Confused about stl's set
http://www.cplusplus.com/reference/stl/set/
Reading in the reference.
Compare: Comparison class: A class that takes two arguments of the same type as the container elements and returns a bool.
My lecturer told a friend of mine, who told me set's Compare class could return an int
+1 if a < b
0 if a = b
-1 if a> b
(or maybe it was the other way around, changing , -1 on the top, and 1 on the bottom)
However, I can't find any help on this subject. That leads me to believe this information could be false. Any help?
Reading in the reference.
Compare: Comparison class: A class that takes two arguments of the same type as the container elements and returns a bool.
My lecturer told a friend of mine, who told me set's Compare class could return an int
+1 if a < b
0 if a = b
-1 if a> b
(or maybe it was the other way around, changing , -1 on the top, and 1 on the bottom)
However, I can't find any help on this subject. That leads me to believe this information could be false. Any help?
Last edited on
| Compare class could return an int. |
That's true to this point since int is convertible to bool. Reference is always right, professors not, and your friend might misunderstood something. Java (from languages which I know of) uses this int method thing in compareTo() method.
For std::set's compare, it expects it to return a bool whether the first is > then second (or something similar). If you return an int, you could return it like that, but if you convert it to a bool, then you would get the wrong information, since 0 = false and non-zero = true, you would get true if they are not equal, and false if they are, creating a possibly weird sorting method.
Last edited on
Thanks a lot for the help, ROmai and firedraco.
still, i wonder how in the world is set going to know when two elements are equal, only having operator < overloaded . Does it compares attribute to attribute? What if the attributes are pointers to memory?(i have the intuition that i should overload operator = , in that case).
still, i wonder how in the world is set going to know when two elements are equal, only having operator < overloaded . Does it compares attribute to attribute? What if the attributes are pointers to memory?(i have the intuition that i should overload operator = , in that case).
Last edited on
It doesn't care whether they are equal (since std::set can only have unique elements).
| how in the world is set going to know when two elements are equal, only having operator < overloaded |
!(a < b) && !(b < a)
| how in the world is set going to know when two elements are equal, only having operator < overloaded |
It doesn't! Set checks for equivalence, not equality and you need to understand the difference. For the case of a std::set<int> it makes no difference. In that case equivalence and equality will mean the same thing. For a set that contains user defined types the difference can be critical. That is a subject worth reading up on when you can make the time.
Topic archived. No new replies allowed.