Optional Code, Approach 2: Pushing Optionality Down In Module Interface

Source

12-conditional-code-approach-2/
├── 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.cpp

• code/libhello/greeter-name.h

• code/blacklist/CMakeLists.txt

• code/blacklist/blacklist.h

• code/blacklist/blacklist.cpp

Remove Too Much CMake Optionality

• Remove optional descending from toplevel CMakeLists.txt

• Remove optional dependency from libhello

• Remove DEMO_USE_BLACKLIST from DemoConfig.h.in

• Test (⟶ macro not defined)

  $ cmake -DUSE_BLACKLIST=ON  ~/work/jfasch-home/trainings/material/soup/cmake/code/
  $ make
  greeter-name.h:8:3: error: #error DEMO_USE_BLACKLIST not defined
  8 | # error DEMO_USE_BLACKLIST not defined
    |   ^~~~~
  

• How to solve? Where to define macro?

TARGET_COMPILE_DEFINITIONS() (backlist Availablility)

Documentation

• TARGET_COMPILE_DEFINITIONS()

• TARGET_COMPILE_OPTIONS()

• Let blacklist announce its availability to its dependers

• ⟶ TARGET_COMPILE_DEFINITIONS()

  blacklist/CMakeLists.txt

  TARGET_COMPILE_DEFINITIONS(blacklist PUBLIC DEMO_USE_BLACKLIST=1)
  

• Test

  • cmake -DUSE_BLACKLIST=ON ... ⟶ works

  • cmake -DUSE_BLACKLIST=OFF ... ⟶ blacklist still used (obviously, because we always announce is as 1)

• ⟶ put optionality in blacklist

Push Optionality Down In blacklist (⟶ INTERFACE Targets)

• Naive approach

  IF (${USE_BLACKLIST})
    ADD_LIBRARY(blacklist blacklist.cpp)
  
    TARGET_INCLUDE_DIRECTORIES(blacklist PUBLIC .)
    TARGET_COMPILE_DEFINITIONS(blacklist PUBLIC DEMO_USE_BLACKLIST=1)
  ELSE()
    TARGET_COMPILE_DEFINITIONS(blacklist PUBLIC DEMO_USE_BLACKLIST=0)
  ENDIF()
  

  $ cmake -DUSE_BLACKLIST=OFF  ~/work/jfasch-home/trainings/material/soup/cmake/code/
  CMake Error at blacklist/CMakeLists.txt:7 (TARGET_COMPILE_DEFINITIONS):
    Cannot specify compile definitions for target "blacklist" which is not
    built by this project.
  
  -- Configuring incomplete, errors occurred!
  

• INTERFACE targets: dependency nodes, but without anything that’s built

• Can have dependencies itself (TARGET_LINK_LIBRARIES())

• Ours has TARGET_COMPILE_DEFINITIONS()

  IF (${USE_BLACKLIST})
    ADD_LIBRARY(blacklist blacklist.cpp)
  
    TARGET_INCLUDE_DIRECTORIES(blacklist PUBLIC .)
    TARGET_COMPILE_DEFINITIONS(
          blacklist INTERFACE DEMO_USE_BLACKLIST=1) # <--- INTERFACE: propagate-only
  ELSE()
    ADD_LIBRARY(blacklist INTERFACE)                # <--- INTERFACE: there is no library here
    TARGET_COMPILE_DEFINITIONS(
          blacklist INTERFACE DEMO_USE_BLACKLIST=0) # <--- INTERFACE: propagate-only
  ENDIF()
  

Good Or Bad?

Is this better than the other approach?

• It is more flexible

• Optionality is local, between provider (blacklist) and consumer (libhello)

• Is it simpler? Probably not!