Step 10: Adding Generator Expressions

Generator expressions are evaluated during build system generation to produce information specific to each build configuration.

Generator expressions are allowed in the context of many target properties, such as LINK_LIBRARIES, INCLUDE_DIRECTORIES, COMPILE_DEFINITIONS and others. They may also be used when using commands to populate those properties, such as target_link_libraries(), target_include_directories(), target_compile_definitions() and others.

Generator expressions may be used to enable conditional linking, conditional definitions used when compiling, conditional include directories and more. The conditions may be based on the build configuration, target properties, platform information or any other queryable information.

There are different types of generator expressions including Logical, Informational, and Output expressions.

Logical expressions are used to create conditional output. The basic expressions are the 0 and 1 expressions. A $<0:...> results in the empty string, and <1:...> results in the content of .... They can also be nested.

A common usage of generator expressions is to conditionally add compiler flags, such as those for language levels or warnings. A nice pattern is to associate this information to an INTERFACE target allowing this information to propagate. Let's start by constructing an INTERFACE target and specifying the required C++ standard level of 11 instead of using CMAKE_CXX_STANDARD.

So the following code:

CMakeLists.txt

# specify the C++ standard
set(CMAKE_CXX_STANDARD 11)
set(CMAKE_CXX_STANDARD_REQUIRED True)

Would be replaced with:

CMakeLists.txt

add_library(tutorial_compiler_flags INTERFACE)
target_compile_features(tutorial_compiler_flags INTERFACE cxx_std_11)

Note: This upcoming section will require a change to the cmake_minimum_required() usage in the code. The Generator Expression that is about to be used was introduced in 3.15. Update the call to require that more recent version:

CMakeLists.txt

cmake_minimum_required(VERSION 3.15)

Next we add the desired compiler warning flags that we want for our project. As warning flags vary based on the compiler we use the COMPILE_LANG_AND_ID generator expression to control which flags to apply given a language and a set of compiler ids as seen below:

CMakeLists.txt

set(gcc_like_cxx "$<COMPILE_LANG_AND_ID:CXX,ARMClang,AppleClang,Clang,GNU,LCC>")
set(msvc_cxx "$<COMPILE_LANG_AND_ID:CXX,MSVC>")
target_compile_options(tutorial_compiler_flags INTERFACE
  "$<${gcc_like_cxx}:$<BUILD_INTERFACE:-Wall;-Wextra;-Wshadow;-Wformat=2;-Wunused>>"
  "$<${msvc_cxx}:$<BUILD_INTERFACE:-W3>>"
)

Looking at this we see that the warning flags are encapsulated inside a BUILD_INTERFACE condition. This is done so that consumers of our installed project will not inherit our warning flags.

Exercise: Modify MathFunctions/CMakeLists.txt so that all targets have a target_link_libraries() call to tutorial_compiler_flags.