C++ Code with abs() does not compile

Question

The code is below. The code does not compile on an online compiler, and I have no idea why. It is short and pretty self-explanatory, please look below for details.

#include <iostream>
#include <cmath> 

using namespace std;
int N;

int distance(int a, int b){
    if(abs(a-b) > N/2){
        return N - abs(a-b);
    }
    return abs(a-b);
}

bool test(int x, int y){
    if(distance(x,y) <=2){
        return true;
    }
    return false;
}

int main()
{
    N = 2;
   cout << "Hello World" << endl; 
   cout << test(3,4) << endl;
   return 0;
}

Error message below:

In file included from /usr/include/c++/4.8.3/bits/stl_algobase.h:65:0,                           
                 from /usr/include/c++/4.8.3/bits/char_traits.h:39,                              
                 from /usr/include/c++/4.8.3/ios:40,                                             
                 from /usr/include/c++/4.8.3/ostream:38,                                         
                 from /usr/include/c++/4.8.3/iostream:39,                                        
                 from main.cpp:1:                                                                
/usr/include/c++/4.8.3/bits/stl_iterator_base_types.h: In instantiation of 'struct std::iterator_
traits<int>':                                                                                    
/usr/include/c++/4.8.3/bits/stl_iterator_base_funcs.h:114:5:   required by substitution of 'templ
ate<class _InputIterator> typename std::iterator_traits<_Iterator>::difference_type std::distance
(_InputIterator, _InputIterator) [with _InputIterator = int]'                                    
main.cpp:15:20:   required from here                                                             
/usr/include/c++/4.8.3/bits/stl_iterator_base_types.h:165:53: error: 'int' is not a class, struct
, or union type                                                                                  
       typedef typename _Iterator::iterator_category iterator_category;                          
                                                     ^                                           
/usr/include/c++/4.8.3/bits/stl_iterator_base_types.h:166:53: error: 'int' is not a class, struct
, or union type                                                                                  
       typedef typename _Iterator::value_type        value_type;                                 
                                                     ^                                           
/usr/include/c++/4.8.3/bits/stl_iterator_base_types.h:167:53: error: 'int' is not a class, struct
, or union type                                                                                  
       typedef typename _Iterator::difference_type   difference_type;                            
                                                     ^                                           
/usr/include/c++/4.8.3/bits/stl_iterator_base_types.h:168:53: error: 'int' is not a class, struct
, or union type                                                                                  
       typedef typename _Iterator::pointer           pointer;                                    
                                                     ^                                           
/usr/include/c++/4.8.3/bits/stl_iterator_base_types.h:169:53: error: 'int' is not a class, struct
, or union type                                                                                  
       typedef typename _Iterator::reference         reference;

***The code does not compile on an online compiler, and I have no idea why.*** They normally give you an error list. — drescherjm, Dec 15 '14 at 17:02
I am new to C++, could not understand the error message which I am putting in the question. — user17902, Dec 15 '14 at 17:03
@user17902 _"I am new to C++, could not understand the error message which is below."_ So how could we help you to understand it if you're not even showing the error message in your question? — πάντα ῥεῖ, Dec 15 '14 at 17:04
Using this http://www.tutorialspoint.com/compile_cpp11_online.php, I realized that your code works as it is expected. Hence I can't get your point. — Christos, Dec 15 '14 at 17:06
That's really strange - I used http://www.tutorialspoint.com/compile_cpp_online.php as well. — user17902, Dec 15 '14 at 17:07
It's mother nature's way of telling you "`using namespace std` is bad for you, don't do that". — n. m. could be an AI, Dec 15 '14 at 17:08
@πάνταῥεῖ so what? there may be another `abs()` (which is a better match) in the global namespace. — Walter, Dec 15 '14 at 17:17
@Walter the error is not about `std::abs`, it's about `std::distance`. — n. m. could be an AI, Dec 15 '14 at 23:53
@n.m. yes, I realised that too (from Mike's answer). My first comment preceded that answer. — Walter, Dec 16 '14 at 09:23

score 7 · Accepted Answer · edited May 23 '17 at 10:28

using namespace std;

This is a bad idea; it dumps anything that's been declared in the std namespace into the global namespace, where it might conflict with anything you declare in the global namespace.

int distance(int a, int b)

This declares a function in the global namespace that conflicts with a function template of the same name in the std namespace.

if(distance(x,y) <=2)

The std::distance template is a better match than your function, according to the arcane rules of overload resolution. Trying to instantiate that, it fails since it can only be instantiated for iterator types, not int.

The best option is to remove the rogue using-directive, and add std:: to anything you use from the standard library. If you don't want to do that for some reason, then qualify your function call to specify the one declared in the global namespace:

if(::distance(x,y) <=2)

@user17902 Welcome on SO! Great that you got your problem sorted. Please consider upvoting all useful answers and accepting the best (should be easy with only one answer). — Walter, Dec 15 '14 at 17:18

C++ Code with abs() does not compile

1 Answers1