Cocos2d-x problem set.

1. do {} while (0) in the source code)

Write a code today:

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

Later I went to the source code:

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;}

After reading it, I wonder, do {} while (0) is not a single execution. What does it mean?

Later, Baidu got the following best explanation:

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:

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 wocould 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 wocould 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 beIf-Statement with two branches:

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

But it wocould be interpreted asIf-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 statementelse                        // 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 blockDoAndWhile (0). Then we have a single statement with the capabilities of a block, but not considered as being a block statement by the compiler. OurIf-Statement now becomes:

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

In general, the purpose of nesting is fixed when the Code uses a macro definition.

Although the above Code is not well reflected, most macro definitions in the source code use the do {} while (0) structure:

#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)