Step 4: 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 $<1:...>
results in the content of ...
. They can also
be nested.
Exercise 1 - Adding Compiler Warning Flags with Generator Expressions¶
A common usage of
generator expressions
is to
conditionally add compiler flags, such as those for language levels or
warnings. A nice pattern is to associate this information to an INTERFACE
target allowing this information to propagate.
Goal¶
Add compiler warning flags when building but not for installed versions.
Helpful Resources¶
Files to Edit¶
CMakeLists.txt
Getting Started¶
Open the file Step4/CMakeLists.txt
and complete TODO 1
through
TODO 4
.
First, in the top level CMakeLists.txt
file, we need to set the
cmake_minimum_required()
to 3.15
. In this exercise we are going
to use a generator expression which was introduced in CMake 3.15.
Next we add the desired compiler warning flags that we want for our project.
As warning flags vary based on the compiler, we use the
COMPILE_LANG_AND_ID
generator expression to control which flags to apply
given a language and a set of compiler ids.
Build and Run¶
Make a new directory called Step4_build
, run the cmake
executable or the cmake-gui
to configure the project
and then build it with your chosen build tool or by using cmake --build .
from the build directory.
mkdir Step4_build
cd Step4_build
cmake ../Step4
cmake --build .
Solution¶
Update the cmake_minimum_required()
to require at least CMake
version 3.15
:
TODO 1: Click to show/hide answer
cmake_minimum_required(VERSION 3.15)
Next we determine which compiler our system is currently using to build
since warning flags vary based on the compiler we use. This is done with
the COMPILE_LANG_AND_ID
generator expression. We set the result in the
variables gcc_like_cxx
and msvc_cxx
as follows:
TODO 2: Click to show/hide answer
set(gcc_like_cxx "$<COMPILE_LANG_AND_ID:CXX,ARMClang,AppleClang,Clang,GNU,LCC>")
set(msvc_cxx "$<COMPILE_LANG_AND_ID:CXX,MSVC>")
Next we add the desired compiler warning flags that we want for our project.
Using our variables gcc_like_cxx
and msvc_cxx
, we can use another
generator expression to apply the respective flags only when the variables are
true. We use target_compile_options()
to apply these flags to our
interface library.
TODO 3: Click to show/hide answer
target_compile_options(tutorial_compiler_flags INTERFACE
"$<${gcc_like_cxx}:-Wall;-Wextra;-Wshadow;-Wformat=2;-Wunused>"
"$<${msvc_cxx}:-W3>"
)
Lastly, we only want these warning flags to be used during builds. Consumers
of our installed project should not inherit our warning flags. To specify
this, we wrap our flags from TODO 3 in a generator expression using the
BUILD_INTERFACE
condition. The resulting full code looks like the following:
TODO 4: Click to show/hide answer
target_compile_options(tutorial_compiler_flags INTERFACE
"$<${gcc_like_cxx}:$<BUILD_INTERFACE:-Wall;-Wextra;-Wshadow;-Wformat=2;-Wunused>>"
"$<${msvc_cxx}:$<BUILD_INTERFACE:-W3>>"
)