Debug vs Release in CMake
Instead of manipulating the CMAKE_CXX_FLAGS strings directly (which could be done more nicely using string(APPEND CMAKE_CXX_FLAGS_DEBUG " -g3") btw), you can use add_compile_options:
add_compile_options(
"-Wall" "-Wpedantic" "-Wextra" "-fexceptions"
"$<$<CONFIG:DEBUG>:-O0;-g3;-ggdb>"
)
This would add the specified warnings to all build types, but only the given debugging flags to the DEBUG build. Note that compile options are stored as a CMake list, which is just a string separating its elements by semicolons ;.
For debug/release flags, see the CMAKE_BUILD_TYPE variable (you pass it as cmake -DCMAKE_BUILD_TYPE=value). It takes values like Release, Debug, etc.
https://gitlab.kitware.com/cmake/community/wikis/doc/cmake/Useful-Variables#compilers-and-tools
cmake uses the extension to choose the compiler, so just name your files .c.
You can override this with various settings:
For example:
set_source_files_properties(yourfile.c LANGUAGE CXX)
Would compile .c files with g++. The link above also shows how to select a specific compiler for C/C++.
A lot of the answers here are out of date/bad. So I'm going to attempt to answer it better. Granted I'm answering this question in 2020, so it's expected things would change.
How do I run CMake for each target type (debug/release)?
First off Debug/Release are called configurations in cmake (nitpick).
If you are using a single configuration generator (Ninja/Unix-Makefiles) you must specify the CMAKE_BUILD_TYPE.
Like this:
# Configure the build
cmake -S . -B build/ -D CMAKE_BUILD_TYPE=Debug
# Actually build the binaries
cmake --build build/
# Configure a release build
cmake -S . -B build/ -D CMAKE_BUILD_TYPE=Release
# Build release binaries
cmake --build build/
For multi-configuration generators it's slightly different (Ninja Multi-Config, Visual Studio)
# Configure the build
cmake -S . -B build
# Build debug binaries
cmake --build build --config Debug
# Build release binaries
cmake --build build --config Release
If you are wondering why this is necessary it's because cmake isn't a build system. It's a meta-build system (IE a build system that build's build systems). This is basically the result of handling build systems that support multiple-configurations in 1 build. If you'd like a deeper understanding I'd suggest reading a bit about cmake in Craig Scott's book "Professional CMake: A Practical Guide
How do I specify debug and release C/C++ flags using CMake?
The modern practice is to use target's and properties.
Here is an example:
add_library(foobar)
# Add this compile definition for debug builds, this same logic works for
# target_compile_options, target_link_options, etc.
target_compile_definitions(foobar PRIVATE
$<$<CONFIG:Debug>:
FOOBAR_DEBUG=1
>
)
NOTE: How I'm using generator expressions to specify the configuration! Using CMAKE_BUILD_TYPE will result in bad builds for any multi-configuration generator!
Further more sometimes you need to set things globally and not just for one target. Use add_compile_definitions, add_compile_options, etc. Those functions support generator expressions. Don't use old style cmake unless you have to (that path is a land of nightmares)
How do I express that the main executable will be compiled with g++ and one nested library with gcc?
Your last question really doesn't make sense.
With CMake, it's generally recommended to do an "out of source" build. Create your CMakeLists.txt in the root of your project. Then from the root of your project:
mkdir Release
cd Release
cmake -DCMAKE_BUILD_TYPE=Release ..
make
And for Debug (again from the root of your project):
mkdir Debug
cd Debug
cmake -DCMAKE_BUILD_TYPE=Debug ..
make
Release / Debug will add the appropriate flags for your compiler. There are also RelWithDebInfo and MinSizeRel build configurations.
You can modify/add to the flags by specifying a toolchain file in which you can add CMAKE_<LANG>_FLAGS_<CONFIG>_INIT variables, e.g.:
set(CMAKE_CXX_FLAGS_DEBUG_INIT "-Wall")
set(CMAKE_CXX_FLAGS_RELEASE_INIT "-Wall")
See CMAKE_BUILD_TYPE for more details.
As for your third question, I'm not sure what you are asking exactly. CMake should automatically detect and use the compiler appropriate for your different source files.