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Full Discussion: question about function object
Top Forums Programming question about function object Post 302497011 by homeboy on Wednesday 16th of February 2011 04:24:34 AM
Old 02-16-2011
question about function object
I have a code as following:
Code:
#include <iostream>
#include <algorithm>
#include <list>
using namespace std;

//the class Nth is a predicates
class Nth{
    private:
         int nth;
         int count;
    public:
         Nth(int n):nth(n),count(0){}
         bool operator()(int){
             return ++count == nth;
        }
};

int main()
{
       list<int> lst;
 
       for(int i=1;i<10;i++)
           lst.push_back(i);

       copy(lst.begin(),lst.end(),ostream_iterator<int>(cout," "));
       cout<<endl;

       list<int>::iterator pos = remove_if(lst.begin(),lst.end(),Nth(3));
       lst.erase(pos,lst.end());

       copy(lst.begin(),lst.end(),ostream_iterator<int>(cout," "));
       cout<<endl;
}

the output is:
1 2 3 4 5 6 7 8 9
1 2 4 5 7 8 9
Not as I expect:
1 2 3 4 5 6 7 8 9
1 2 4 5 6 7 8 9
The book explained:
This happens because the usual implementation of the algorithm copies the predicate internally during the algorithm:
Code:
template<class ForwIter,class Predicate>
ForwIter std::remove_if(ForwIter beg,ForeIter end,Predicate op)
{
       ForwIter pos = find_if(beg,end,op);
       if(pos == end)
            return beg;
       else{
            ForwIter next = beg;
            return remove_copy_if(++next,end,beg,op);
       }
}

and it says it is possible to avoid this surprise behavior like this:
Code:
template<class ForwIter,class Predicate>
ForwIter std::remove_if(ForwIter beg,ForeIter end,Predicate op)
{
       while(beg != end && !op(*beg))
            beg++;
       if(pos == end)
            return beg;
       else{
            ForwIter next = beg;
            return remove_copy_if(++next,end,beg,op);
       }
}

I don't know what the difference between the two implementations is.I think i was able to understand the first one,but why the second one works fine?I thought they were the same.Any help will be appreciated.
 

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LEARN ABOUT DEBIAN

paps

PAPS(1) 						      General Commands Manual							   PAPS(1)

NAME

       paps - UTF-8 to PostScript converter using Pango

SYNOPSIS

       paps [options] files...

DESCRIPTION

       paps reads a UTF-8 encoded file and generates a PostScript language rendering of the file. The rendering is done by creating outline curves
       through the pango ft2 backend.

OPTIONS

       These programs follow the usual GNU command line syntax, with long options starting with  two  dashes  (`-').   A  summary  of  options	is
       included below.

       --landscape
	      Landscape output. Default is portrait.

       --columns=cl
	      Number of columns output. Default is 1.
	      Please notice this option isn't related to the terminal length as in a "80 culums terminal".

       --font=desc
	      Set the font description. Default is Monospace 12.

       --rtl  Do right to left (RTL) layout.

       --paper ps
	      Choose paper size. Known paper sizes are legal, letter and A4. Default is A4.

       Postscript points
	      Each postscript point equals to 1/72 of an inch. 36 points are 1/2 of an inch.

       --bottom-margin=bm
	      Set bottom margin. Default is 36 postscript points.

       --top-margin=tm
	      Set top margin. Default is 36 postscript points.

       --left-margin=lm
	      Set left margin. Default is 36 postscript points.

       --right-margin=rm
	      Set right margin. Default is 36 postscript points.

       --gutter-width=gw
	      Set gutter width. Default is 40 postscript points.

       --help Show summary of options.

       --header
	      Draw page header for each page.

       --markup
	      Interpret the text as pango markup.

       --lpi  Set the lines per inch. This determines the line spacing.

       --cpi  Set the characters per inch. This is an alternative method of specifying the font size.

       --stretch-chars
	      Indicates  that  characters  should be stretched in the y-direction to fill up their vertical space. This is similar to the texttops
	      behaviour.

AUTHOR

       paps was written by Dov Grobgeld <dov.grobgeld@gmail.com>.

       This manual page was written by Lior Kaplan <kaplan@debian.org>, for the Debian project (but may be used by others).

								  April  17, 2006							   PAPS(1)
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