C++: A misguided case of function overloading

Question

I wrote and executed the following code in c++:

    int calc(int x=10,int y=100);int a,b,s,p,m;void main()
{clrscr();cout<<"enter the two numbers:";cin>>a>>b;s=calc(a,b);p=calc(a);`m=calc();cout<<"\n sum:"<<s;cout<<"\n product:"<<p;cout<<"\n subtraction:"<<m;getch();}
int calc(int a)
{int t;b=100;t=a*b;return t;}
int calc(int a,int b)
{int t;t=a+b;return t;}
int calc()
{int t;t=b-a;return t;}

I See that only one function is being called and is giving the correct output. For example: 3 and 4 will give 7, but from multiply it will be 101. I am not very good with c++ concepts. An explanation would be useful. Regards.

If `main` would also see declaration of `calc(int)` and `calc()`, you would have ambiguous call with `calc(int = 10, ,int = 100)` — Jarod42, Jun 24 '16 at 10:10
From the way your question is worded now, it is completely unclear what your problem is (and why formatting should have anything to do with it). — MikeMB, Jul 09 '16 at 11:24
You provided default values. So, if you just pass one value how does it know to use the single param version or the two param version with the second defaulted. — Andrew Truckle, Jul 09 '16 at 12:22

score 1 · Accepted Answer · answered Jun 24 '16 at 10:08

1

Point is

s=calc(a,b); p=calc(a); m=calc(); All match function

int calc(int a, int b)
  {
  int t;
  t=a+b;
 return t;
  }

Because it has defined default values, in case you don's specify input:

int calc(int x=10, int y=100);

It means that if you use

calc(1);

It will use

calc(1, 100);

BTW, this does not even compile on VisualStudio 2015, because with error:

error C2668: 'calc': ambiguous call to overloaded function

answered Jun 24 '16 at 10:08

MaciekGrynda

583
2
13

Thanks a lot. Actually this was asked by one of my students. Thank you for clearing the concept – Sky's On Ur Head Jun 24 '16 at 10:11
But from MSDN : _If two candidates are judged to be equally good, preference goes to a candidate that does not have optional parameters for which arguments were omitted in the call. This is a consequence of a general preference in overload resolution for candidates that have fewer parameters_ Which is different than the behaviour described in the question. Do you have any idea why? REference [http://stackoverflow.com/questions/2674417/conflicting-overloaded-methods-with-optional-parameters] – Avantika Saini Jun 24 '16 at 10:16
This was not compiled with VS so VS features don't matter – Irminsul Jun 24 '16 at 10:21
@Benoit - it was example Edit. I'm not smarter than people designing VS and it's compiler, so If they say it's wrong it's most likely wrong. And they give explanation. – MaciekGrynda Jun 24 '16 at 10:26
@AvantikaSaini Link you provided is for C#. C++ has different rules http://stackoverflow.com/questions/9943596/function-overloading-two-functions-only-differ-by-a-default-parameter – MaciekGrynda Jun 24 '16 at 10:26
@MaciekGrynda oops! sorry my bad – Avantika Saini Jun 24 '16 at 10:29
@MaciekGrynda : my comment was adressed to AvantikaSaini because his link came from msdn. I didn't mean to be agressive. An didnt notice your last remark, neither the fact the link was about c#. My bad. – Irminsul Jun 24 '16 at 10:34
I dont see why people have downvoted my question.. :( .. I see so many good answers and explainations.. – Sky's On Ur Head Jun 24 '16 at 10:44
I don't know but you should generally at least indent properly your code, and leave one blanck line bewteen your different function/classes if there are some. – Irminsul Jun 24 '16 at 11:03

score 0 · Answer 2 · answered Jun 24 '16 at 10:05

0

because you gave default values. When you call calc(a) with a=3, your prgramm actually runs calc(3,100)

answered Jun 24 '16 at 10:05

Irminsul

171
9

score 0 · Answer 3 · answered Jun 24 '16 at 10:46

0

Code is so strange at my first sight.

First of all, you should know global a = 3, b = 4 all the time. both parameters are provided the first time call calc so the result is 7. the second time, only one paramter provided, so a = 3, b = 100 and the last time no paramater, a = 10, b = 100.

int calc(int x = 10, int y = 100)
{
    int t;
    t = x + y;
    return t;
}

this may be easier to understand?

answered Jun 24 '16 at 10:46

Z.Ding

7
2

Thanks. It was actually written by a guy who actually thought he was quite expert at messing with logic. Before giving any answers to him I just wanted to be sure of the answer myself . That's why i put the question on my behalf and not his.The end result turned out to be pretty good! – Sky's On Ur Head Jun 24 '16 at 12:13

C++: A misguided case of function overloading

3 Answers3