Can We Emit Compile-Time Warnings Like `static_assert` in C ?

Can I Implement a "Static Warning" to Emit Warnings at Compile Time?

Static_assert in C 11 is a valuable tool for identifying errors at compile time. However, it only generates compilation errors, not warnings. This question explores the possibility of implementing a "static_warning" that emits warnings instead of aborting compilation.

Possible Implementation Using Macros

Drawing inspiration from Michael E's comment, a sophisticated solution is proposed using macros:

<code class="cpp">#define DEPRECATE(foo, msg) foo __attribute__((deprecated(msg)))  // GCC
#define DEPRECATE(foo, msg) __declspec(deprecated(msg)) foo  // MSVC</code>

Additional macros are introduced to facilitate the creation of static warnings:

<code class="cpp">#define STATIC_WARNING(cond, msg) ...
#define STATIC_WARNING_TEMPLATE(token, cond, msg) ...</code>

Usage Examples

The macros can be employed at various scopes:

<code class="cpp">STATIC_WARNING(1==2, "Failed with 1 and 2");
STATIC_WARNING(..., "2 and 3 worked");

struct Foo {
  STATIC_WARNING(..., "2 and 3: oops");
};

void func() {
  STATIC_WARNING(..., "Not so good on 3 and 4");
}

template <typename T> struct wrap {
  STATIC_WARNING_TEMPLATE(WRAP_WARNING1, ..., "A template warning");
};</code>

Output in Different Compilers

Compiling the example code with various compilers generates warnings as expected:

• GCC 4.6:

  warning: ‘void static_warning1::_(const detail::false_type&)’ is deprecated ...

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• Visual C 2010:

  warning C4996: 'static_warning1::_': Failed with 1 and 2

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• Clang 3.1:

  warning: '_' is deprecated: Failed with 1 and 2 ...

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Conclusion

The presented macros provide a sophisticated approach to implementing a static_warning feature that emits warnings rather than causing compilation errors. It is a useful tool for debugging and tracing complex template specializations during compilation.

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