在关于C的一个有信誉的来源中,在讨论&操作符后给出了以下信息:

... 有点不幸的是,术语[地址的]仍然存在,因为它混淆了那些不知道地址是关于什么的人,并误导了那些知道地址的人:将指针视为地址通常会导致悲伤……

我读过的其他材料(来自同样有名望的来源,我想说)总是毫不掩饰地将指针和&操作符作为内存地址。我很愿意继续寻找事情的真相,但当有信誉的消息来源不同意时,这有点困难。

现在我有点困惑了——如果指针不是内存地址,那么它到底是什么?

P.S.

作者后来说:……不过,我将继续使用“地址”这个术语,因为发明一个不同的(术语)会更糟糕。


当前回答

简单地说,指针实际上是分割机制的偏移部分,分割后转换为线性地址,分页后转换为物理地址。物理地址实际上是从ram中寻址的。

       Selector  +--------------+         +-----------+
      ---------->|              |         |           |
                 | Segmentation | ------->|  Paging   |
        Offset   |  Mechanism   |         | Mechanism |
      ---------->|              |         |           |
                 +--------------+         +-----------+
        Virtual                   Linear                Physical

其他回答

它说“因为它让那些不知道地址是什么的人感到困惑”——而且,这是真的:如果你知道地址是什么,你就不会困惑了。从理论上讲,指针是一个指向另一个变量的变量,实际上保存着一个地址,即它所指向的变量的地址。我不知道为什么要隐瞒这个事实,这又不是什么高深的科学。如果你理解了指针,你就离理解计算机的工作原理更近了一步。去吧!

指针是表示内存位置的抽象。请注意,这句话并没有说把指针当作内存地址是错误的,它只是说它“通常会导致悲伤”。换句话说,它会让你产生错误的期望。

The most likely source of grief is certainly pointer arithmetic, which is actually one of C's strengths. If a pointer was an address, you'd expect pointer arithmetic to be address arithmetic; but it's not. For example, adding 10 to an address should give you an address that is larger by 10 addressing units; but adding 10 to a pointer increments it by 10 times the size of the kind of object it points to (and not even the actual size, but rounded up to an alignment boundary). With an int * on an ordinary architecture with 32-bit integers, adding 10 to it would increment it by 40 addressing units (bytes). Experienced C programmers are aware of this and put it to all kinds of good uses, but your author is evidently no fan of sloppy metaphors.

There's the additional question of how the contents of the pointer represent the memory location: As many of the answers have explained, an address is not always an int (or long). In some architectures an address is a "segment" plus an offset. A pointer might even contain just the offset into the current segment ("near" pointer), which by itself is not a unique memory address. And the pointer contents might have only an indirect relationship to a memory address as the hardware understands it. But the author of the quote cited doesn't even mention representation, so I think it was conceptual equivalence, rather than representation, that they had in mind.

你是对的,是理智的。通常,指针只是一个地址,因此您可以将其强制转换为整数并进行任何算术运算。

但有时指针只是地址的一部分。在一些体系结构上,指针被转换为一个增加了基数的地址或使用另一个CPU寄存器。

但是现在,在PC和ARM架构上,使用平面内存模型和原生编译的C语言,可以认为指针是指向一维可寻址RAM中某个位置的整数地址。

C或c++指针与简单内存地址的另一个不同之处是,我在其他答案中没有看到不同的指针类型(尽管考虑到它们的总大小,我可能忽略了它)。但它可能是最重要的一个,因为即使是经验丰富的C/ c++程序员也会被它绊倒:

编译器可能会假设不兼容类型的指针不指向相同的地址,即使它们很明显指向相同的地址,这可能会导致简单的pointer==address模型不可能出现的行为。考虑以下代码(假设sizeof(int) = 2*sizeof(short)):

unsigned int i = 0;
unsigned short* p = (unsigned short*)&i;
p[0]=p[1]=1;

if (i == 2 + (unsigned short)(-1))
{
  // you'd expect this to execute, but it need not
}

if (i == 0)
{
  // you'd expect this not to execute, but it actually may do so
}

注意,char*有一个例外,所以使用char*操作值是可能的(尽管不是很可移植)。

指针只是另一个变量,用来保存内存位置的地址(通常是另一个变量的内存地址)。