Question

I have a self-define class using template, like this:

template<class T>
class foo 
{
    public:
    T a;
    bool operator<(const foo<T> &f);
    //other functions...
}

template<class T>
bool foo<T>::operator<(const foo<T> &f)
{return a - f.a;}

Now, I new some foos and give them value, then I want to sort this array:

foo<int>* fp = new foo<int>[3];
//give each element value
sort(fp, fp+3); //run-time error

When I use sort function, I got a run-time error.

Did I do something wrong? Please help me.

Answer 1

Probably like this:

template<class T>
bool foo<T>::operator<(const foo<T> &f)
{return a < f.a;}

std::sort requires that the comparison function (your less-than operator in this case) defines a strict weak ordering. Your implementation does not, because it is possible for both A < B and B < A to be true.