CMake: Managing Build Type Flags (Without CMAKE_NOT_USING_CONFIG_FLAGS)

What it is

• It's a boolean flag (can be either TRUE or FALSE).
• CMAKE_NOT_USING_CONFIG_FLAGS is an internal variable used by CMake's build system generators.

What it does

• These flags are typically used by some generators (like Visual Studio) to control aspects of the build process based on the chosen build type.
• When set to TRUE, it instructs the generators to skip processing configuration flags associated with the build type (e.g., Debug, Release).

Why it's used (Internal implementation detail)

• However, it's likely used in scenarios where the generator might already have its own mechanisms for handling build type flags, and CMake wants to avoid applying them twice or potentially interfering with the generator's approach.
• The exact reason for this variable's existence is not publicly documented as it's an internal detail of CMake's implementation.

When you might encounter it

• It's primarily used by CMake internally to manage the build process communication between itself and the specific generator you're using.
• You're unlikely to directly encounter or modify CMAKE_NOT_USING_CONFIG_FLAGS in your CMakeLists.txt files.

• You typically don't need to be aware of its existence in most CMake projects.
• It controls whether build type flags are processed by the generator.
• It's an internal variable, not intended for user modification.

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1. Internal CMake Management
   CMake itself sets and manages this variable based on the generator and build system interaction. Users don't need to control it.

2. Generator-Specific Flags
   CMake generators (like Visual Studio, Xcode) handle build types through their own mechanisms. CMAKE_NOT_USING_CONFIG_FLAGS might be used internally by CMake to avoid applying these flags twice if the generator already manages them.

cmake_minimum_required(VERSION 3.0)

project(MyProject)

# Set build type using environment variable (optional)
if(NOT CMAKE_BUILD_TYPE)
  set(CMAKE_BUILD_TYPE Debug CACHE STRING "Choose the build type")
endif()

# Different compiler flags based on build type (example)
if(CMAKE_BUILD_TYPE MATCHES Debug)
  message(STATUS "Enabling debugging symbols")
  add_compile_options(-g)
else()
  message(STATUS "Optimizing for performance")
  add_compile_options(-O3)
endif()

add_executable(my_executable main.cpp)

• Finally, we create an executable named my_executable from the source file main.cpp.
• Inside an if statement, we check the build type.
  • If it's Debug, we enable debugging symbols using add_compile_options(-g).
  • Otherwise, we optimize for performance using add_compile_options(-O3).
• We optionally set the build type using the CMAKE_BUILD_TYPE environment variable. This allows users to choose a build type from their command line (e.g., cmake -DCMAKE_BUILD_TYPE=Release).
• We create a project named MyProject.
• We set the minimum required CMake version.

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Leverage Built-in Build Types

• Use the CMAKE_BUILD_TYPE variable to select the desired type:
• CMake provides predefined build types like Debug, Release, RelWithDebInfo (release with debug information), and MinSizeRel (release with minimal size).

cmake_minimum_required(VERSION 3.0)

project(MyProject)

set(CMAKE_BUILD_TYPE Debug CACHE STRING "Choose the build type")  # Optional, default is Debug

# Example: Different flags based on build type
if(CMAKE_BUILD_TYPE MATCHES Debug)
  message(STATUS "Enabling debugging symbols")
  add_compile_options(-g)
else()
  message(STATUS "Optimizing for performance")
  add_compile_options(-O3)
endif()

add_executable(my_executable main.cpp)

Custom Build Types (Advanced)

• If the built-in types don't suffice, you can create custom ones using the cmake_minimum_required with a version greater than 3.1:

cmake_minimum_required(VERSION 3.1)

project(MyProject)

# Define a custom build type (e.g., Profiling)
add_compile_definitions(CMAKE_BUILD_TYPE_PROFILING)

# Example: Use the custom type for specific flags
if(CMAKE_BUILD_TYPE MATCHES Profiling)
  message(STATUS "Enabling profiling flags")
  add_compile_options(-pg)
endif()

add_executable(my_executable main.cpp)

• You can use environment variables to pass build configuration options during the build process. This provides flexibility but can be less portable across different build environments.