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Alex Vinokur · Guest

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 <vector>
#include <iostream>
#include <sstream>
#include <algorithm>
#include <iterator>
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
public :
Foo (vector<int> x, vector<int> 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 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 const vector<int> 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 (AT) connect (DOT) to
http://mathforum.org/library/view/10978.html
news://news.gmane.org/gmane.comp.lang.c++.perfometer
=====================================

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Andrey Tarasevich · Guest

Alex Vinokur wrote:

DrPizza · Guest

""Alex Vinokur"" <alexvn (AT) bigfoot (DOT) com> wrote