Can we implement a custom `static_warning` functionality in C ?

Does there exist a static_warning?

Question:

It is known that Boost provides a "STATIC WARNING" feature. However, this question aims to specifically explore the possibility of implementing a custom static_warning functionality that operates similarly to static_assert but issues a warning at compile time instead of aborting the compilation.

Answer:

Yes, it is possible to implement a custom static_warning feature using either GCC or MSVC. The implementation leverages the macro DEPRECATE to define a warning-emitting function and employs a series of nested macros to create the desired functionality.

Usage:

The custom static_warning can be used like this:

<code class="cpp">STATIC_WARNING(condition, "Warning message here");</code>

…

For instance, this code will issue a warning:

<code class="cpp">STATIC_WARNING(true, "This warning is intended");</code>

Implementation:

The implementation relies on macros to achieve the gewünschten behavior:

<code class="cpp">#define DEPRECATE(foo, msg) foo __attribute__((deprecated(msg)))
#define STATIC_WARNING(cond, msg) ...

...

struct true_type {};
struct false_type {};
template<int test> struct converter : public true_type {};
template<> struct converter<0> : public false_type {};

...

STATIC_WARNING(cond, msg) {
  DEPRECATE(void _(const detail::false_type&), msg) {};
  void _(const detail::true_type& ) {};
  PP_CAT(static_warning,__LINE__)() {_(::detail::converter<(cond)>>());}
}</code>

Example:

Consider the following code:

<code class="cpp">STATIC_WARNING(1 == 1, "This is not a warning");
STATIC_WARNING(1 != 1, "This should generate a warning");</code>

When compiled with the appropriate warning level, the second line will trigger a warning.

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