cannot sort structure !

i am unable to compile this code..can any 1 help

Code:

---

#include <cstdio>
#include <cmath>
#include <algorithm>
using namespace std;

struct fraction
{
        int num,den;
       
        fraction(){}
        fraction(int a,int b) {insert(a,b);}
       
        void insert(int a,int b)
        {
                int g=__gcd(a,b);
                if(g!=0) {a/=g;b/=g;} // reducing to lowest terms !
                if(b<0)  {a=-a;b=-b;} // changing the sign !
                num=a;den=b;
        }
        bool operator <(fraction A)
        {
                return (num*A.den-den*A.num<0);
        }
        bool operator >(fraction A)
        {
                return (num*A.den-den*A.num>0);
        }
        bool operator ==(fraction A)
        {
                return (num*A.den-den*A.num==0);
        }
};

int main()
{
        fraction a[100];
        sort(a,a+100);
        return 0;
}

---

Thank you !

If you were unable to compile the code, then presumably there was some message involved.

Code:

---

bool operator <(const fraction &A) const

---

changing to the above works .but donno why ?

Because you can't call non-const functions on constant or temporary objects. (Temporary would be the key here, I'm guessing.)

It might depend on the implementation. You can look it up from the error, where it happens in particular. With GCC (first) error occurs in this function:

Code:

---

  template<typename _Tp>
    inline const _Tp&
    __median(const _Tp& __a, const _Tp& __b, const _Tp& __c)

---

As you can notice your object is passed by const reference to a function that uses your operator<.

In general, it is absolutely necessary that comparing objects doesn't change them (imagine objects were changing while you are sorting them!), and C++ uses the const keyword to enforce that this requirement is met.