Beyond Variables: Alternative Approaches in CMake

Variables in CMake

Variables are fundamental building blocks in CMake, allowing you to store and manage project-specific information during the build process. They provide a way to customize the build based on your needs and environment.

Key Characteristics

String-based
All variables are stored internally as strings, even if they represent numerical values.
Alphanumeric and Underscore
Variable names can only contain letters, numbers, and underscores (_).
Case-sensitive
Names are treated differently based on capitalization (e.g., MY_VAR is distinct from my_var).

Types of Variables

User-defined variables
You create these to manage project-specific data such as paths, build flags, or other settings.

Creating User-defined Variables

There are two main ways to create user-defined variables:

Simple assignment
```
set(MY_VAR "value")
```
This creates a variable named MY_VAR and assigns the string value "value" to it.
Using the cmake_minimum_required command
```
cmake_minimum_required(VERSION 3.10)
set(ANOTHER_VAR TRUE)  # Can store booleans as well
```
This ensures that CMake version 3.10 (or later) is used and then creates a variable named ANOTHER_VAR with the boolean value TRUE.

Accessing Variables

Once you've created a variable, you can use it throughout your CMakeLists.txt file using the following syntax:

${MY_VAR}: Accesses the value of the variable MY_VAR.

Example

set(PROJECT_NAME "MyAwesomeProject")
set(SOURCE_DIR "src")
set(BUILD_DIR "build")

project(${PROJECT_NAME})

add_executable(${PROJECT_NAME}
               "${SOURCE_DIR}/main.cpp")

target_link_libraries(${PROJECT_NAME} some_external_library)

set_target_properties(${PROJECT_NAME} PROPERTIES OUTPUT_DIRECTORY "${BUILD_DIR}")

In this example:

The variables are used to define the project name, source code directory, build directory, and link an external library.
PROJECT_NAME, SOURCE_DIR, and BUILD_DIR are user-defined variables.

Advanced Features
CMake offers advanced variable manipulation capabilities such as appending strings, performing conditional assignments, and using variable scoping with modules. Explore the CMake documentation for details.
Caching
CMake caches variable values to improve build performance. You can control caching behavior using commands like cmake_policy.
Scope
Variables can be local to a specific CMakeLists.txt file or global, accessible from any file. Global variables are typically defined at the top level of your project's main CMakeLists.txt file.

Conditional Compilation Based on Variable

set(DEBUG_MODE OFF)  # Change to ON for debug build

if(${DEBUG_MODE})
  message(STATUS "Building in debug mode")
  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -g")  # Add debug flags
else()
  message(STATUS "Building in release mode")
  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -O3")  # Optimize for release
endif()

add_executable(my_program main.cpp)

This code defines a DEBUG_MODE variable. Based on its value, different compiler flags are set during compilation.

Using Variables with Lists

set(SOURCE_FILES
    main.cpp
    utility.cpp
    network.cpp)

set(INCLUDE_DIRS
    "${PROJECT_SOURCE_DIR}/include"  # Assuming PROJECT_SOURCE_DIR is defined elsewhere
    "${THIRD_PARTY_DIR}/headers")  # Assuming THIRD_PARTY_DIR is defined elsewhere

add_executable(my_program ${SOURCE_FILES})
target_include_directories(my_program PUBLIC ${INCLUDE_DIRS})

This code defines a list of source files (SOURCE_FILES) and a list of include directories (INCLUDE_DIRS). Both lists are used for creating an executable target.

Looping over a List

set(LIBRARIES sfml-graphics sfml-system)

foreach(LIBRARY IN LISTS LIBRARIES)
  find_package(${LIBRARY} REQUIRED)
  target_link_libraries(my_program ${LIBRARY})
endforeach()

This code iterates through a list of libraries (LIBRARIES) using a foreach loop. Inside the loop, it finds each library and links it with the executable my_program.

Variable Expansion

set(PROJECT_VERSION "1.2.3")
configure_file(version.h.in version.h COPYONLY)
string(REPLACE "@PROJECT_VERSION@" "${PROJECT_VERSION}" version.h.in version.h)

This code defines a PROJECT_VERSION variable. It then uses configure_file to copy a template file (version.h.in). Within the copy process, it replaces the placeholder @PROJECT_VERSION@ with the actual value from the variable.

Advanced: Passing Variables to Functions

cmake_minimum_required(VERSION 3.10)

function(create_library NAME TARGET_SOURCES)
  add_library(${NAME} STATIC ${TARGET_SOURCES})
  target_include_directories(${NAME} PUBLIC "${PROJECT_SOURCE_DIR}/include")
  # Set other properties or perform actions based on variables
endfunction()

create_library(my_library main.cpp utility.cpp)

This code defines a custom function create_library that takes two arguments: a name for the library and a list of source files. These arguments are variables passed to the function. Inside the function, the library is created using the provided variables.

Preprocessor Macros

Disadvantages
- Less organized and maintainable compared to CMake variables, especially for complex build configurations.
- Macros require modifying source code, which can be less flexible for build-time configuration.
Advantages
- May already be familiar to developers with C/C++ experience.
- Can be used for conditional compilation based on defined macros.
Description
You can use preprocessor macros defined in your source code to influence the build process. These macros can be set from the command line using compiler flags like -D.

Environment Variables

Disadvantages
- Can lead to conflicts with other system-wide environment variables.
- Not as project-specific as CMake variables.
Advantages
- May be useful if you need to share build configurations across different projects or environments.
Description
You can use environment variables set on the system to influence the build process. These variables are accessible within your CMakeLists.txt using set(ENV{VAR_NAME} value).

Command-Line Arguments

Disadvantages
- Not ideal for complex configurations or frequent modifications.
- Can clutter the command line and make builds less repeatable.
Advantages
- Useful for one-time configuration changes or overrides without modifying project files.
Description
You can pass arguments directly when invoking CMake, and access them within your CMakeLists.txt using cmake_parse_arguments.

CMake Modules and Functions

Disadvantages
- Requires additional development effort to create and manage modules.
Advantages
- Promotes modularity and code reuse.
- Can enhance project organization and maintainability.
Description
While not directly a replacement for variables, you can create reusable modules or functions to encapsulate logic or configuration options. These modules or functions can accept arguments and perform actions based on them.

Modules and functions can be a powerful way to organize complex build logic, but require more effort to set up.
Command-line arguments are best suited for one-off configuration changes.
Preprocessor macros and environment variables might be considered for compatibility with existing code or sharing settings across projects, but with caution due to potential drawbacks.
For most build configuration needs, variables offer the best balance of clarity, maintainability, and flexibility.