“Configured” Header Files

Source

10-configure-file/
├── CMakeLists.txt
├── DemoConfig.h.in
├── bin
│   ├── CMakeLists.txt
│   ├── hello-first.cpp
│   └── hello-second.cpp
└── 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/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/bin/CMakeLists.txt

• code/bin/hello-second.cpp

• code/bin/hello-first.cpp

Problem

Documentation

• PROJECT()

• CONFIGURE_FILE()

• Want to output project version in executables

• ⟶ Set version using PROJECT()

• Output using MESSAGE()

  PROJECT(Demo VERSION 42.666)
  MESSAGE(DEBUG "Major: ${Demo_VERSION_MAJOR}")
  MESSAGE(DEBUG "Major: ${Demo_VERSION_MINOR}")
  

• Question: how to make those available to compiled code?

• Answer: CONFIGURE_FILE()

Configured Files

Documentation

• CONFIGURE_FILE()

• CMAKE_CURRENT_BINARY_DIR

• Header file template that contains CMake variable substitutions

  code/DemoConfig.h.in

  // This file is generated, do not modify
  
  #define DEMO_MAJOR @Demo_VERSION_MAJOR@
  #define DEMO_MINOR @Demo_VERSION_MINOR@
  

• Let CMake perform substitutions

  CONFIGURE_FILE(DemoConfig.h.in DemoConfig.h)
  

• Converted to ${CMAKE_CURRENT_BINARY_DIR}/DemoConfig.h

Use In Compiled Code

Documentation

• INCLUDE_DIRECTORIES()

• Pick up CMake variables from configure file (code/bin/hello-first.cpp and code/bin/hello-second.cpp)

  #include <DemoConfig.h>
  // ...
  std::cout << "Version " << DEMO_MAJOR << '.' << DEMO_MINOR << std::endl;
  

• Does not compile ⟶ include path missing

  bin/hello-first.cpp:4:10: fatal error: DemoConfig.h: No such file or directory
      4 | #include <DemoConfig.h>
        |          ^~~~~~~~~~~~~~
  

• Solution: set include path for all (to add a dependency would be overkill but could be done)

  Toplevel CMakeLists.txt

  INCLUDE_DIRECTORIES(${CMAKE_CURRENT_BINARY_DIR})