Re: Modifying the value of key of a map.

Mark P <>
Tue, 30 May 2006 06:42:26 GMT
Milind wrote:


I have a map of the type:

std::map<A, B> mymap_A;
std::map<X, A> mymap_X;

where A, B and X are user defined types (i.e. my own clases)

At the beging of the program i do a insert on the maps. Later, at some
point i come across a situation where the value of some member of A
changes. e.g:

class A {
int a_member;

when i did an insert a_member was 2; now it is supposed to be 4.


my first, and absolutely naive approach was:

std::map<A,B>::iterator i = mymap.find(valueofkey);
i->first = 4;

But this is not allowed as i->first is read only.

Now, the possible solution for this would be to either create a new
pair (and necessarily copy everything in the value part... which is
going to be very costly) and delete old and insert new pair.

Am I missing something too obvious??

Any pointers??


The Key type of a map is necessarily a constant. If you want to
"modify" it, the most direct approaches are to delete and re-insert as
you suggested, or use a layer of indirection and let your key have a
pointer to some other data which you can modify freely.


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