CMake: Checking Variables with if() Instead of variable_requires()

• Behavior
  
  • If any variable wasn't set to TRUE or wasn't set at all (meaning its value was NOTFOUND), it would report an error.
  • If all variables were set to TRUE or not set, it did nothing.
• Arguments
  It took a list of variables as arguments.
• Purpose
  It checked if certain variables were set to specific values.

Replacement

if(NOT MY_VARIABLE OR NOT ANOTHER_VARIABLE)
  message(ERROR "Variables MY_VARIABLE and ANOTHER_VARIABLE are required")
endif()

This code checks if both MY_VARIABLE and ANOTHER_VARIABLE are set to TRUE. If either is not set, it displays an error message.

---

Example 1: Checking a single variable

Imagine you have a variable USE_GUI that determines if your project uses a graphical user interface. You can check if it's set to ON before including a GUI library:

# Old way (deprecated)
# variable_requires(USE_GUI)

# New way (using if())
if(NOT USE_GUI)
  message(WARNING "Building without GUI support")
endif()

find_package(FLTK QUIET)  # Only try to find FLTK if USE_GUI is ON
if(FLTK_FOUND)
  include_directories(${FLTK_INCLUDE_DIRS})
  target_link_libraries(your_target FLTK)
endif()

Example 2: Checking multiple variables

Suppose you need a specific compiler and a specific library for your project. You can check for both:

# Old way (deprecated)
# variable_requires(CMAKE_CXX_COMPILER CMAKE_REQUIRED_LIBRARIES)

# New way (using if())
if(NOT CMAKE_CXX_COMPILER OR NOT CMAKE_REQUIRED_LIBRARIES)
  message(FATAL_ERROR "Requires a C++ compiler and libraries")
endif()

Example 3: Checking variable values

You can also check if a variable has a specific value:

# Old way (not directly possible with variable_requires())

# New way (using if())
if(NOT MY_OPTION EQUAL "VALUE")
  message(STATUS "Option MY_OPTION is not set to 'VALUE'")
endif()

---

Using if() for Variable Checks

• Basic check for existence
  

if(NOT MY_VARIABLE)
  message(ERROR "Variable MY_VARIABLE is required")
endif()

This checks if MY_VARIABLE is defined (not NOTFOUND).

• Checking for specific value
  

if(NOT MY_OPTION EQUAL "VALUE")
  message(STATUS "Option MY_OPTION is not set to 'VALUE'")
endif()

This verifies if MY_OPTION has the value "VALUE".

• Checking multiple variables
  

if(NOT CMAKE_CXX_COMPILER OR NOT CMAKE_REQUIRED_LIBRARIES)
  message(FATAL_ERROR "Requires a C++ compiler and libraries")
endif()

This ensures both CMAKE_CXX_COMPILER and CMAKE_REQUIRED_LIBRARIES are defined.

Additional Control Flow options with if()

• elseif()
  This allows for checking additional conditions within the if() block.
• else() and endif()
  These define alternative code blocks if the condition isn't met and the end of the conditional block, respectively.

• Integration
  if() can be easily combined with other control flow statements like else() and while() for more intricate logic.
• Readability
  The syntax of if() is generally considered more readable and easier to understand.
• Flexibility
  if() allows for more complex conditional logic compared to the limited functionality of variable_requires().