Compiler flags and warnings

Your code should compile with warnings enabled, but only for your code, and with flags that exist on the compiler you are using. This lesson shows the per-target way to do it.

The per-target command

target_compile_options(app PRIVATE -Wall -Wextra)

Same grammar as everything else. Attach to one target, PRIVATE so it does not leak into libraries you link.

Why

add_compile_options(-Wall) is global. It would also apply to every add_subdirectory target you do not own, including third-party code whose warnings you cannot fix. Per-target options mean your warnings, your code.

Flags are compiler-specific

-Wall is GCC and Clang. MSVC wants /W4. Writing one line that works everywhere means branching on the compiler. The official tutorial shows the pattern, checking the frontend variant, which also catches clang-cl, since it uses the MSVC frontend:

if(CMAKE_CXX_COMPILER_FRONTEND_VARIANT MATCHES "MSVC")
    target_compile_options(app PRIVATE /W4)
elseif(CMAKE_CXX_COMPILER_FRONTEND_VARIANT MATCHES "GNU|Clang")
    target_compile_options(app PRIVATE -Wall -Wextra)
endif()

Tip

Wrap it in an interface library (lesson 13) so every target in your project gets the same warnings with target_link_libraries(app PRIVATE warnings). That is the pattern larger projects use instead of repeating the if block.

Warnings as errors. Do not default to it

Turning warnings into errors (-Werror / /WX) makes CI fail on any warning, which sounds great. But it breaks the moment a new compiler version adds a warning to your dependencies’ headers. The docs are explicit. Projects should not turn warnings-as-errors flags on by default.

# OK for your own code, as an opt-in for CI. Not the default.
option(APP_ENABLE_WERROR "Treat warnings as errors" OFF)
if(APP_ENABLE_WERROR)
    if(MSVC)
        target_compile_options(app PRIVATE /WX)
    else()
        target_compile_options(app PRIVATE -Werror)
    endif()
endif()

Definitions

-DFOO=bar has a per-target form too:

target_compile_definitions(app PRIVATE VERSION=2 NDEBUG)

The old global add_definitions(-DVERSION=2) is the same reasoning as before. Attached to a target, it cannot leak.

Note

CMake itself leaves MSVC-style warning flags out of CMAKE_<LANG>_FLAGS by default (policy CMP0092, NEW since 3.15), precisely so that you decide per-target. If you see a tutorial setting CMAKE_CXX_FLAGS "-Wall", that is the old style.

Pitfall

Do not put compiler flags in CMAKE_CXX_FLAGS for your project. It is global, it affects every target, it persists in the build directory’s cache even after you delete the line, and it breaks the one-target-one-requirement model. Flags belong on targets.

Read the docs

• target_compile_options
• In-Depth CMake Target Commands, official tutorial
• Policy CMP0092