"solartimba" <ka*****@hotmail.com> wrote in message
news:1a**************************@posting.google.c om...
I am using the advance() auxiliary STL iterator function to move a map
iter forward, but I read that there is no check to keep the iter from
pointing past the end of the map. How is this problem normally
solved?
By doing the checking yourself. With a container which has
no random-access iterators (and thus no 'iterator arithmetic')
such as 'std::map', the obvious solution of e.g. checking the
'advance count' against 'end() - iterator' is not possible, so
something like 'std::distance' is needed to find out how far
you can go. But this function requires successively incrementing
the iterator, which is what advance does, so you've at least
doubled the processing. So imo one should only use 'advance'
with containers which support random access iterators where the
typically more efficient iterator arithmetic is possible (unless
your logic can somehow otherwise generate the 'max' advance value
beforehand.)
But anyway, just for fun, here's a 'bounds-checked advance' function:
#include <iostream>
#include <iterator>
#include <map>
#include <string>
template<typename Iter, typename Dist>
void checked_advance(Iter& it, // iterator to advance
const Iter& last, // 'highest' allowed iterator value
Dist dist) // requested advance distance
{
Dist maxdist(std::distance(it, last));
std::advance(it, dist > maxdist ? maxdist : dist);
}
int main()
{
std::map <int, std::string> numbers;
numbers[1] = "one";
numbers[2] = "two";
numbers[3] = "three";
numbers[4] = "four";
numbers[5] = "five";
std::map<int, std::string>::size_type count = 10;
std::map<int, std::string>::iterator it(numbers.begin()),
en(numbers.end()),
orig(it);
for(std::map<int, std::string>::size_type i = 0;
i < count; ++i, it = orig)
{
std::cout << "advance " << i << ":\n";
checked_advance(it, en, i);
std::cout << (it != en ? it->second : "[end]") << '\n';
}
return 0;
}
Output:
advance 0:
one
advance 1:
two
advance 2:
three
advance 3:
four
advance 4:
five
advance 5:
[end]
advance 6:
[end]
advance 7:
[end]
advance 8:
[end]
advance 9:
[end]
One could also change 'checked_advance()' to return the actual
distance advanced, if that value is useful.
HTH,
-Mike
