 44 55 60 70 } sum_ = 590 (after for_each-3) // sum_ changed { vect_ = 11 22 33 44 55 60 70 } ====== Compilation & Run : END ======== -- ===================================== Alex Vinokur mailto:alexvn@connect.to http://mathforum.org/library/view/10978.html news://news.gmane.org/gmane.comp.lang.c++.perfometer ===================================== --- [ comp.std.c++ is moderated. 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