C语言宏的妙用

C语言宏的妙用

C程序的编译分为预处理，编译，汇编，链接4个阶段。预处理会展开所有的宏。宏的强大在于可以构造各种语法糖，精简代码，不但使代码看起来更简洁优雅，而且没有任何执行成本（不同于函数）。举个例子，我们要创建A对象，然而在创建A对象时，可能需要创建a1,a2,a3,a4,a5,…等各种对象。只有全部a(i)对象创建成功，A才算成功。例如下面的代码(code1)：

// code1.c
int createAobject (A * pA)
{
  a1 = create_obj();
  a2 = create_obj();
  a3 = create_obj();
  a4 = create_obj();
  a5 = create_obj();

  a = create_objA(a1, a2, a3, a4, a5);

  // success
  *pA = a;
  return 0; 
}

上面的code1代码最大的问题是没有错误检查，这是不允许的，下面加上错误检查code2：

// code2.c
int createAobject (A * pA)
{
  a1 = create_obj();
  if (! a1) {
      // failed
    return -1;
  }
  a2 = create_obj();
  if (! a2) {
      delete_obj(a1);
    return -1;
  }
  a3 = create_obj();
  if (! a3) {
      delete_obj(a2);
      delete_obj(a1);
    return -1;
  }
  a4 = create_obj();
  if (! a4) {
      delete_obj(a3);
    delete_obj(a2);
      delete_obj(a1);
    return -1;
  }
  a5 = create_obj();
  if (! a5) {
      delete_obj(a4);
    delete_obj(a3);
    delete_obj(a2);
      delete_obj(a1);
    return -1;
  }

  a = create_objA(a1, a2, a3, a4, a5);
  if (! a) {
      // failed
      delete_obj(a5);
      delete_obj(a4);
    delete_obj(a3);
    delete_obj(a2);
      delete_obj(a1);
    return -1;
  }
  // success
  *pA = a;
  return 0; 
}

code2 处理的全部错误的逻辑，代码立即变得不可读了。于是再改编到code3:

// code3.c
int createAobject (A * pA)
{
    a=a1=a2=a3=a4=0;
    int oka5 = 0;

    a1 = create_obj();
    if (! a1) {
        // failed
        goto onerror_exit;
    }
    a2 = create_obj();
    if (! a2) {
        goto onerror_exit;
    }
    a3 = create_obj();
    if (! a3) {
        goto onerror_exit;
    }
    a4 = create_obj();
    if (! a4) {
        goto onerror_exit;
    }
    
    if (init_obj(&a5)) {
        goto onerror_exit;
    }
    oka5 = 1;

    a = create_objA(a1, a2, a3, a4, a5);
    if (! a) {
       goto onerror_exit;
    }
    // success
    *pA = a;
    return 0;
    
onerror_exit:
    // failed
    if (!oka5 ) {
        uninit_obj(a5);
    }
    if (a4) {
        delete_obj(a4);
    }
    if (a3) {
        delete_obj(a3);
    }
    if (a2) {
        delete_obj(a2);
    }
    if (a1) {
        delete_obj(a1);
    }
    if (a) {
        delete_obj(a);
    }
    return -1;
}

code3已经比code2强多了，尤其对象数目更多的时候。但是还是太啰嗦，而且依赖a(i)都是指针对象，有时候a(i)根本不是指针，就只能加个标记值oka5，例如上面的a5。于是再升级代码到code4：

// code4.c
int createAobject (A * pA)
{
  int err, errlevel = 0;
  // 创建对象的次序: a0,a1,a2,a3,a4,a5,a
  a0 = create_obj();
  check_null_object(a0, 0);

  a1 = create_obj();
  check_null_object(a1, 1);
  
  a2 = create_obj();
  check_null_object(a2, 2);
  
  a3 = create_obj();
  check_null_object(a3, 3);
  
  a4 = create_obj();
  check_null_object(a4, 4);
  
  err = init_obj(&a5);
  check_ret_error(err, 5);
  
    a = create_objA(a0, a1, a2, a3, a4, a5);
  check_null_object(a, 6);

  // success
  *pA = a;
  return 0;

onerror_exit:
  //  删除对象的次序: a,a5,a4,a3,a2,a1,a0
  onerror_case_begin(7, destroyAobject(a));
  onerror_case_level(6, delete_obj(a));
  onerror_case_level(5, uninit_obj(a5));
  onerror_case_level(4, delete_obj(a4));
  onerror_case_level(3, delete_obj(a3));  
  onerror_case_level(2, delete_obj(a2));
  onerror_case_level(1, delete_obj(a1));
  onerror_case_level(0, delete_obj(a0));
  onerror_case_end();
  return -1;
}

void destroyAobject(a)
{
  uninit_obj(a->a5);
  delete_obj(a->a4));
  delete_obj(a->a3));  
  delete_obj(a->a2));
  delete_obj(a->a1));
  delete_obj(a->a0));
  delete_obj(a);
}

code4就用到了宏，从而使代码更优雅，而且确保对象创建的次序和删除的次序正好相反，宏如下：

#define check_null_object(ob, level)  if (! (ob)) goto_onerror_exit(level)

#define check_ret_error(err, level)  if (err) goto_onerror_exit(level)

#define goto_onerror_exit(level)  do { \
        errlevel = (level); \
        goto onerror_exit; \
    } while(0)

#define onerror_case_begin(maxlevel, freeallobjects) do { \
        if (errlevel == maxlevel) { \
            (freeallobjects); \
        } else { \
            while (errlevel-- > 0) { \
                if (errlevel < 0) break

#define onerror_case_level(level, freeobject) \
                else if (errlevel == level) (freeobject)

#define onerror_case_end() \
            } \
        } \
    } while(0)

code4展示了C语言宏对代码的友好。好好利用它！