comparision function in stl::sort()

class node{
public:
  unsigned long long int value;
  int index;
};


bool comp2(node& a,node& b){
  if(a.value < b.value) { return true; }
  return false;
}

vector <node*>llis开发者_StackOverflowt,rlist;
sort(llist.begin(),llist.end(),comp2);

Above code was giving me some weired error that is too in some other lines(places latter in code), but when i changed comp2 function to following all error disappeared .

bool comp2(node* a,node* b){
  assert(a && b && "comp2 - null argument");
  if(a->value < b->value){ return true; }
  return false;
}

Any rationale on this ?

ERROR:/usr/include/c++/4.4/bits/stl_algo.h|124|error: invalid initialization of reference of type ‘node&’ from expression of type ‘node* const’|

If this(bellow) works then above should also work

using namespace std;

void rep(int& a,int& b){

int c;
c=a;
a=b;
b=c;

}

int main(void){

int a=3,b=4;

rep(a,b);
cout<<a<<" "<<b;
return 0;
}

You have defined a std::vector of node *. Therefore, all the elements are node *, and all operations that the vector performs will be on node *. You can't give sort a comparison function of a different type.

The rational is that your vector contains values of type node*, so the comparison functions needs to compare values of type node*.

What you probably meant to say in the first place was vector<node>. If you wanted pointers to nodes, then the second comparison function is reasonable.