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Hopefully my last STL dumb question.

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Here's a toy example. Say I have a std::vector of points. that, themselves, contain an index 'i'. I map pointers to those points together with pointers to those points index. Shouldn't the map remain sorted with the new index if I modify it AFTER having inserted them in the list ? My functor is ok, and if I switch the index switching code BEFORE the insertion in the map, it works fine.
#include <iostream>
#include <vector>
#include <map>
#include <algorithm>

struct ltind
  bool operator()(const int* a, const int* b) const
	return (*a) < (*b);

struct Point {
	int i;
	int x;
	int y;
	Point(int _i, int _x, int _y) { i= _i; x = _x; y=_y;}

int main() {
	std::vector<Point> test;
	std::multimap<int*, Point*, ltind> zeList;
        zeList.insert(std::pair<int*, Point*>(&(test[0].i),&test[0]));
	zeList.insert(std::pair<int*, Point*>(&(test[1].i),&test[1]));
	zeList.insert(std::pair<int*, Point*>(&(test[2].i),&test[2]));
	test[0].i = 3;
	test[1].i = 2;
	test[2].i = 1;
	for (std::multimap<int*, Point*, ltind>::iterator it = zeList.begin(); it != zeList.end(); ++it) {
		std::cout << "  [" << *it->first << ", (" << (*it).second->x << "," << (*it).second->y << ")]" << std::endl;

	return 0;
While the output should be [1, (3,3)] [2, (2,2)] [3, (1,1)] It is actually [3, (1,1)] [2, (2,2)] [1, (3,3)] I must be doing something wrong yet I can't find it. [edited by - xMcBaiNx on May 29, 2004 2:53:59 PM]

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std::map and std::multimap expect the logical value of the indexers to be immutable, which is why they declare the value type of the key to be const. When you use a pointer as a key type, you subvert the logical constness of the key. Don''t do that. If you need to change the key in a std::map, delete the node and reinsert it.

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Hmm.. Guess I had skipped the const keyword eh?

The problem however, since it is a multimap, is that there might be a lot of entries with the same key. That makes it impossible to find the right one back unless i loop trough those with the said key to compare the pointers. That makes it kindof slow...

Well I might do this. We''ll see.


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