可变参数宏:粘贴标记的扩展 [英] Variadic macros: expansion of pasted tokens

问题描述

我想知道是否可以嵌套"可变参数宏调用.我只真正关心 GCC 和 Clang.我的宏定义如下所示:

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__); 
    })

我使用它来方便地在我的 OO 框架中调用实例方法"(https://github.com/jdolan/客观地):

I use this to conveniently call "instance methods" in my OO framework (https://github.com/jdolan/objectively):

$(array, addObject, obj);

工作老板.不幸的是，我还没有找到允许嵌套这些调用的方法，这在某些情况下会非常有用；例如:

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));
        }
    }
}

上面的嵌套可变参数宏调用无法编译，因为内部调用从未扩展.是否有可能解决这个问题，或者我已经将预处理器滥用到了极限?:)

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? :)

推荐答案

这是嵌套预处理器宏的常见问题.预处理器扩展规则相当晦涩；相关的 tl;dr 是宏是分层扩展的.解决方法是添加一个可以扩展参数的间接层:

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)))

旁注:注意 $ 不是 C 的基本源字符集的一部分；使用它可能不便携.

Side note: be aware that $ is not part of C's basic source character set; using it will probably not be portable.

这篇关于可变参数宏:粘贴标记的扩展的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！