Function Overloading Example Showing Error in C++

Question

I am trying to run an example of Function Overloading in C++. But Its showing me following error

 function.cpp(21): error C2084: function 'double abs(double)' already has a body include\math.h(495) : see previous definition of 'abs' function.cpp(26): error C2084: function 'long abs(long)' already has a body include\stdlib.h(467) : see previous definition of 'abs'

Program

#include<iostream> using namespace std; int abs(int i); double abs(double d); long abs(long l); int main() { cout << abs(-10); cout << abs(-20.2); cout << abs(-30L); return 0; } int abs(int i) { cout << "Using int \n"; return i<0 ? -i:i; } double abs(double d) { cout << "Using Double \n"; return d<0.0 ?-d:d; } long abs(long l) { cout << "Using Long\n"; return l<0?-l:l; }

I have copied the same code as given in book C++ Complete Reference , Fourth Edition by Herbert Schildt

You should avoid using namespace std to make sure you aren't getting one of the std::abs functions. And put your abs in a namespace to make sure you aren't getting any of the C standard library abs functions. — juanchopanza
– juanchopanza, Commented Nov 22, 2013 at 10:39
Herbert Schildt is notorius in certain quarters for writing books that describe a language that is almost but not quite the same as C++. — john
– john, Commented Nov 22, 2013 at 10:40
I removed using namespace std; line and now its showing more errors. But I am using modern compiler so it should work with using namespace std — Akash Sharma
– Akash Sharma, Commented Nov 22, 2013 at 10:42
@AkashSharma No it shouldn't. Just because using namespace std; works in one program does not mean it will work in another. You should realise (it gets said often enough) that using namespace std; is dangerous, if you don't understand the danger then you should not use it. — john
– john, Commented Nov 22, 2013 at 10:43
@AkashSharma Please describe the new errors, I would guess that you need to replace cout with std::cout. — john
– john, Commented Nov 22, 2013 at 10:43

ForEveR · Accepted Answer · 2013-11-22 11:07:19Z

9

~~You should remove using namespace std, since there is already abs function in standard-library in cmath header, or you can wrap your own functions into some namespace.~~

So, since you have errors from math.h - you should use another names of functions, or trying to wrap functions into your own namespace.

edited Nov 22, 2013 at 11:07

answered Nov 22, 2013 at 10:39

ForEveR

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7 Comments

Akash Sharma Over a year ago

I removed using namespace line and added new header <cmath> but no solution.

ForEveR Over a year ago

@AkashSharma you shouldn't include cmath. You must qualify cout with std:: prefix.

Akash Sharma Over a year ago

I tried std:: prefix but still same errors. hlinker.com/stackoverflow/errors.png

ForEveR Over a year ago

@AkashSharma ok. you should trying to wrap your functions into namespace, since there is abs in global ns.

ForEveR Over a year ago

@user2708138 it's not strange. It's not reglamented which header should be include in another header in standard library.

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user2768863 · Accepted Answer · 2013-11-22 13:21:01Z

#include<iostream> using namespace std; int f(int d) { return d<0.0 ?-d:d; } double f(double d) { return d<0.0 ?-d:d; }`enter code here` int main() { cout<< f(2); cout<< f(2.3); return 0; }

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