Clang Flag: -Wclass-varargs / -Wno-class-varargs

The Clang compiler flags -Wclass-varargs and -Wno-class-varargs are used to control warnings related to passing class objects through variadic functions. Variadic functions are functions (such as printf) that can take an unspecified number of arguments. In C++, variadic functions can only safely handle POD (Plain Old Data) types due to the standard calling conventions. The -Wclass-varargs flag, when enabled, activates warnings if an object of a class type is passed through a variadic function. Such operations are risky, as non-POD class types may have complex construction, destruction, and copying semantics that variadic functions are not equipped to handle, potentially leading to runtime errors or undefined behavior. This flag is inactive by default and must be explicitly turned on if developers wish to be notified of such usages in their code. On the other hand, -Wno-class-varargs suppresses warnings related to passing non-POD class types through variadic functions. This flag can be useful in suppressing noisy warnings during compilation, especially if the warnings are considered non-critical or if the codebase is known to manage such situations correctly. This flag is active by default, indicating that Clang will not warn about non-POD class types passed to variadic functions unless the developer opts in with -Wclass-varargs. Developers utilizing Clang for compilation should carefully consider the implications of these flags, particularly if their projects make use of variadic functions. Awareness and management of these compilation flags can aid in maintaining the portability, stability, and reliability of C++ codebases.

AI Generated

Subroups

• -Wnon-pod-varargs (4 elements)

Warnings/Remarks

Default Active (Deactivate with -Wno-class-varargs)

warn_cannot_pass_non_pod_arg_to_vararg	error: cannot pass object of non-POD non-trivial type B through variadic function block method constructor ; call will abort at runtime
warn_non_pod_vararg_with_format_string	error: cannot pass non-POD non-trivial object of type B to variadic function block method constructor ; expected type from format string was D
warn_second_parameter_to_va_arg_not_pod	error: second argument to 'va_arg' is of non-POD type A
warn_second_parameter_to_va_arg_ownership_qualified	error: second argument to 'va_arg' is of ARC ownership-qualified type A

Default Inactive (Activate with -Wclass-varargs)

warn_pass_class_arg_to_vararg

warning: passing object of class type A through variadic function block method constructor   ; did you mean to call 'D'?