介绍一个C++奇巧淫技

你能实现这样一个函数吗：

  MyType type;
  HisType htype;
  serialize_3(11, type, htype);
  serialize_4(type, htype ,type, htype);
  serialize_4(11, type , htype, htype);

参数类型自由，个数自由，怎么做呢？往下看：

[xiaochu.yh@OB macro]$ cat auto_type.cpp
/*
 * (C) 1999-2013 Alibaba Inc.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as
 * published by the Free Software Foundation.
 *
 *
 * Version:  auto_type.cpp,  09/04/2013 08:02:17 PM Yu Huang Exp $
 *
 * Author:
 *   Huang Yu <xiaochu.yh@alipay.com>
 * Description:
 *   auto type match
 *
 */

#include <stdio.h>

void serialize()
{
  return;
}

class HisType
{
  public:
    HisType(const char *i) : value_(i) { }
    ~HisType() { }
    void serialize() const
    {
      printf("HisType: s(%s)\n", value_);
    }
  private:
    const char *value_;
};

class MyType
{
  public:
    MyType(int i) : value_(i) { }
    ~MyType() { }
    void serialize() const
    {
      printf("MyType: f(%d)\n", value_);
    }
  private:
    int value_;
};

void serialize(const int arg0)
{
  printf("int: %d\n", arg0);
}
void serialize(const float arg0)
{
  printf("float: %f\n", arg0);
}

template<typename Arg0>
void serialize(const Arg0 &arg0)
{
  arg0.serialize();
}

template<typename Arg0>
void serialize_1(const Arg0 &arg0)
{
  serialize(arg0);
}

#define JOIN(x,y) JOIN2(x,y)
#define JOIN2(x,y) x##y

#define DECVAL_1 0
#define DECVAL_2 1
#define DECVAL_3 2
#define DECVAL_4 3
#define DEC_VAL(n) DECVAL_##n

// recursively expanding macro
#define ARG_TN0
#define ARG_TN1  typename Arg0
#define ARG_TN2  ARG_TN1, typename Arg1
#define ARG_TN3  ARG_TN2, typename Arg2
#define ARG_TN4  ARG_TN3, typename Arg3

#define ARG_PN0
#define ARG_PN1  const Arg0 & arg0
#define ARG_PN2  ARG_PN1, const Arg1 & arg1
#define ARG_PN3  ARG_PN2, const Arg2 & arg2
#define ARG_PN4  ARG_PN3, const Arg3 & arg3

#define ARG_AN0
#define ARG_AN1  arg0
#define ARG_AN2  ARG_AN1, arg1
#define ARG_AN3  ARG_AN2, arg2
#define ARG_AN4  ARG_AN3, arg3

#define ARG_CN0
#define ARG_CN1  arg0
#define ARG_CN2  arg1
#define ARG_CN3  arg2
#define ARG_CN4  arg3

#define SERIALIZE_DECLARE(NUM_ARG) \
  template<JOIN(ARG_TN, NUM_ARG)> \
void JOIN(serialize_, NUM_ARG)(JOIN(ARG_PN, NUM_ARG))

SERIALIZE_DECLARE(2);
SERIALIZE_DECLARE(3);
SERIALIZE_DECLARE(4);
#define SERIALIZE_DEFINE(NUM_ARG) \
  template<JOIN(ARG_TN, NUM_ARG)> \
void JOIN(serialize_, NUM_ARG)(JOIN(ARG_PN, NUM_ARG)) \
{ \
  JOIN(serialize_, DEC_VAL(NUM_ARG))(JOIN(ARG_AN, DEC_VAL(NUM_ARG))); \
  serialize(JOIN(ARG_CN, NUM_ARG)); \
}

SERIALIZE_DEFINE(2);
SERIALIZE_DEFINE(3);
SERIALIZE_DEFINE(4);

int main()
{
  MyType type(4234);
  HisType htype("home");
  //先来个见面礼, 1是int类型，10.2f是float类型，type是自定义类型
  serialize_4(1,10.2f,3, type);
  printf("==============\n");
  serialize_3(type,11, htype);  // <== 注意下面的参数个数，以及参数顺序，完全自由！
  printf("==============\n");
  serialize_3(11 ,type, htype);
  printf("==============\n");
  serialize_3(htype ,type, htype);
  printf("==============\n");
  return 0;
}

编译运行结果：

[xiaochu.yh@OB macro]$ g++ auto_type.cpp
[xiaochu.yh@OB]$ ./a.out
int: 1
float: 10.200000
int: 3
MyType: f(4234)
==============
MyType: f(4234)
int: 11
HisType: s(home)
==============
int: 11
MyType: f(4234)
HisType: s(home)
==============
HisType: s(home)
MyType: f(4234)
HisType: s(home)
==============

该技术是从曲山同学的代码中学习来的，曲山对宏的运用真是炉火纯青！这里最神奇的就是下面一段代码，至今不明：

#define JOIN(x,y) JOIN2(x,y)
#define JOIN2(x,y) x##y

JOIN和JOIN2不是等价的吗？不过还真不是。如果只写JOIN2，在宏展开阶段会有比较诡异的事情发生。不信你试试。但是为什么呢？我也不知道。@曲山，求助啊~~

更全面的代码见OceanBase源码oceanbase/src/common/ob_rpc_stub.h和oceanbase/src/common/ob_rpc_macros.h

=============================================

UPDATE：

这篇帖子发到了内网，得到了@探晴同学指点，加上@元启 同学的解释，基本弄明白了JOIN的机制。

原因的确很简单。 
#define MY_VALUE 2 
#define JOIN(A,B) A##B
JOIN(hello, world)的输出结果就是 helloworld，
JOIN(MY_VALUE, b)的输出结果就是 MY_VALUEb。尽管MY_VALUE是个宏，你期待它展开成2b。

如何成为一个2b呢？ 这么做：
#define JOIN(a, b) JOIN_EXPAND_PARAM(a,b)
#define JOIN_EXPAND_PARAM(a,b) a##b

JOIN(MY_VALUE, b)的展开过程是：
1. JOIN(MY_VALUE, b)展开成 JOIN_EXPAND_PARAM(2, b)
2. JOIN_EXPAND_PARAM(2, b) 展开成 2b

这句话：Macro arguments are completely macro-expanded before they are substituted into a macro body, unless they are stringified or pasted with other tokens.

参考： http://gcc.gnu.org/onlinedocs/cpp/Argument-Prescan.html