#include <iostream.h>

int main(int argc, char* argv[])
{
    cout<<"Hello World\n";
    return 0;
};

Компилирую gcc hello.cpp (gcc 2.95.2 под Solaris7)
Ошибка:

Undefined                       first referenced
 symbol                             in file
cout                                /var/tmp/ccA5q6Z8.o
ostream::operator<<(char const *)   /var/tmp/ccA5q6Z8.o
ld: fatal: Symbol referencing errors. No output written to a.out
collect2: ld returned 1 exit status