Thanks Gassa for help. The working variant is such:

#define file "taskname"	
#ifdef ONLINE_JUDGE
#define in \
        //file name in task
	freopen(file".in", "r", stdin); \
	freopen(file".out", "w", stdout);
#else
#define in \
        //your file if you use files as input
	freopen("in.txt", "r", stdin);
        //freopen("out.txt", "w", stdout); 
#endif

int main()
{
   in;
}

It will make your code for I/O shorter.