Question about using for_each

[Yan]

Hi,

I don't seem to be able to use for_each if it should replace a 'for'
loop in a method (constructor in my case) and inside that 'for' loop a
class member variable is being accessed. The presence of this member
variable prevents me from using a static or global method to be passed
as a third parameter to for_each, and mem_fun doesn't seem to work for
me either as I am not going to execute a method of an iterator but
pass an iterator as a parameter. I am not sure that explanation makes
a lot of sense, so below is a sample code. I would like to be able to
replace the 'for' loop inside C class constructor with a for_each.
Please don't pay attention to what is actually happening with these
two vectors of ints, it's just for illustration purposes. Thanks.

-----------------------------------------------------------

#include <iostream>
#include <algorithm>
#include <vector>

using namespace std;

typedef vector<int> vectorInts;
typedef vector<int>::iterator vectorIntsIter;

class C {
public:

C(pair<vectorIntsIter, vectorIntsIter> aPair) {
for (vectorIntsIter iter = aPair.first; iter != aPair.second; +
+iter) {
// do something that involves member variable
// which seems to prevent using a static method
// such as for example this:
ints.push_back(*iter + 10);
}
}

void printAllElements() {
// here a static method 'print' is successfully used
for_each(ints.begin(), ints.end(), print);
}

private:
static void print(int number) {
cout << number << endl;
}


private:
vectorInts ints;
};

int main() {
vectorInts ints;

ints.push_back(0);
ints.push_back(1);
ints.push_back(2);
ints.push_back(3);

pair<vectorIntsIter, vectorIntsIter> aPair(ints.begin(),
ints.end());
C c(aPair);
c.printAllElements();

return 0;
}

 

[int2str]

Hi,

I don't seem to be able to use for_each if it should replace a 'for'
loop in a method (constructor in my case) and inside that 'for' loop a
class member variable is being accessed. The presence of this member
variable prevents me from using a static or global method to be passed
as a third parameter to for_each, and mem_fun doesn't seem to work for
me either as I am not going to execute a method of an iterator but
pass an iterator as a parameter. I am not sure that explanation makes
a lot of sense, so below is a sample code. I would like to be able to
replace the 'for' loop inside C class constructor with a for_each.
Please don't pay attention to what is actually happening with these
two vectors of ints, it's just for illustration purposes. Thanks.

[snipped original code using for()]

How about something like this:

#include <iostream>
#include <iterator>
#include <vector>
#include <algorithm>

using namespace std;

class push_back_double
{
public:
push_back_double( vector<int> & ints ) : p_ints( &ints ) {}

void operator() ( const int & i )
{
p_ints->push_back( i*2 );
}

private:
vector<int> *p_ints;
};

class Test
{
public:
Test( vector<int>::iterator first, vector<int>::iterator last )
{
for_each( first, last, push_back_double(ints));
}

void Dump()
{
copy( ints.begin(), ints.end(),
ostream_iterator<int>(cout,"\n"));
}

private:
vector<int> ints;
};

int main()
{
vector<int> test;
test.push_back( 1 );
test.push_back( 2 );
test.push_back( 3 );

Test t( test.begin(), test.end() );
t.Dump();
}

Though I'm pretty sure this could be done cleaner with mem_fun...
I'll try that next.

Cbeers,
Andre

 

[Barry Ding]

Hi,

I don't seem to be able to use for_each if it should replace a 'for'
loop in a method (constructor in my case) and inside that 'for' loop a
class member variable is being accessed. The presence of this member
variable prevents me from using a static or global method to be passed
as a third parameter to for_each, and mem_fun doesn't seem to work for
me either as I am not going to execute a method of an iterator but
pass an iterator as a parameter. I am not sure that explanation makes
a lot of sense, so below is a sample code. I would like to be able to
replace the 'for' loop inside C class constructor with a for_each.
Please don't pay attention to what is actually happening with these
two vectors of ints, it's just for illustration purposes. Thanks.

[snipped original code using for()]

How about something like this:

#include <iostream>
#include <iterator>
#include <vector>
#include <algorithm>

using namespace std;

class push_back_double
{
public:
push_back_double( vector<int> & ints ) : p_ints( &ints ) {}

void operator() ( const int & i )
{
p_ints->push_back( i*2 );
}

private:
vector<int> *p_ints;

};

class Test
{
public:
Test( vector<int>::iterator first, vector<int>::iterator last )
{
for_each( first, last, push_back_double(ints));
}

void Dump()
{
copy( ints.begin(), ints.end(),
ostream_iterator<int>(cout,"\n"));
}

private:
vector<int> ints;

};

int main()
{
vector<int> test;
test.push_back( 1 );
test.push_back( 2 );
test.push_back( 3 );

Test t( test.begin(), test.end() );
t.Dump();

}

Though I'm pretty sure this could be done cleaner with mem_fun...
I'll try that next.

Cbeers,
Andre

the operator() in the functor is always better a const member function

 

[Greg Herlihy]

I don't seem to be able to use for_each if it should replace a 'for'
loop in a method (constructor in my case) and inside that 'for' loop a
class member variable is being accessed. The presence of this member
variable prevents me from using a static or global method to be passed
as a third parameter to for_each, and mem_fun doesn't seem to work for
me either as I am not going to execute a method of an iterator but
pass an iterator as a parameter. I am not sure that explanation makes
a lot of sense, so below is a sample code. I would like to be able to
replace the 'for' loop inside C class constructor with a for_each.
Please don't pay attention to what is actually happening with these
two vectors of ints, it's just for illustration purposes. Thanks.

-----------------------------------------------------------

#include <iostream>
#include <algorithm>
#include <vector>

using namespace std;

typedef vector<int> vectorInts;
typedef vector<int>::iterator vectorIntsIter;

class C {
public:

C(pair<vectorIntsIter, vectorIntsIter> aPair) {
for (vectorIntsIter iter = aPair.first; iter != aPair.second; +
+iter) {
// do something that involves member variable
// which seems to prevent using a static method
// such as for example this:
ints.push_back(*iter + 10);
}
}

A std::for_each() is not the best choice for implementing the loop in the
constructor. I would use std::copy() instead:

copy( aPair.first, aPair.second, back_inserter(ints) );

Greg