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David Levy
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 Posted: Thu Dec 23, 2004 9:13 pm Post subject: Replace a loop with an STL algorithm? |
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Item 43 of Effective STL suggests that I prefer alogorithm calls to
hand-written loops. The suggestion has been very helpful in general, but
this loop has me stumped.
The code below works fine with the hand-written loop.
void Board::FindCellsToDel(void)
{
set<int> toBeDel;
//code to populate set omitted
for (set<int>::const_iterator i=toBeDel.begin(); i!=toBeDel.end(); ++i)
DelCell(*i);
}
void Board::DelCell(int itemID)
{
//code to delete items omitted
}
I want to use the for_each algorithm. If DelCell were a free function rather
than a member function of Board I could write:
for_each(toBeDel.begin(), toBeDel.end(), DelCell);
but DelCell needs access to much private data in class Board.
I don't understand how to call the member function and pass the deferenced
iterator. I have been experimenting with combinations of mem_fun_ref and
bind2nd, but nothing seems to compile.
Can anyone straighten me out?
Thanks,
David
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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dave_abrahams
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Carl Barron
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| Posted: Fri Dec 24, 2004 9:51 am Post subject: Re: Replace a loop with an STL algorithm? |
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David Levy <dml.pubic.nospam (AT) sbcglobal (DOT) net> wrote:
| Quote: | Item 43 of Effective STL suggests that I prefer alogorithm calls to
hand-written loops. The suggestion has been very helpful in general, but
this loop has me stumped.
The code below works fine with the hand-written loop.
void Board::FindCellsToDel(void)
{
set<int> toBeDel;
//code to populate set omitted
for (set<int>::const_iterator i=toBeDel.begin(); i!=toBeDel.end(); ++i)
DelCell(*i);
}
void Board::DelCell(int itemID)
{
//code to delete items omitted
}
I want to use the for_each algorithm. If DelCell were a free function rather
than a member function of Board I could write:
for_each(toBeDel.begin(), toBeDel.end(), DelCell);
the short answer is: |
std::for_each(toBeDel.begin(),toBeDel.end(),
std::bind1st(std::mem_fun(&Board::DelCel),this));
first create an functor that takes a Board * and and int as arguments
say F1, and then create a second functor that takes a int as its
qrgument, calling a instance of F1 with the first parameter fixed as the
this pointer. [Looks worse than it really is] what you end up with is
a functor that is called with an int argument executing
(this->*DelCel)(x), for an int x.
if this is 'too messy'
class MyFunc
{
Board *p_board;
public:
explicit MyFunc(Board *a):p_board(a){}
void operator () (int x)
{
(p_boara->*DelCel)(x);
}
};
is what you are creating with the std functions mem_fun,bind1st above.
std::for_each(toBeDel.begin(),toBeDel.end(),MyFunc(this)); is what the
for_each reduces to.
Hope this helps some
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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