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The #warning directive emits a user-specified warning message at compile time. It doesn't stop compilation. This directive is available starting in C23 and C++23.
Syntax
#warning token-string
Remarks
The warning message is the token-string parameter. The token-string parameter isn't subject to macro expansion and can be optionally enclosed in quotes.
Use this directive to inform the developer of a nonfatal issue or to communicate other important information during compilation.
The following example shows how to use the #warning directive:
#if defined(_LEGACY_FEATURE_FLAG)
#warning "_LEGACY_FEATURE is deprecated and should not be used."
#endif