Re: STL save reference inside container
Rares Vernica wrote:
John Harrison wrote:
Rares Vernica wrote:
Hi,
How can I save a reference inside a container?
For example I have:
map<string, unsigned> X;
I would like to be able to save a reference to a position inside X.
For a vector, the reference would be the index inside the vector. For
map the reference would be the key (in this case a string). The
problem is that I don't want to store the string as it might be too
long. I would like to have a smaller size reference, like an unsigned
or a pointer.
Thanks,
Ray
Use an iterator, that's what they're for.
typedef map<string, unsigned> su_map;
su_map X;
X["abc"] = 1;
X["def"] = 2;
X["ghj"] = 3;
X["wer"] = 4;
su_map::iterator i = X.find("def");
i is now a 'reference' to the position of ("def", 2) inside the map.
You really need to read up on iterators in the STL, yu're not going to
get very far without them.
john
Hi,
I tried the iterators. Here is the next phase of my problem.
Assume I get references inside map<stirng, unsigned>. Now, I need to
store this references inside a set, that is set<reference_type>.
If reference_type is map<...>::iterator then the problem is that
elements of reference_type cannot be stored in a set as they need to be
comparable with "<".
So, the full problem is that I need to store references to a map<...> in
a set<...>.
Thanks a lot,
Ray
When you need a set or map, but the elements of the set or map doesn't
have a predefined operator< (or the predefined one isn't what you want)
all you have to do is define your own.
It looks complicated but really is quite simple once you know the form.
Here's an example, you can look up the details in a book on the STL.
#include <map>
#include <set>
#include <string>
#include <functional>
using namespace std;
typedef map<string, unsigned> su_map;
typedef su_map::iterator su_map_iter;
struct su_map_iter_less_then : public binary_function<su_map_iter,
su_map_iter, bool>
{
bool operator()(su_map_iter x, su_map_iter y) const
{
return x->first < y->first;
}
};
int main()
{
su_map m;
m["abc"] = 1;
m["def"] = 2;
set<su_map_iter, su_map_iter_less_then> s;
s.insert(m.find("abc"));
s.insert(m.find("def"));
}
The class su_map_iter_less_then is what defines 'less than' for the set
s in the function main. It's an example of what's called a function
object or functor.
john