问题描述
这是错误的,因为可变大小的对象可能未初始化
This is wrong since a variable sized object may not be initialized
int size = 4;
int array[size] = {1};
size
是一个变量,编译器在创建数组
(不是 size
时间?)?之后,让 size
更改为什么会是一个问题?我的意思是,这些是连续的指令,在声明数组之前,可能会改变 size
的值?
size
is a variable, but doesn't the compiler know its value when it creates array
(Isn't size
assigned an initial value of 4 at compile-time?)? Let size
change after that, why would it be an issue? I mean, these are consecutive instructions, what could possibly alter the value of size
before the array is declared?
第二个问题:
为什么不允许这样:
Second question:Why is this not allowed:
const int size = 4;
int array[size] = {1};
我将 size
声明为const。我知道const!=只读,并且声明 size
作为宏是正确的方式去做它。但是如果我保证编译器使用const不改变 size
的值,为什么不允许?
I am declaring size
as a const. I know that const != read-only, and that declaring size
as a macro is the correct way to go about it. But if I promise the compiler using const that I wont change the value of size
, why is it not allowed?
推荐答案
第一个问题的答案是因为语言规范说。虽然编译器可以能够推断数组的大小,但这样做需要一些静态分析,当数组大小不是编译时常数表达式时,这是不重要的。
The answer to the first question is "because the language specification says so". Although a compiler may be able to infer the size of the array, doing so requires some static analysis which is not trivial when the array size is not a compile-time constant expression.
至于为什么不允许VLA的初始化:一个原因你的编译器生活在一个洞穴中(例如,微软的工程师们 - 他们的C编译器是广泛使用的编译器的一个罕见例子, the writers of your compiler are living in a cave (for example, the engineers over at Microsoft do -- their C compiler is one rare example of a widely used compiler that still does not support C99, 15 years after its standardization.). Any modern, decent C compiler should enable you to use variable-length arrays which are present in C99. Compilers that already implement C11 may or may not opt to support VLAs (as it's an optional feature of the latest standard).
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