cocos2d-x 問題集。

1、源代碼裏的do{}while(0)

今天寫到一個代碼：

	if(!CCScene::init())
	{
		return false;
	}

後來就去看源代碼：

bool CCScene::init()
{
    bool bRet = false;
     do 
     {
         CCDirector * pDirector;
         CC_BREAK_IF( ! (pDirector = CCDirector::sharedDirector()) );
         this->setContentSize(pDirector->getWinSize());
         // success
         bRet = true;
     } while (0);
     return bRet;
}

看後，我就奇怪，do{}while(0)不就是執行一次麼，是什麼意思？

後來百度，得到如下最佳解釋：

FAQ FROM CSDN:

FAQ/DoWhile0

Why do a lot of #defines in the kernel use do { ... } while(0)?

There are a couple of reasons:

• (from Dave Miller) Empty statements give a warning from the compiler so this is why you see #define FOO do { } while(0).

• (from Dave Miller) It gives you a basic block in which to declare local variables.

• (from Ben Collins) It allows you to use more complex macros in conditional code. Imagine a macro of several lines of code like:

  #define FOO(x) /
          printf("arg is %s/n", x); /
          do_something_useful(x);
  

  Now imagine using it like:

  if (blah == 2)
          FOO(blah);
  

  This interprets to:

  if (blah == 2)
          printf("arg is %s/n", blah);
          do_something_useful(blah);;
  

  As you can see, the if then only encompasses the printf(), and the do_something_useful() call is unconditional (not within the scope of the if), like you wanted it. So, by using a block likedo { ... } while(0), you would get this:

  if (blah == 2)
          do {
                  printf("arg is %s/n", blah);
                  do_something_useful(blah);
          } while (0);
  

  Which is exactly what you want.

• (from Per Persson) As both Miller and Collins point out, you want a block statement so you can have several lines of code and declare local variables. But then the natural thing would be to just use for example:

  #define exch(x,y) { int tmp; tmp=x; x=y; y=tmp; }
  

  However that wouldn't work in some cases. The following code is meant to be an if-statement with two branches:

  if (x > y)
          exch(x,y);          // Branch 1
  else  
          do_something();     // Branch 2
  

  But it would be interpreted as an if-statement with only one branch:

  if (x > y) {                // Single-branch if-statement!!!
          int tmp;            // The one and only branch consists
          tmp = x;            // of the block.
          x = y;
          y = tmp;
  }
  ;                           // empty statement
  else                        // ERROR!!! "parse error before else"
          do_something();
  

  The problem is the semi-colon (;) coming directly after the block. The solution for this is to sandwich the block between do and while (0). Then we have a single statement with the capabilities of a block, but not considered as being a block statement by the compiler. Our if-statement now becomes:

  if (x > y)
          do {
                  int tmp;
                  tmp = x;
                  x = y;
                  y = tmp;
          } while(0);
  else
          do_something();

總的來說，解決了代碼使用宏定義時，要無誤嵌套的目的。

雖然上述代碼沒能很好地體現，但在源代碼裏的大多數宏定義的地方，都是使用了do{}while(0)的結構：

#define CC_SAFE_DELETE(p)            do { if(p) { delete (p); (p) = 0; } } while(0)
#define CC_SAFE_DELETE_ARRAY(p)     do { if(p) { delete[] (p); (p) = 0; } } while(0)
#define CC_SAFE_FREE(p)                do { if(p) { free(p); (p) = 0; } } while(0)
#define CC_SAFE_RELEASE(p)            do { if(p) { (p)->release(); } } while(0)
#define CC_SAFE_RELEASE_NULL(p)        do { if(p) { (p)->release(); (p) = 0; } } while(0)
#define CC_SAFE_RETAIN(p)            do { if(p) { (p)->retain(); } } while(0)

2、純虛函數 virtual=0

       在看代碼的時候，發現C++中有不少地方使用到了純虛函數等於0的。我覺得這樣的意思是在其聲明類中無需實現，需要在子類中實現，類似Java的接口。

       後來，百度得到答案如下：

------------>

       在C++中的一種函數申明被稱之爲：純虛函數(pure virtual function).它的申明格式如下：

       class CShape

       {

       public:

           virtual void Show() =0;

       };

       注意紅色部分，在普通的虛函數後面加上" =0"這樣就聲明瞭一個pure virtual function.

 

          在什麼情況下使用純虛函數(pure vitrual function)?

       1，當想在基類中抽象出一個方法，且該基類只做能被繼承，而不能被實例化；

       2，這個方法必須在派生類(derived class)中被實現；

       （原文請參看：http://www.oschina.net/question/565065_86228）

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        綜上答案：也就如我的猜測一樣，類似Java的接口方法一樣，其所在類只能被繼承（C++中不存在implements），且該方法必須在派生類中被實現。