Sorting STL list by custom comperator

[danalovesc]

Hey guys, i looked at some sorting examples, and i thought i jsut had to build a comperator which returns bool, in order to keep my list sorted by that value.

here's the relevent code:

bool CityHall::EmployeeComp (Employee *e1,Employee *e2) const
{
if (e1->GetID()<e2->GetID())
	return true;
return false;
}

char** CityHall::PrintAllWorkersByLastName() const
{
	if (m_cityHallWorkers->size()==0)
		 return NULL;
	 m_cityHallWorkers->sort(EmployeeComp);
.
.
.

and i get:

Error 1 error C3867: 'CityHall::Empl oyeeComp': function call missing argument list; use '&CityHall::Emp loyeeComp' to create a pointer to member.
Error 2 error C2660: 'std::list<_Ty> ::sort' : function does not take 1 arguments.

I know the comperator takes to args, but in all the examples they didn't send any, what should i send it if i should?!

Thank you very much for ur help and time

[hype261]

Here is a page on the sort function for a list. The sort method can take a custom comparitor for sorting the list, but why would you? You sort your list by the < operator just like the default sorting method does. Why don't you just define the < operator for your Employee class?

http://www.cplusplus.com/reference/stl/list/sort/

[newb16]

I know the comperator takes to args.

I don't get it. What does comparator takes to them? Anyway, the error message is quite clear - "use '&CityHall::Emp loyeeComp' to create a pointer to member." -
will

m_cityHallWorkers->sort(&CityHall::EmployeeComp);

work?

[Banfa]

Your problem is that you have tried to call a member function of CityHall. However to call a member function you need to have an instance of the class to call it on and there is no way to pass that instance into list::sort.

There is a solution:

Remove EmployeeComps dependency on having an object of type CityHall. This is easy make it a static member function which do not require an object to all and pass it to list::sort through the class CityHall::Emplo yeeComp . An alternative is to move the method out of the CityHall class entirely but then that puts it into global scope which is probably not good.

[danalovesc]

hype261 --> I will try defining < for my class, i was just wondering why doesn't it work, and what should be the syntax for it to work

newb16-->
when u do that u get:


Error 2 error C2064: term does not evaluate to a function taking 2 arguments

and it jumps a file called "xutility" where it show the error in it

[danalovesc]

@ Banfa --> how do i turn it into static, should i just add
"static" before the func decleration at cityhall.h?

[Banfa]

Yes .

[danalovesc]

and then i get:


Error 1 error C2272: 'EmployeeComp' : modifiers not allowed on static member functions

same code as before, just added static at the h file

[Banfa]

Like I said you will need to access it through the class so

m_cityHallWorke rs->sort(CityHall: :EmployeeComp);

[danalovesc]

this is how it is written in the h file:
static bool EmployeeComp (Employee *e1,Employee* e2) const;

still gives the same error when i did:

m_cityHallWorke rs->sort(CityHall: :EmployeeComp);

Error 1 error C2272: 'EmployeeComp' : modifiers not allowed on static member functions

[newb16]

remove const modifier.

[danalovesc]

well if only i knew where do i even modify a const here..

anyway i think i'll just go with definind < operator

thank you all!!

if someone can still point what the problem is i'll be happy to hear!:)

[hype261]

Remove the const at the end of this statement.

static bool EmployeeComp (Employee *e1,Employee* e2) const;

[danalovesc]

thnx alot, this works!