Optional Code, Approach 1: Optional blacklist, C Macro (Global Flags)

Source

11-conditional-code-approach-1/
├── CMakeLists.txt
├── DemoConfig.h.in
├── bin
│   ├── CMakeLists.txt
│   ├── hello-first.cpp
│   └── hello-second.cpp
├── blacklist
│   ├── blacklist.cpp
│   ├── blacklist.h
│   └── CMakeLists.txt
└── libhello
    ├── CMakeLists.txt
    ├── greeter.h
    ├── greeter-name.cpp
    ├── greeter-name.h
    ├── greeter-simple.cpp
    └── greeter-simple.h

• code/CMakeLists.txt

• code/DemoConfig.h.in

• code/bin/CMakeLists.txt

• code/bin/hello-first.cpp

• code/bin/hello-second.cpp

• code/libhello/CMakeLists.txt

• code/libhello/greeter.h

• code/libhello/greeter-simple.h

• code/libhello/greeter-simple.cpp

• code/libhello/greeter-name.h

• code/libhello/greeter-name.cpp

• code/blacklist/CMakeLists.txt

• code/blacklist/blacklist.h

• code/blacklist/blacklist.cpp

Optional blacklist, C Macro

• Descend into blacklist directory optionally

  Toplevel CMakeLists.txt

  IF (${USE_BLACKLIST})
    ADD_SUBDIRECTORY(blacklist)
  ENDIF()
  

• Provide C macro for occasional C macro orgies

  Toplevel CMakeLists.txt

  # for configure-file macro, define parallel variable with real bool
  # values
  IF (${USE_BLACKLIST})
    SET(MACRO_USE_BLACKLIST 1)
  ELSE()
    SET(MACRO_USE_BLACKLIST 0)
  ENDIF()
  

  DemoConfig.h.in

  #define DEMO_USE_BLACKLIST @MACRO_USE_BLACKLIST@
  

Optional Dependency In libhello

• Optional TARGET_LINK_LIBRARIES()

  libhello/CMakeLists.txt

  IF (${USE_BLACKLIST})
    TARGET_LINK_LIBRARIES(hello blacklist)
  ENDIF()
  

Note

This is a massacre!

Optional Dependency In libhello Header File

Note

This is a software design choice. Putting optional code in header file leads to messy code, no matter which build system.

• Designer’s / Architect’s choice: dependency in greeter-name.h header file

• ⟶ massacre

• All users (“dependers”) of libhello need to have include path to blacklist.h

• Macro-conditionals all over the place

#include <DemoConfig.h>                                // <--- dependency in header file!!!
#ifndef DEMO_USE_BLACKLIST
# error DEMO_USE_BLACKLIST not defined                 // <--- guard against build system bugs
#endif
#if DEMO_USE_BLACKLIST == 1
#  include <blacklist.h>                               // <--- dependency in header file!!!
#endif

#include <string>

class NameGreeter : public Greeter
{
private:
#if DEMO_USE_BLACKLIST == 1                            // <--- dependency in header file!!!
    Blacklist _blacklist;
#endif
};

Note

All users of libhello need to be aware!

Optional Dependency In libhello CPP File

• If one likes macros, then this is normal business

• No implications on users, as opposed to conditional code in header file

void NameGreeter::sayhello()
{
#if DEMO_USE_BLACKLIST == 1
    if (_blacklist.is_forbidden(_name))
        std::cout << "Goodbye " << _name << '\n';
#else
    std::cout << "Hello " << _name << '\n';
#endif
}

Good Or Bad?

What is this approach?

• ⟶ Using “globals” a lot

  • USE_BACKLIST defined toplevel, and used in libhello to conditionally add dependency on blacklist

  • C macro defined toplevel - in DemoConfig.h.in

Is there a better way?

• Almost always 🥷