How is it possible to declare nothing inside main() in C++ and yet have a working application after compilation?

Question

In an interview I was confronted with a question such as this:

Your friend has given you a single source code file which prints the Fibonacci numbers on the console. Note that the main() block is empty and doesn't have any statements inside it.

Explain how this is possible (hint: global instance!)

I really want to know about this, how such a thing can be even possible!

Answer 1

It is most likely implemented as (or a variant of it):

 void print_fibs()   {        //implementation  }   int ignore = (print_fibs(), 0);   int main() {} 

In this code, the global variable ignore has to be initialized before entering into main() function. Now in order to initialize the global, print_fibs() needs to be executed where you can do anything — in this case, compute fibonacci numbers and print them! A similar thing I've shown in the following question (which I had asked long back):

• Is main() really start of a C++ program?

Note that such code is not safe and should be best avoided in general. For example, the std::cout object may not be initialized when print_fibs() is executed, if so then what would std::cout do in the function? However, if in other circumstances, it doesn't depend on such initialization order, then it is safe to call initialization functions (which is a common practice in C and C++).

Answer 2

Hope this helps

class cls {   public:     cls()     {       // Your code for fibonacci series     } } objCls;  int main() { } 

So as soon as a global variable of the class is declared, the constructor is called and there you add the logic to print out the Fibonacci series.