Re: STL algorithm member function problem
In article <fZX6h.44807$uv5.305674@twister1.libero.it>, Alberto Ganesh
Barbati <AlbertoBarbati@libero.it> wrote:
Carl Barron ha scritto:
struct remove_by_rule
{
std::vector<std::string> &fileList;
remove_by_rule(std::vector<std::string> &a):fileList(a){}
void operator () (RulePtr p)
{
fileList.erase
(
std::remove_if(fileList.begin(),fileList.end(),*p),
fileList.end()
);
}
};
int main()
{
// ...
std::for_each(m_rules.begin(),m_rules.end(),
remove_rule(fileList) );
// ...
}
easier to read in my opinion and avoids explicitly naming iterator
types for iterators local to the loops actually performed. Holding
a non constant reference in the functor remove_by_rule assures the
same vector will be used no matter how many copies of remove_by_rule
are preformed and those copies will be 'cheap'. It also avoids copying
after the for_each() is performed.
Except that it doesn't work because *p is an abstract class and that
it's very inefficient because strings may be moved many times. It's
better to keep remove_if/erase as the "outer loop" rather than keep it
in the "inner loop".
A solution using boost/tr1 function<bool(string const &)> avoids any
slicing discusion, but I am not so sure there is ever a conversion from
Rule & to Rule in the expansion of remove_if. [shared_ptr::operator *()
returns a Rule & not a Rule].
As far as copying strings, there is the same # of copies performed
whether a home made loop is done or for_each is performed since the
container is held by reference, The # of copies from std::remove_if is
the same if the erase is performed immediately or delayed by using an
iterator pair and erasing upon exit from for_each. If the copying is a
problem use a list<string> and list<string>::remove_if() member
function and no copies will be performed and erasure is automatic.
I don't see it being seriously inefficient since references are passed
and not containers and the code is merely replacing a homemade loop
with with explicit iterator types and template type deduction not to
clutter the code with those nested types.
By the way it compiles with CW9.
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