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

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++).