C++17 and C++20 standard code is used throughout projects of all sizes, perhaps with limited-feature fallback to older language standards. Some standards certifications require a specific language standard. High reliability and safety-critical projects may require specific language standards. Examples include FACE and MISRA C++.

To enforce a specific language standard be limited, consider in a header used throughout the project as follows. This example limits the language standard to C++14 or earlier by halt the build if a higher standard is detected:

#if __cplusplus >= 201703L
#error "C++14 or earlier required"
#endif

For C code say no higher than C99, consider in a header used throughout the project, which will halt the build if a higher standard is detected:

#if __STDC_VERSION__ >= 201112L
#error "C99 or earlier required"
#endif