当代码流是这样的:

if(check())
{
  ...
  ...
  if(check())
  {
    ...
    ...
    if(check())
    {
      ...
      ...
    }
  }
}

我通常看到这种方法可以避免上述混乱的代码流:

do {
    if(!check()) break;
    ...
    ...
    if(!check()) break;
    ...
    ...
    if(!check()) break;
    ...
    ...
} while(0);

有什么更好的方法可以避免这种工作/黑客,从而使其成为更高级别(行业级别)的代码?

任何跳出盒子的建议都是欢迎的!


当前回答

对我来说,当(0)是好的时候做{…}。如果您不想看到do{…}while(0),您可以为它们定义替代关键字。

例子:

SomeUtilities.hpp:

#define BEGIN_TEST do{
#define END_TEST }while(0);

SomeSourceFile.cpp:

BEGIN_TEST
   if(!condition1) break;
   if(!condition2) break;
   if(!condition3) break;
   if(!condition4) break;
   if(!condition5) break;
   
   //processing code here

END_TEST

我认为编译器将在二进制版本的do{…}while(0)中删除不必要的while(0)条件,并将断点转换为无条件跳转。您可以检查它的汇编语言版本来确定。

使用goto还可以生成更简洁的代码,并且使用条件-然后跳转的逻辑非常简单。您可以执行以下操作:

{
   if(!condition1) goto end_blahblah;
   if(!condition2) goto end_blahblah;
   if(!condition3) goto end_blahblah;
   if(!condition4) goto end_blahblah;
   if(!condition5) goto end_blahblah;
   
   //processing code here

 }end_blah_blah:;  //use appropriate label here to describe...
                   //  ...the whole code inside the block.
 

注意标签放在结束}之后。这是避免一个可能的问题,在goto中,不小心将代码放在中间,因为你没有看到标签。现在就像do{…}while(0)没有条件代码。

为了让代码更清晰,更容易理解,你可以这样做:

SomeUtilities.hpp:

#define BEGIN_TEST {
#define END_TEST(_test_label_) }_test_label_:;
#define FAILED(_test_label_) goto _test_label_

SomeSourceFile.cpp:

BEGIN_TEST
   if(!condition1) FAILED(NormalizeData);
   if(!condition2) FAILED(NormalizeData);
   if(!condition3) FAILED(NormalizeData);
   if(!condition4) FAILED(NormalizeData);
   if(!condition5) FAILED(NormalizeData);

END_TEST(NormalizeData)

有了这个,你可以做嵌套块,并指定你想要退出/跳出的位置。

BEGIN_TEST
   if(!condition1) FAILED(NormalizeData);
   if(!condition2) FAILED(NormalizeData);

   BEGIN_TEST
      if(!conditionAA) FAILED(DecryptBlah);
      if(!conditionBB) FAILED(NormalizeData);   //Jump out to the outmost block
      if(!conditionCC) FAILED(DecryptBlah);
  
      // --We can now decrypt and do other stuffs.

   END_TEST(DecryptBlah)

   if(!condition3) FAILED(NormalizeData);
   if(!condition4) FAILED(NormalizeData);

   // --other code here

   BEGIN_TEST
      if(!conditionA) FAILED(TrimSpaces);
      if(!conditionB) FAILED(TrimSpaces);
      if(!conditionC) FAILED(NormalizeData);   //Jump out to the outmost block
      if(!conditionD) FAILED(TrimSpaces);

      // --We can now trim completely or do other stuffs.

   END_TEST(TrimSpaces)

   // --Other code here...

   if(!condition5) FAILED(NormalizeData);

   //Ok, we got here. We can now process what we need to process.

END_TEST(NormalizeData)

杂乱的代码不是goto的错,而是程序员的错。不使用goto仍然可以生成意大利面条代码。

其他回答

当然不是答案,而是一个答案(为了完整起见)

而不是:

do {
    if(!check()) break;
    ...
    ...
    if(!check()) break;
    ...
    ...
    if(!check()) break;
    ...
    ...
} while(0);

你可以这样写:

switch (0) {
case 0:
    if(!check()) break;
    ...
    ...
    if(!check()) break;
    ...
    ...
    if(!check()) break;
    ...
    ...
}

这仍然是一个伪装的goto,但至少它不再是一个循环。这意味着你不需要非常仔细地检查是否有一些继续隐藏在块的某处。

构造也非常简单,您可以希望编译器会优化它。

正如@jamesdlin所建议的,你甚至可以把它隐藏在一个宏后面

#define BLOC switch(0) case 0:

然后像这样使用它

BLOC {
    if(!check()) break;
    ...
    ...
    if(!check()) break;
    ...
    ...
    if(!check()) break;
    ...
    ...
}

这是可能的,因为C语言语法期望在开关之后有一个语句,而不是一个括号中的块,并且您可以在该语句之前放置一个case标签。到目前为止,我还没有看到允许这样做的意义,但在这种特殊情况下,将开关隐藏在一个漂亮的宏后面是很方便的。

如果根据故障发生的位置需要不同的清理步骤,另一个模式很有用:

    private ResultCode DoEverything()
    {
        ResultCode processResult = ResultCode.FAILURE;
        if (DoStep1() != ResultCode.SUCCESSFUL)
        {
            Step1FailureCleanup();
        }
        else if (DoStep2() != ResultCode.SUCCESSFUL)
        {
            Step2FailureCleanup();
            processResult = ResultCode.SPECIFIC_FAILURE;
        }
        else if (DoStep3() != ResultCode.SUCCESSFUL)
        {
            Step3FailureCleanup();
        }
        ...
        else
        {
            processResult = ResultCode.SUCCESSFUL;
        }
        return processResult;
    }

我会推荐一种类似于Mats答案的方法,减去不必要的goto。只在函数中放入条件逻辑。任何始终运行的代码都应该在调用方调用函数之前或之后执行:

void main()
{
    //do stuff always
    func();
    //do other stuff always
}

void func()
{
    if (!condition)
        return;
    ...
    if (!other condition)
        return;
    ...
    if (!another condition)
        return;
    ... 
    if (!yet another condition)
        return;
    ...
}

I'm adding an answer for the sake of completeness. A number of other answers pointed out that the large condition block could be split out into a separate function. But as was also pointed out a number of times is that this approach separates the conditional code from the original context. This is one reason that lambdas were added to the language in C++11. Using lambdas was suggested by others but no explicit sample was provided. I've put one in this answer. What strikes me is that it feels very similar to the do { } while(0) approach in many ways - and maybe that means it's still a goto in disguise....

earlier operations
...
[&]()->void {

    if (!check()) return;
    ...
    ...
    if (!check()) return;
    ...
    ...
    if (!check()) return;
    ...
    ...
}();
later operations

你可以使用一个简单的bool变量的延续模式:

bool goOn;
if ((goOn = check0())) {
    ...
}
if (goOn && (goOn = check1())) {
    ...
}
if (goOn && (goOn = check2())) {
    ...
}
if (goOn && (goOn = check3())) {
    ...
}

这个执行链将在checkN返回false时停止。由于&&操作符短路,将不再执行进一步的检查…()调用。此外,优化编译器足够聪明,可以识别将goOn设置为false是单行道,并为您插入缺少的goto结尾。因此,上面代码的性能将与do/while(0)相同,只是对其可读性没有严重的影响。