Why Does Variadic Macro Expansion Differ Between MSVC and GCC?

Variadic Macro Expansion in MSVC

The variadic macro mechanism in MSVC behaves differently from the one in GCC, leading to difficulties in expanding macros with multiple arguments. Consider the following macros:

<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)

#define FULLY_EXPANDED(count, ...) \
  MAC ## count (__VA_ARGS__)

#define SEMI_EXPANDED(count, ...) FULLY_EXPANDED(count, __VA_ARGS__)

#define EXPAND_THESE(...) SEMI_EXPANDED(VA_NARGS(__VA_ARGS__), __VA_ARGS__)

#define ACTUAL_MACRO(x) parent->GetProperty<x>();
#define MAC1(a) ACTUAL_MACRO(a)
#define MAC2(a,b) MAC1(a) ACTUAL_MACRO(b)
#define MAC3(a,b,c) MAC2(a,b) ACTUAL_MACRO(c)
#define MAC4(a,b,c,d) MAC3(a,b,c) ACTUAL_MACRO(d)
#define MAC5(a,b,c,d,e) MAC4(a,b,c,d) ACTUAL_MACRO(e)</code>

In GCC, this macro would expand to:

<code class="cpp">struct MyStructure
{
  void Foo()
  {
    parent->GetProperty<Property1>(); 
    parent->GetProperty<Property2>(); 
    parent->GetProperty<Property3>(); 
    parent->GetProperty<Property4>();
  }

  Base * parent;
}</code>

However, in MSVC , it expands to:

<code class="cpp">struct MyStructure
{
  void Foo()
  {
    parent->GetProperty<Property1, Property2, Property3, Property4>();
  }

  Base * parent;
}</code>

Explanation

The issue arises because MSVC applies macro expansion level-by-level, while GCC expands macros fully in one pass. Consider the macro invocation:

<code class="cpp">EXPAND_THESE(Property1, Property2, Property3, Property4)</code>

• MSVC :

  1. VA_NARGS(Property1, Property2, Property3, Property4) -> 4
  2. SEMI_EXPANDED(4, Property1, Property2, Property3, Property4) -> MAC4(Property1, Property2, Property3, Property4)
  3. EXPAND_THESE(MAC4(Property1, Property2, Property3, Property4)) -> MAC4(Property1, Property2, Property3, Property4)

• GCC:

  1. VA_NARGS(Property1, Property2, Property3, Property4) -> 4
  2. SEMI_EXPANDED(4, Property1, Property2, Property3, Property4) -> MAC4(Property1, Property2, Property3, Property4)
  3. EXPAND_THESE(MAC4(Property1, Property2, Property3, Property4)) -> MAC4(Property1, Property2, Property3, Property4)
     --> Further unfolds the MAC4 macro
     --> Equivalent to multiple lines of ACTUAL_MACRO calls

Workaround

To achieve similar behavior to GCC, one can employ Jeff Walden's approach, which involves creating additional macro helpers and using the COUNT_ARGS_MAX5 macro to determine the number of arguments:

<code class="cpp">#define FOO_CHOOSE_HELPER1(count) FOO##count
#define FOO_CHOOSE_HELPER2(count) FOO_CHOOSE_HELPER1(count)
#define FOO_CHOOSE_HELPER(count) FOO_CHOOSE_HELPER2(count)

#define ERROR(...) GLUE(FOO_CHOOSE_HELPER(COUNT_ARGS_MAX5(__VA_ARGS__)), (__VA_ARGS__))

#define ASSERT1(expr) singleArgumentExpansion(expr)
#define ASSERT2(expr, explain) twoArgumentExpansion(expr, explain)
#define ASSERT(...) GLUE(FOO_CHOOSE_HELPER(COUNT_ARGS_MAX5(__VA_ARGS__)), (__VA_ARGS__))</code>

Alternatively, the abstraction provided by the "GLUE" macro can be used to simplify the workaround:

<code class="cpp">#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__))

#define ERROR1(title) printf("Error: %s\n", title);
#define ERROR2(title, message) \
    ERROR1(title); \
    printf("Message: %s\n", message);
#define ERROR(...) CALL_OVERLOAD(ERROR, __VA_ARGS__);

#define ASSERT1(expr) singleArgumentExpansion(expr)
#define ASSERT2(expr, explain) twoArgumentExpansion(expr, explain)
#define ASSERT(...) CALL_OVERLOAD(ASSERT, __VA_ARGS__);</code>

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