arrays - 为什么我们在 C 中指定 size_t 数组大小而不是仅仅使用整数？这样做的好处是什么？

Question

我开始学习 C，我的一项家庭作业要求我创建一个数组 size_t 常量 N_MAX = 10；双 v1[N_MAX]； 我知道 size_t 代表 v1 数组长度，但我仍然不明白 size_t 类型在这里有什么贡献，而不是使用 int N_MAX？

Answer 1

size_t can store the maximum size of a theoretically possible object of any type (including array).

size_t is commonly used for array indexing and loop counting. Programs that use other types, such as unsigned int, for array indexing may fail on, e.g. 64-bit systems when the index exceeds UINT_MAX or if it relies on 32-bit modular arithmetic.

( source )

这意味着 size_t 保证能够在您为其编译的任何平台上保存任何大小/索引数。

它通常是根据您编译的目标平台来定义的。例如，Windows SDK 中包含的 intsafe.h header 定义如下:

#ifdef _WIN64
typedef __int64             ptrdiff_t;
typedef unsigned __int64    size_t;
#else
typedef int            ptrdiff_t;
typedef unsigned int   size_t;
#endif

直接使用 size_t 意味着当您为不同的架构(例如，x86 与 x86- 64).

编辑:正如@Eric Postpischil 在评论中提到的，C 标准的实际措辞与我从 cppreference 链接的解释不同。

查看the standard我们可以看到一些关于 size_t 的提及:

6.5.3.4 The sizeof and alignof operators

[...]

5 The value of the result of both operators is implementation-defined, and its type (an unsigned integer type) is size_t, defined in <stddef.h> (and other headers)

[...]

7.19 Common definitions <stddef.h>

[...]

size_t which is the unsigned integer type of the result of the sizeof operator;

size_t 是 sizeof 运算符返回值的类型这一事实是我们可以假设“size_t 可以存储最大大小的原因理论上可能的任何类型的对象”，但实际上在标准中没有任何地方强制要求。该标准还说明了以下关于 sizeof 的内容:

The sizeof operator yields the size (in bytes) of its operand, which may be an expression or the parenthesized name of a type. The size is determined from the type of the operand. The result is an integer. If the type of the operand is a variable length array type, the operand is evaluated; otherwise, the operand is not evaluated and the result is an integer constant

我的解释(可能是错误的)是你不能拥有一个大小不适合 size_t 的对象，因为在那种情况下你可以没有有效的 sizeof 运算符。

编辑 2:

我将在这里引用 Eric Postpischil:

There are simply limits as to what we can do with computers, and sometimes calculations go out of bounds. It is generally easy on modern hardware to define size_t so that sizeof always works, and you obviously cannot malloc a larger object, but you might construct one with a static array, in some esoteric C implementation with 16-bit size_t but 22-bit addresses.