Exploring Alternatives to CheckVariableExists in CMake

Purpose

• Useful for conditionally including code blocks based on the presence or absence of certain variables.
• Determines if a specific variable exists within the current CMake project's cache.

Usage

CHECK_VARIABLE_EXISTS(VAR VARIABLE)

• VARIABLE: The name of a new internal cache variable that will be set to 1 if VAR exists, and 0 otherwise.
• VAR: The name of the variable you want to check for.

Behavior

1. Check Existence
   
   • If VAR is defined in the CMake cache, the VARIABLE is set to 1.
2. Optional Compilation Test (C Variables Only)
   
   • If no definition is found in the cache, the macro attempts to compile a simple C source file to verify if the variable is used within the code.
   • This is useful for variables that might be defined in external header files.
   • Note that this step is only performed for C variables, not other variable types.

Optional Flags

• CMAKE_REQUIRED_QUIET: Suppresses informational messages during the check.
• CMAKE_REQUIRED_LIBRARIES: Libraries to link with for the test compilation.
• CMAKE_REQUIRED_LINK_OPTIONS: Linker options for the test compilation.
• CMAKE_REQUIRED_DEFINITIONS: Preprocessor definitions for the test compilation (list of -DFOO=bar macros).
• CMAKE_REQUIRED_FLAGS: Additional compiler flags for the test compilation (space-delimited string).

Example

# Check for a variable named MY_LIBRARY
CHECK_VARIABLE_EXISTS(MY_LIBRARY HAS_MY_LIBRARY)

if(HAS_MY_LIBRARY)
  message(STATUS "Variable MY_LIBRARY exists")
  # Use MY_LIBRARY for linking or other purposes
else()
  message(STATUS "Variable MY_LIBRARY doesn't exist")
  # Handle the case where MY_LIBRARY is not available
endif()

• The optional compilation test is intended for variables from external header files that might not be explicitly defined in the cache. If the variable is purely internal, setting it directly might be more efficient.
• The CheckVariableExists macro is designed for checking C variables. It won't work for other variable types like strings or booleans.

---

# Optional flags for the test compilation (if needed)
set(CMAKE_REQUIRED_FLAGS "-Wall")

CHECK_VARIABLE_EXISTS(USE_CUSTOM_MATH HAS_CUSTOM_MATH)

if(HAS_CUSTOM_MATH)
  message(STATUS "Variable USE_CUSTOM_MATH exists (using compilation test)")
  # Use custom math library if available
else()
  message(STATUS "Variable USE_CUSTOM_MATH doesn't exist")
  # Use standard math library
endif()

• The if block executes if HAS_CUSTOM_MATH is true (meaning the variable exists or was found in a header during compilation).
• The macro checks for USE_CUSTOM_MATH and sets HAS_CUSTOM_MATH accordingly.
• We set CMAKE_REQUIRED_FLAGS with -Wall to enable compiler warnings during the test compilation (optional).

This example checks for a boolean variable named ENABLE_LOGGING without the compilation test:

# No compilation test needed here

CHECK_VARIABLE_EXISTS(ENABLE_LOGGING HAS_LOGGING)

if(HAS_LOGGING)
  message(STATUS "Variable ENABLE_LOGGING exists (boolean)")
  # Enable logging based on the variable value
else()
  message(STATUS "Variable ENABLE_LOGGING doesn't exist, assuming logging disabled")
  # Disable logging or set default behavior
endif()

• The if block checks the value of HAS_LOGGING to determine whether logging should be enabled.
• The macro checks for ENABLE_LOGGING and sets HAS_LOGGING to 1 if defined (true), or 0 otherwise.
• We don't set any compilation flags as the variable is likely not defined in a header file.

---

Direct if Statement (CMake 3.3 or later)

• If you're using CMake 3.3 or later, you can leverage the improved if statement capabilities. This approach works for any variable type (cache, environment, or custom variables).

if(DEFINED MY_VARIABLE)
  message(STATUS "Variable MY_VARIABLE exists")
  # Use MY_VARIABLE
else()
  message(STATUS "Variable MY_VARIABLE doesn't exist")
  # Handle the case where MY_VARIABLE is not available
endif()

• DEFINED checks if a variable of any type exists (cache, environment, or custom).

Custom Function (More Complex Checks)

• If you need more complex checks or logic beyond simple existence, a custom function can be defined. This approach allows for tailored behavior based on your requirements.

function(check_variable_and_value VAR VALUE REQUIRED)
  if(NOT DEFINED ${VAR})
    message(STATUS "Variable ${VAR} is not defined")
    if(${REQUIRED})
      message(FATAL_ERROR "Variable ${VAR} is required")
    endif()
    return()
  endif()

  if(NOT EQUAL ${${VAR}} "${VALUE}")
    message(STATUS "Variable ${VAR} has value '${${VAR}}', expected '${VALUE}'")
    if(${REQUIRED})
      message(FATAL_ERROR "Variable ${VAR} must have value '${VALUE}'")
    endif()
  endif()
endfunction(check_variable_and_value)

check_variable_and_value(MY_VARIABLE "CUSTOM_VALUE" REQUIRED)
# Use MY_VARIABLE

• You can extend this function with additional checks or error handling as needed.
• The function checks if the variable is defined (DEFINED) and optionally verifies its value using EQUAL.
• The custom function check_variable_and_value takes three arguments:
  • VAR: The name of the variable to check.
  • VALUE: The expected value of the variable (optional).
  • REQUIRED: A boolean flag indicating if the variable is required (optional).

• CheckVariableExists remains a viable option, especially if you need compilation testing for C variables or prefer its specific behavior.
• For more complex checks or logic, a custom function provides flexibility.
• For simple existence checks, the direct if statement is the most concise solution (CMake 3.3+).