inserting on a pointer to std::set

Question

Inserting values on Set with and without pointer behaves differently. What is wrong with this code? The first for loop is insertion on set using pointer and the second is without pointer. But apart from that everything else is exactly same.

#include <iostream>
#include <set>

using namespace std;

typedef struct{
    int local;
    int global;
}Node;
Node CreateNode(int global, int local) {
    Node n; n.local=local; n.global=global;
    return(n);
}
bool compare(Node a, Node b){
    a.global < b.global;
}

int main()
{
   std::pair<std::set<Node>::iterator,bool> itr;

   set<Node,bool(*)(Node,Node)> *graph_set_pointer = new set<Node,bool(*)(Node,Node)>(compare);

   for(int i=10;i>0;--i){
     itr = graph_set_pointer->insert(CreateNode(i,i));
     cout << "global = " << i << " local = " << i ;
     cout << " inserted_global = " << (*itr.first).global << endl;
   }

   cout << "Number of items in pointer set = " << graph_set_pointer->size() << "\n\n";

   set<Node,bool(*)(Node,Node)> graph_set_object(compare);
   for(int i=10;i>0;--i){
     itr = graph_set_object.insert(CreateNode(i,i));
     cout << "global = " << i << " local = " << i ;
     cout << " inserted_global = " << (*itr.first).global << endl;
   }

   cout << "Number of items in non pointer set = " << graph_set_object.size() <<"\n";

   delete graph_set_pointer;

   return 0;
}

Output:

global = 10 local = 10 inserted_global = 10
global = 9 local = 9 inserted_global = 9
global = 8 local = 8 inserted_global = 8
global = 7 local = 7 inserted_global = 7
global = 6 local = 6 inserted_global = 7
global = 5 local = 5 inserted_global = 7
global = 4 local = 4 inserted_global = 7
global = 3 local = 3 inserted_global = 7
global = 2 local = 2 inserted_global = 7
global = 1 local = 1 inserted_global = 7
Number of items in pointer set = 4

global = 10 local = 10 inserted_global = 10
global = 9 local = 9 inserted_global = 9
global = 8 local = 8 inserted_global = 8
global = 7 local = 7 inserted_global = 7
global = 6 local = 6 inserted_global = 6
global = 5 local = 5 inserted_global = 5
global = 4 local = 4 inserted_global = 4
global = 3 local = 3 inserted_global = 3
global = 2 local = 2 inserted_global = 2
global = 1 local = 1 inserted_global = 1
Number of items in non pointer set = 10

Which platform are you working in? I am seeing sane behavior with g++ 4.7.3. — R Sahu, Apr 21 '14 at 18:23
[Is this what you're supposed to see](http://ideone.com/17tuZs)? That is copied verbatim from your code to ideone.com — WhozCraig, Apr 21 '14 at 18:25
i missed the return in the comparator. Try that on http://www.compileonline.com/ — mvairavan, Apr 21 '14 at 19:33

Adam Wulkiewicz · Accepted Answer · 2014-04-21T19:27:18.970

0

The problem may be the compare() function which doesn't return the result of comparison. Try:

bool compare(Node a, Node b){
    return a.global < b.global;
}

In the future you may consider passing -Wall parameter to GCC (4.7.3). The compiler will warn you about such mistakes. clang (3.2) warns about them by default, VC++ (2010) reports an error.

edited Apr 21 '14 at 19:27

answered Apr 21 '14 at 19:20

Adam Wulkiewicz

2,068
18
24

How stupid of me. It worked after adding return, but its still odd that it worked for non pointer and dint work for pointer. Any idea why that might be? – mvairavan Apr 21 '14 at 19:31
@mvairavan - If no return is given on a function that expects a value to be returned, the behavior is `undefined`. – PaulMcKenzie Apr 21 '14 at 19:40

inserting on a pointer to std::set

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