Preprocessor Directives

Preprocessor directives can appear anywhere in a source file, but they apply only to the remainder of the source file.

#error：Error directives produce compiler-time error messages.
#if !defined(__cplusplus)
#error C++ compiler required.
#endif

#import
Used to incorporate information from a type library. The content of the type library is converted into C++ classes, mostly describing the COM interfaces.     
Header Files Created by Import
#import creates two header files that reconstruct the type library contents in C++ source code. The primary header file is similar to that produced by the Microsoft Interface Definition Language (MIDL) compiler, but with additional compiler-generated code and data. The primary header file has the same base name as the type library, plus a .TLH extension. The secondary header file has the same base name as the type library, with a .TLI extension. It contains the implementations for compiler-generated member functions, and is included (#include) in the primary header file.
If importing a dispinterface property that uses byref parameters, #import will not generate __declspec(property) statement for the function.
Both header files are placed in the output directory specified by the /Fo (name object file) option. They are then read and compiled by the compiler as if the primary header file was named by a #include directive.

The following compiler optimizations come with the #import directive:
    * The header file, when created, is given the same timestamp as the type library.
    * When #import is processed, the compiler first checks if the header exists and is up to date. If yes, then it does not need to be re-created.
The #import directive also participates in minimal rebuild and can be placed in a precompiled header file. See Creating Precompiled Header Files for more information.

#undef：Removes (undefines) a name previously created with #define.

#elif

#if DLEVEL == 0
    #define STACK 0
#elif DLEVEL == 1
    #define STACK 100
#elif DLEVEL > 5
    display( debugptr );
#else
    #define STACK 200
#endif

#include ：The #include directive tells the preprocessor to treat the contents of a specified file as if those contents had appeared in the source program at the point where the directive appears.