Question

So I was doing some simple C++ exercises and I noticed an interesting feat. Boiled down to bare metal one could try out compiling the following code:

class nice
{
  public:
    nice() {}
};

int main()
{
  nice n;
  return 0;
};

The result is a compilation error that goes something like this:

<file>.cpp: In function ‘int main()’:
<file>.cpp:11: error: expected `;' before ‘n’
<file>.cpp:11: warning: statement is a reference, not call, to function ‘nice’
<file>.cpp:11: warning: statement has no effect

And this was using regular g++ on Max OS X, some of my friends have tried in on Ubuntu as well, yielding the same result.

The feat seems to lie in the word “nice”, because refactoring it allows us to compile. Now, I can’t find the “nice” in the keyword listings for C++ or C, so I was wondering if anyone here had an idea?

Also, putting

class nice n;

instead of

nice n;

fixes the problem.

P.S. I’m a relative C++ newbie, and come from the ActionScript/.NET/Java/Python world.

Update:

Right, my bad, I also had an

#include <iostream>

at the top, which seems to be the root of the problem, because without it everything works just fine.

Answer 1

It is a namespace problem but not with namespace std. The header <iostream> is pulling in <unistd.h>

If you try

class nice
{
  public:
      nice() {};
};

int main(int argc, char *argv[])
{
    nice n;

    return 0;
}

there is no problem.

Simply add

#include <unistd.h>

and you will get the “expected ‘;’ before ‘n’” error. Namespace std does not enter the picture.

So the solution is the same as before – put class nice in its own namespace and it will not clash with the global ::nice().