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Enforcing Objective-C++ Standards in CMake Projects: CMAKE_OBJCXX_STANDARD_REQUIRED and Beyond


What it is

  • It acts as a default value for the OBJCXX_STANDARD_REQUIRED property of targets in your CMake project.
  • It's a CMake variable introduced in version 3.16.

What it does

  • By setting CMAKE_OBJCXX_STANDARD_REQUIRED, you ensure that all targets in your project (unless explicitly overridden) will be compiled with the specified minimum language standard.
  • The OBJCXX_STANDARD_REQUIRED property specifies the minimum required Objective-C++ language standard for compiling source files associated with a target.

Benefits of using it

  • Helps avoid potential compilation errors or warnings that might arise due to using features incompatible with older standards.
  • Enforces a consistent language standard across your project, leading to better code maintainability and compatibility.

Example usage

# Set the minimum required standard to Objective-C++14 (or a different standard as needed)
set(CMAKE_OBJCXX_STANDARD_REQUIRED 14)

# Create a target (source files will be compiled with at least Objective-C++14)
add_executable(my_program main.mm)

How it interacts with "Variables" in CMake

  • This means you can set CMAKE_OBJCXX_STANDARD_REQUIRED once, and it will apply to all subsequent CMake runs for your project unless you explicitly change it.
  • Cache variables are stored in the CMake cache, which is a file that persists between CMake invocations.
  • CMAKE_OBJCXX_STANDARD_REQUIRED is a special type of variable called a "cache variable."
  • CMake variables are settings that control the behavior of the build process.
  • You can still override the default standard for specific targets by setting the OBJCXX_STANDARD_REQUIRED property directly on the target.

Example 1: Setting a Default Standard

# Set the default minimum standard to Objective-C++17
set(CMAKE_OBJCXX_STANDARD_REQUIRED 17)

# Create multiple targets that inherit the default standard
add_executable(program1 main.mm)
add_library(library1 my_functions.mm)

# You can still verify the inherited standard for a target
message(STATUS "Target 'program1' standard: ${OBJCXX_STANDARD_REQUIRED}")

In this example, all targets (program1 and library1) will be compiled with Objective-C++17 by default because of the CMAKE_OBJCXX_STANDARD_REQUIRED setting. The message command demonstrates how to retrieve the actual standard used for a specific target.

Example 2: Overriding the Default

# Set the default minimum standard to Objective-C++14
set(CMAKE_OBJCXX_STANDARD_REQUIRED 14)

# Create a target with a higher required standard
add_executable(program2 main2.mm PROPERTIES OBJCXX_STANDARD_REQUIRED 17)

# Create a target that doesn't require any specific standard
add_executable(program3 no_standard.mm)

Here, program2 explicitly sets its OBJCXX_STANDARD_REQUIRED property to 17, overriding the default of 14. program3 doesn't specify any standard, so it will likely inherit the default from the compiler itself (which might be lower than 14).


Compiler Flags

  • This approach offers fine-grained control but requires manual intervention for each target and might be less portable across different compilers.
  • Consult your compiler's documentation for the specific flags related to the Objective-C++ standard (e.g., -std=c++17 for GCC).
  • You can directly set the compiler flags that specify the language standard using the COMPILE_FLAGS property on targets.

Example

# Set the default compiler flags (replace with your compiler's flags)
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=c++14")
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++14")

# Override for a specific target (replace with your compiler's flags)
add_executable(program1 main.mm PROPERTIES COMPILE_FLAGS "-std=c++17")

Target Compile Features

  • This method offers more control over individual language features but might be less straightforward for simple standard enforcement.
  • You can use this in conjunction with features introduced in particular standards.
  • CMake provides the target_compile_features command to enable or disable specific language features.

Example

# Enable features introduced in C++17 (modify as needed)
target_compile_features(my_target PRIVATE CPP17)

# You can combine this with compiler flags for a more granular approach

Custom Modules or Functions

  • However, it requires more upfront effort and can increase project complexity.
  • This approach allows for reusability and potentially more complex decision-making based on project or target characteristics.
  • You can create custom CMake modules or functions that encapsulate the logic for setting the language standard.

Choosing the Right Approach

The best approach depends on your project's requirements and complexity:

  • For complex scenarios with reusable logic, custom modules might be suitable.
  • If you need more control over individual targets or compiler flags, consider using COMPILE_FLAGS or target_compile_features.
  • For simple, project-wide enforcement of a standard, CMAKE_OBJCXX_STANDARD_REQUIRED is the most straightforward option.