Why does MSVC treat VA_ARGS as a single argument in variadic macro expansion, and how can this be worked around?

Variadic Macro Expansion in MSVC

In the world of C programming, macros offer a convenient way to define reusable code snippets. However, when exploring variadic macros in Microsoft's Visual C (MSVC ) compiler, a stumbling block may arise. This article delves into the issue raised by a developer and presents a workaround to enable variadic macro expansion similar to that found in GCC.

Problem Statement

The issue at hand involves a macro that flawlessly expands in GCC but encounters difficulties in MSVC . Specifically, the macro in question malfunctions by treating all VA_ARGS (variable arguments) as a single argument. Below is an excerpt of the problem code:

<code class="cpp">#define VA_NARGS_IMPL(__1, __2, __3, __4, __5, N, ...) N
#define VA_NARGS(...) VA_NARGS_IMPL(__VA_ARGS__, 5, 4, 3, 2, 1)</code>

This macro is designed to count the number of arguments passed to it. However, MSVC erroneously interprets all VA_ARGS as one argument, resulting in an incorrect count.

Workaround

While the inquirer sought a technique to replicate the desired behavior of GCC in MSVC , a brilliant solution was proposed by Jeff Walden in the source thread. This workaround requires the declaration of a helper macro for each variadic macro, as seen below:

<code class="cpp">#define FOO_CHOOSE_HELPER1(macro, count, ...) macro##count(__VA_ARGS__)
#define FOO_CHOOSE_HELPER2(macro, count, ...) FOO_CHOOSE_HELPER1(macro, count)
#define FOO(...) FOO_CHOOSE_HELPER2(FOO, COUNT_ARGS(__VA_ARGS__), __VA_ARGS__)</code>

The COUNT_ARGS macro is used to determine the number of arguments passed to the variadic macro FOO, and the helper macros FOO_CHOOSE_HELPER1 and FOO_CHOOSE_HELPER2 select the appropriate overload of FOO based on the argument count.

Conclusion

While the workaround effectively addresses the variadic macro expansion issue in MSVC , it necessitates the declaration of a helper macro for each variadic macro, which can become cumbersome for large codebases. Nonetheless, it provides a viable solution for those encountering similar challenges in their MSVC projects.

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