c++ std::set insert causing segmentation fault

Question

All-

I can't quite figure out why this piece of code is resulting in a segmentation fault... Any help would be great.

#include <iostream>
#include <set>

using namespace std;

class A{
public:
    int _x;

    A(int x){
        _x = x;
    }
};

bool fncomp(A a1, A a2){
    return a1._x < a2._x;
}

int main(){
    bool(*fn_pt)(A,A) = fncomp;

    set<A, bool(*)(A,A)> testSet;
    for(int i=0; i<10; i++){
        cout << i << endl;
        A a(i);
        testSet.insert(a);
    }
}

The output is:

0
1
Segmentation Fault

Answer 1

Well, look at your code. You declared a function fncomp, but are you really using that function anywhere? You initialize fn_pt with it, but fn_pt is not used anywhere. Doesn't it seem strange to you? How do you expect your testSet object to know that you want it to use your fncomp as the comparator, if you never ask your set object to use that function?

You declared your set testSet with an ordinary function pointer type bool(*)(A,A) as a comparator type. That's the type of the comparator. Now, you have to pass the actual value of the comparator to your set object through the constructor parameter

set<A, bool(*)(A,A)> testSet(fn_pt);

or

set<A, bool(*)(A,A)> testSet(fncomp);

(You don't really need that intermediate pointer fn_pt).

You forgot to do that, and the set object used the default constructor argument value for the comparator, which in this case is a null pointer. So, every time your testSet object tries to compare two elements, it performs a function call through a null pointer. No wonder it crashes.