There is more

Most of the time, flags are compiler-specific. Our current example will only work with GCC and Clang; compilers from other vendors will not understand many, if not all, of those flags. Clearly, if a project aims at being truly cross-platform, this problem has to be solved. There are three approaches to this.

The most typical approach will append a list of desired compiler flags to each configuration type CMake variable, that is, to CMAKE_<LANG>_FLAGS_<CONFIG>. These flags are set to what is known to work for the given compiler vendor, and will thus be enclosed in 
if-endif clauses that check the CMAKE_<LANG>_COMPILER_ID variable, for example:

if(CMAKE_CXX_COMPILER_ID MATCHES GNU)
  list(APPEND CMAKE_CXX_FLAGS "-fno-rtti" "-fno-exceptions")
  list(APPEND CMAKE_CXX_FLAGS_DEBUG "-Wsuggest-final-types" "-Wsuggest-final-methods" "-Wsuggest-override")
  list(APPEND CMAKE_CXX_FLAGS_RELEASE "-O3" "-Wno-unused")
endif()

if(CMAKE_CXX_COMPILER_ID MATCHES Clang)  
  list(APPEND CMAKE_CXX_FLAGS "-fno-rtti" "-fno-exceptions" "-Qunused-arguments" "-fcolor-diagnostics")
  list(APPEND CMAKE_CXX_FLAGS_DEBUG "-Wdocumentation")
  list(APPEND CMAKE_CXX_FLAGS_RELEASE "-O3" "-Wno-unused")
endif()

A more refined approach does not tamper with the CMAKE_<LANG>_FLAGS_<CONFIG> variables at all and rather defines project-specific lists of flags:

set(COMPILER_FLAGS)
set(COMPILER_FLAGS_DEBUG)
set(COMPILER_FLAGS_RELEASE)

if(CMAKE_CXX_COMPILER_ID MATCHES GNU)
  list(APPEND CXX_FLAGS "-fno-rtti" "-fno-exceptions")
  list(APPEND CXX_FLAGS_DEBUG "-Wsuggest-final-types" "-Wsuggest-final-methods" "-Wsuggest-override")
  list(APPEND CXX_FLAGS_RELEASE "-O3" "-Wno-unused")
endif()

if(CMAKE_CXX_COMPILER_ID MATCHES Clang)  
  list(APPEND CXX_FLAGS "-fno-rtti" "-fno-exceptions" "-Qunused-arguments" "-fcolor-diagnostics")
  list(APPEND CXX_FLAGS_DEBUG "-Wdocumentation")
  list(APPEND CXX_FLAGS_RELEASE "-O3" "-Wno-unused")
endif()

Later on, it uses generator expressions to set compiler flags on a per-configuration and per-target basis:

target_compile_option(compute-areas
  PRIVATE
    ${CXX_FLAGS}
    "$<$<CONFIG:Debug>:${CXX_FLAGS_DEBUG}>"
    "$<$<CONFIG:Release>:${CXX_FLAGS_RELEASE}>"
  )

We have shown both approaches in the current recipe and have clearly recommended the latter (project-specific variables and target_compile_options) over the former (CMake variables).

Both approaches work and are widely used in many projects. However, they have shortcomings. As we have already mentioned, CMAKE_<LANG>_COMPILER_ID is not guaranteed to be defined for all compiler vendors. In addition, some flags might become deprecated or might have been introduced in a later version of the compiler. Similarly to CMAKE_<LANG>_COMPILER_ID, the CMAKE_<LANG>_COMPILER_VERSION variable is not guaranteed to be defined for all languages and vendors. Although checking on these variables is quite popular, we think that a more robust alternative would be to check whether a desired set of flags works with the given compiler, so that only effectively working flags are actually used in the project. Combined with the use of project-specific variables, target_compile_options, and generator expressions, this approach is quite powerful. We will show how to use this check-and-set pattern in Recipe 3, Writing a function to test and set compiler flags, in Chapter 7, Structuring Projects.