Error(s):

source_file.cpp(25): error C2628: 'Base' followed by 'int' is illegal (did you forget a ';'?)
source_file.cpp(26): error C2259: 'Base': cannot instantiate abstract class
source_file.cpp(26): note: due to following members:
source_file.cpp(26): note: 'void Base::f(void)': is abstract
source_file.cpp(7): note: see declaration of 'Base::f'
source_file.cpp(26): error C3874: return type of 'main' should be 'int' instead of 'Base'
Microsoft (R) C/C++ Optimizing Compiler Version 19.00.23506 for x64