How to Achieve Variadic Macro Expansion in MSVC ?

MSVC Variadic Macro Expansion: The Basics and a Workaround

In the realm of C programming, macro expansion can be a powerful tool for manipulating and generating code on the fly. However, not all compilers handle variadic macros, which allow for a variable number of arguments, in the same way. Microsoft's Visual C (MSVC ) compiler, specifically, has distinct behavior when it comes to such macros.

The Problem: Unintended Argument Concatenation

Consider a variadic macro defined as follows in GCC:

<code class="c++">#define VA_NARGS(...) VA_NARGS_IMPL(__VA_ARGS__, 5, 4, 3, 2, 1)</code>

This macro is intended to count the number of arguments passed to it. However, when expanded in GCC, it correctly treats each argument as individual, giving us the desired count. In MSVC , however, all arguments are concatenated into a single argument.

The Workaround: Nested Macros and Overloading

To achieve variadic macro expansion in MSVC , we can employ a technique that involves nesting macros and overloading. The following code demonstrates this approach:

<code class="c++">#define GLUE(x, y) x y

#define RETURN_ARG_COUNT(_1_, _2_, _3_, _4_, _5_, count, ...) count
#define EXPAND_ARGS(args) RETURN_ARG_COUNT args

#define COUNT_ARGS_MAX5(...) EXPAND_ARGS((__VA_ARGS__, 5, 4, 3, 2, 1, 0))

#define OVERLOAD_MACRO2(name, count) name##count
#define OVERLOAD_MACRO1(name, count) OVERLOAD_MACRO2(name, count)
#define OVERLOAD_MACRO(name, count) OVERLOAD_MACRO1(name, count)

#define CALL_OVERLOAD(name, ...) GLUE(OVERLOAD_MACRO(name, COUNT_ARGS_MAX5(__VA_ARGS__)), (__VA_ARGS__))</code>

Example Usage:

With this workaround, we can define variadic macros that behave consistently across compilers. For instance, the following error reporting macro:

<code class="c++">#define ERROR(...) CALL_OVERLOAD(ERROR, __VA_ARGS__)</code>

can be used to output errors with a variable number of arguments:

<code class="c++">ERROR("Error: %s", errorMessage);  // single argument
ERROR("Error: %s", errorMessage, "Additional details");  // two arguments</code>

Key Insights:

• MSVC handles variadic macro expansion differently from GCC and other compilers.
• Nested macros and overloading can be used to work around MSVC 's limitations.
• This technique allows for the portable creation of variadic macros that are properly expanded in both GCC and MSVC .

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