Why does MSVC variadic macro expansion differ from GCC, and how can we overcome this discrepancy?-C++-php.cn

Why does MSVC variadic macro expansion differ from GCC, and how can we overcome this discrepancy?

Barbara Streisand

Release： 2024-11-06 09:10:02

Original

795 people have browsed it

Why does MSVC variadic macro expansion differ from GCC, and how can we overcome this discrepancy?

MSVC Variadic Macro Expansion

Variadic macros offer versatile expansion capabilities, allowing you to handle multiple arguments within macros. In GCC, macros like the following work as expected:

<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>

Copy after login

MSVC Expansion Behavior

However, in Microsoft's C Compiler (MSVC ), the macro expansion behaves differently. Instead of expanding each argument separately, MSVC concatenates all arguments into a single expression.

struct MyStructure<br>{<br>  void Foo()<br>  {</p>
<div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">EXPAND_THESE(Property1, Property2, Property3, Property4)

Copy after login

struct MyStructure<br>{<br> void Foo()<br> {</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">parent->GetProperty<Property1>(); parent->GetProperty<Property2>(); parent->GetProperty<Property3>(); parent->GetProperty<Property4>();

<code class="cpp">#define ERRORn(...) ERROR_CHOOSE_HELPERn(COUNT_ARGS_MAX5(__VA_ARGS__), __VA_ARGS__) #define ERROR_CHOOSE_HELPER1(count) ERROR1 #define ERROR_CHOOSE_HELPER2(count) ERROR2 #define ERROR_CHOOSE_HELPER(count) ERROR_CHOOSE_HELPER##count</code>

<code class="cpp">#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>

<code class="cpp">#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>