No match for operator > when using with a function

Question

I have overloaded the following greater than operator:

bool operator > (Person & a, Person & b)
{
   //firstname is a string data type
   return (a.FirstName > b.FirstName);      
}

Which works fine if I have something like the following:

Person a = myPersonA;
Person b = myPersonB;

return myPersonA > myPersonB;

However, within my Person class I have defined a Person getByID(int id) function, which returns an instance of a Person, by the given ID. If I try to use my operator with the returned values from this function like this:

bool whosGreater = listPeople.getById(1) > listPeople.getById(2);

I get "Error: no match for operator >(Person&, Person&)"

but if I do the following it works fine:

Person a = listPeople.getById(1);
Person b = listPeople.getById(2);
bool whosGreater = a > b;

Is there something I am not seeing here? It seems to me it should work.

PS: This is for a homework, so I could really get away with declaring the variables and assign them what the functions return and get away with it, but I would like to know what is happening so that I can learn. I have tried googling it, but I can't come up with the right question.

Answer 1

The returned value from a function is a temporary value, not a "normal" Person object. Temporary values can only be passed in as const parameter references, so changing your parameters to const references should work well;

bool operator > (const Person & a, const Person & b)
{
   //firstname is a string data type
   return (a.FirstName > b.FirstName);      
}

Answer 2

Make it:

bool operator > (Person const & a, Person const & b)
{
   //firstname is a string data type
   return (a.FirstName > b.FirstName);      
}

Normal references cannot bind to temporary objects (as the one returned by getById()). And you are not mutating passed in objects, so make the references const.