宏魔法

• 下载宏 - 1.2 KB

引言

认为宏很简单、枯燥且用处不大，是吗？

背景

C++ 模板 并非 C 宏 的替代品，而是两者结合起来才能创造出强大的工具，让经验丰富的程序员实现有趣的惯用语，例如 如果成员函数存在则执行它。

Using the Code

那么，我们能用宏做什么？

1. 多行

   // Literally equal to the #define MIN_(A, B) ((A) < (B) ? (A) : (B))
   #define MIN_(A, B) (\
                       (A) < (B) ? \
                       (A) : (B)\
                      )

2. 可变参数

   // Using PRINT_ERROR_("%s they be stealin my %s!", "oh noes", "bucket")
   //  will print "oh noes they be stealin my bucket" in to the standard error stream
   #define PRINT_ERROR_(...) fprintf(stderr, __VA_ARGS__)

3. 串联

   作为字符串

   #define HELLO_ "say hello to my little"
   #define SPACE_ " "
   #define WORLD_ "friend"
   
   // Will be ptr. to the compile-time constant C str. "say hello to my little friend"
   static auto const HELLO_WORLD_ = HELLO_""SPACE_""WORLD_;

   作为标记

   #define NUMBER_FROM_DIGITS_(D1, D2, D3) D1 ## D2 ## D3
   
   static const auto MY_NUMBER_ = NUMBER_FROM_DIGITS_(1, 2, 3); // will be 123

4. 字符串化

   #define MAKE_STR_(STR1, STR2, STR3) #STR1""#STR2""#STR3
   
   // Will be ptr. to the compile-time constant C str. "paint_it_black"
   static auto const MY_STR_ = MAKE_STR_(paint, _it_, black);

5. 调用其他宏

   #define M_S_(arg) #arg // stringify
   #define MAKE_STR__(arg) M_S_(arg)
   
   // Will be ptr. to the compile-time constant C str.
   // In MS VS 2013: "+2,147,483,647" (for 32 bit)
   // In modern GCC: "INT_MAX"
   static auto const MY_STR__ = MAKE_STR__(INT_MAX);

6. 函数式 AND 递归

   int f() throw() {return 1;}
   
   // Only expanded if appears with parentheses
   #define f() (f() + 1) // self-reference is NOT considered a macro call (passed unchanged)
   
   static const auto VAL_1_ = f(); // will be 2
   
   static auto const VAL_2_ = f; // will ptr. to the function

7. 唯一定义规则

   #define SUM(A, B) ((A) + (B))
   #define SUM(A, B) ((A) + (B)) // NO compile error here [the rule doesn't applied to macros]
   
   #define SUM(A, B) ((A)+(B)) // compile warning here (allowed redefinition)
   
   #define SUM(A, B) (  (A) + (B)   )
   #define SUM(A, B) ( (A   ) + 
   	(   B) ) // NO compile warning here (if spacers, their amount is ignored)
   #define SUM(A, B) ( (A   ) /*what if*/ + 
   	/*add some comments here*/ (   B) ) // NO redefinition here: comments count as spacers

8. “即时”重新定义

   #define SUM(A, B) ((A) + (B))
   
   // Will be 1 + 3 + 2 = 6
   static const auto VAL_ = SUM(1
   #undef SUM
   #define SUM + 3
   SUM, // new definition is used for argument pre-expansion
   2);  // original definition is used for argument replacement

测试

在 Ideone 在线编译器中测试代码

std::cout << MIN_(1 + 2, 3 + 4) << std::endl;                   // prints "3"
PRINT_ERROR_("%s they be stealin my %s!", "oh noes", "bucket"); // "oh noes they be stealin my bucket!"
std::cout << HELLO_WORLD_ << std::endl;                         // "say hello to my little friend"
std::cout << MY_NUMBER_ + 1 << std::endl;                       // prints 124
std::cout << MY_STR_ << std::endl;                              // prints "paint_it_black"
std::cout << MY_STR__ << std::endl;                             // prints "INT_MAX"
std::cout << VAL_1_ << std::endl;                               // prints "2"
std::cout << VAL_2_ + 10 << std::endl;                          // prints "1" (as a non-nil pointer)
std::cout << VAL_ << std::endl;                                 // prints "6"

关注点

警告！结果可能因使用的编译器而异！

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历史

• 2016年3月5日：初始版本