可变宏：扩展粘贴的令牌 [英] Variadic macros: expansion of pasted tokens

问题描述

我想知道是否可以嵌套可变宏调用。我只是真正关心GCC和Clang。我的宏定义如下所示：

  / ** 
 * @brief调用实例方法。 
 * / 
 #define $（obj，method，...）\ 
（{\ 
 typeof（obj）_obj = obj; \ 
 _obj-> interface->方法（_obj，## __VA_ARGS__）; \ 
}）
  

我用它在我的OO框架中方便地调用实例方法（ https://github.com/ jdolan / objectively ）：

  $（array，addObject，obj）; 
  

作品老板。不幸的是，我还没有想出一种方法来嵌套这些调用，这在某些情况下非常有用;例如：

  / ** 
 * @see MutableSetInterface :: addObjectsFromArray（MutableSet *，const Array *）
 $ / 
 static void addObjectsFromArray（MutableSet * self，const Array * array）{
 
 if（array）{
 for（size_t i = 0; i< array - > count; i ++）{
 $（self，addObject，$（array，objectAtIndex，i））; 
 
 
 
 code 
 $ b 
上面的嵌套可变宏调用无法编译，因为内部调用永远不会展开。是否有可能解决这个问题，或者我是否已经将预处理器滥用到了极限？ ：）
 
解决方案
这是嵌套预处理器宏的常见问题。预处理器扩展规则相当神秘;相关的tl; dr是宏分层展开的。解决方法是添加一个可以扩展参数的间接层： 
 
 
  #define MI（obj，method ,. ..）\ 
（{\ 
 typeof（obj）_obj = obj; \ 
 _obj-> interface->方法（_obj，## __VA_ARGS__）; \ 
}）
 
 #define M（obj，method，...）MI（obj，method，__VA_ARGS__）
 
 //现在将正确扩展。 
 M（self，addObject，M（array，objectAtIndex，M（foo，bar，i）））
  
注意：注意 $ 不是C的基本源字符集的一部分;使用它可能不会便携。
 
I'm wondering if it's possible to "nest" variadic macro invocations. I'm only truly concerned with GCC and Clang. My macro definition looks like this:
/**
 * @brief Invoke an instance method.
 */
#define $(obj, method, ...) \
    ({ \
        typeof(obj) _obj = obj; \
        _obj->interface->method(_obj, ## __VA_ARGS__); \
    })
I use this to conveniently call "instance methods" in my OO framework (https://github.com/jdolan/objectively):
$(array, addObject, obj);
Works boss. Unfortunately, I haven't yet figured out a way to allow nesting of these calls, which would be very useful in some situations; e.g.:
/**
 * @see MutableSetInterface::addObjectsFromArray(MutableSet *, const Array *)
 */
static void addObjectsFromArray(MutableSet *self, const Array *array) {

    if (array) {
        for (size_t i = 0; i < array->count; i++) {
            $(self, addObject, $(array, objectAtIndex, i));
        }
    }
}
The nested variadic macro invocation above fails to compile because the inner invocation is never expanded. Is it possible to fix this, or have I already abused the preprocessor to its limits? :)
解决方案
This is a common problem with nested preprocessor macros. Preprocessor expansion rules are fairly arcane; the relevant tl;dr is that macros are expanded in layers. The workaround is to add a layer of indirection in which the parameter can be expanded:
#define MI(obj, method, ...) \
  ({ \
    typeof(obj) _obj = obj; \
    _obj->interface->method(_obj, ## __VA_ARGS__); \
  })

#define M(obj, method, ...) MI(obj, method, __VA_ARGS__)

// This will now expand properly.
M(self, addObject, M(array, objectAtIndex, M(foo, bar, i)))
Side note: be aware that $ is not part of C's basic source character set; using it will probably not be portable.

                        
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