扩展的结构 [英] Extending a structure

问题描述

typedef struct A 
{
  int x;
}A;

typedef struct B 
{
  A a;
  int d;
}B;

void fn()
{
  B *b;
  ((A*)b)->x = 10;
}

我SO看了上面的code片段。 （（A *）B） - 方式＆gt; X 不是良好的编程风格 B-＆GT; AX 好编程风格。因为任何人添加语句之前的东西A一;在结构B，它不会工作。我不明白为什么？我想这一点。有什么建议吗？

I read the above code snippet in SO. ((A*)b)->xis not good programming style.b->a.x is good programming style. Because anybody adds something before the statement "A a;" in structure b , it will not work. I don't understand why? I tried it too. Any suggestions please?

推荐答案

这招是用来模拟在继承C.它使可能通过地址A或B的功能它期望指向一个。

That trick is used to emulate inheritance in C. It makes possible to pass address A or B to function which expects pointer to A.

这工作，因为C保证有结构的第一个成员之前没有填充。因此，如果A是B的第一构件，在B点的开始存储器布局总是相同为A

This works because C guarantees that there is no padding before the first member of struct. So if A is first member of B, memory layout at the beginning of B is always same as A.

int doStuff(A * a) {
    return a->x + 1;
}

...

B b;
doStuff((A*)&b); // Will work because b and b.a have the same start address

如果您将变动b声明：

typedef struct B 
{
    int d;
    A a;
}B;

这将不再工作，为（A *）和b 将返回地址 BD ，而不是 BA 。

this would no longer work as (A*)&b would return address of b.d, not b.a.

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