Understanding OBJCXX and Environment Variables in CMake for Objective-C++ Compilation

OBJCXX in CMake

• Behavior
  
  • CMake checks for the OBJCXX environment variable only during the first configuration of your project's build tree.
  • If OBJCXX is set, CMake attempts to use the specified compiler.
  • If OBJCXX is not set or the compiler path is invalid, CMake searches for a suitable Objective-C++ compiler based on its internal mechanisms.
  • Once a compiler is found (either from OBJCXX or internal search), CMake stores the information in its cache as the CMAKE_OBJCXX_COMPILER variable.
  • In subsequent configurations (e.g., cmake .), CMake uses the cached compiler path from CMAKE_OBJCXX_COMPILER and ignores the OBJCXX environment variable, unless you explicitly clear the CMake cache.
• Purpose
  The OBJCXX environment variable in CMake specifies the preferred executable for compiling Objective-C++ source files (files with the .mm extension). It points CMake to the Objective-C++ compiler to be used during the build process.

Key Points

• Precedence
  If both OBJCXX is set and CMAKE_OBJCXX_COMPILER is already cached, CMake prioritizes the cached value.
• First Configuration
  OBJCXX only affects the initial compiler selection. Subsequent builds use the cached value.
• Optional
  Setting OBJCXX is optional. If not set, CMake will search for a suitable compiler on its own.

Example Usage

If you have a preferred Objective-C++ compiler installed in a non-standard location, you can set the OBJCXX environment variable before running cmake:

export OBJCXX=/path/to/your/objcxx_compiler
cmake .

---

Setting OBJCXX Environment Variable

# Before running cmake:
export OBJCXX=/path/to/your/objcxx_compiler

# CMakeLists.txt
project(MyOBJCXXProject)

# CMake will check the OBJCXX environment variable during the first configuration.
find_package(Objective-C REQUIRED)  # Find Objective-C libraries (optional)

add_executable(my_objcxx_program main.mm)
target_link_libraries(my_objcxx_program ObjectiveC)  # Link with Objective-C libraries

• The CMakeLists.txt remains standard, using find_package(Objective-C REQUIRED) for linking with Objective-C libraries.
• We set the OBJCXX environment variable to the path of your desired compiler before running cmake.

Using find_package for Compiler Selection

# CMakeLists.txt
project(MyOBJCXXProject)

# Explicitly specify the compiler using find_package
find_package(Clang REQUIRED COMPONENTS ClangC ClangCXX)  # Replace with GCC if needed

add_executable(my_objcxx_program main.mm)
target_link_libraries(my_objcxx_program ObjectiveC)  # Link with Objective-C libraries

• CMake will attempt to find the Objective-C++ compiler (ClangCXX in this case) and link it to your project.
• This approach uses find_package(Clang REQUIRED COMPONENTS ClangC ClangCXX) to search for the Clang compiler suite (replace with GCC for the GNU Compiler Collection).

# CMakeLists.txt (for Apple platforms)
project(MyOBJCXXProject)

# Use the system-provided Apple Clang compiler on Apple platforms
if(APPLE)
  set(CMAKE_CXX_COMPILER clang++)
endif()

add_executable(my_objcxx_program main.mm)
target_link_libraries(my_objcxx_program ObjectiveC)  # Link with Objective-C libraries

• CMake will use the specified compiler for both C++ and Objective-C++ code.
• The if(APPLE) block checks for the Apple platform and sets CMAKE_CXX_COMPILER accordingly.
• This example leverages the CMAKE_CXX_COMPILER variable, which often points to the system-provided Apple Clang compiler on Apple platforms (macOS, iOS, etc.).

---

find_package with Specific Compilers

• Instead of using the generic Objective-C package, use find_package commands for specific compilers like Clang or GCC:

find_package(Clang REQUIRED COMPONENTS ClangC ClangCXX)
# or
find_package(GCC REQUIRED COMPONENTS CXX)

• This approach explicitly searches for the desired compiler suite (Clang or GCC) and links the necessary components (ClangC/ClangCXX or GCC for C++ compilation).

CMAKE_CXX_COMPILER (for Apple Clang)

• You can leverage this variable in your CMakeLists.txt:
• On Apple platforms (macOS, iOS, etc.), CMake typically sets the CMAKE_CXX_COMPILER variable to point to the system-provided Apple Clang compiler.

if(APPLE)
  set(CMAKE_CXX_COMPILER clang++)
endif()

• This approach utilizes the pre-configured Apple Clang compiler, simplifying the process.

Custom Toolchain File

• Include this file during the CMake configuration step using the -DCMAKE_TOOLCHAIN_FILE option:
• Create a custom toolchain file that defines the compiler path and other settings specific to your environment.

cmake -DCMAKE_TOOLCHAIN_FILE=path/to/your/toolchain.cmake .

• The toolchain file can contain commands like set(CMAKE_CXX_COMPILER /path/to/your/compiler).

• For complex environments or custom configurations, a custom toolchain file provides the most flexibility.
• On Apple platforms, leveraging CMAKE_CXX_COMPILER is convenient for using the system-provided Apple Clang.
• If you have a strong preference for Clang or GCC, using find_package for those specific compilers offers more control.