可一个const变量用来声明C中的数组的大小？ [英] Can a const variable be used to declare the size of an array in C?

问题描述

为什么以下code抛出一个错误？

Why does the following code throw an error?

const int a = 5;
int b[a]={1,2,3,4,5};

和也，当我试图编译上面的code没有常量关键字，我得到了同样的错误：

And also when I tried to compile the above code without "const" keyword, I got the same error:

int a = 5; 
int b[a]={1,2,3,4,5};

为什么会这样？
什么是我在这里做什么错误了吗？

why is it so? What is the mistake that I am doing here?

和另一种问题：
当常数与它们的实际值替换在code，即如果我声明一个变量说：const int的X = 5;
我知道，没有内存分配在RAM中的变量x，但在ROM常变量区域存放值5和x被简单地由5处处x会出现在code中的值替换。但是，当出现这种情况？编译时间？开机时间？ preprocessing时间？

And also another question: When are constants replaced with their actual values in a code, i.e if I declare a variable say: const int x= 5; I know that no memory is allocated in RAM for the variable x, but constant variable area in ROM holds the value 5 and that x is simply replaced by the value 5 everywhere x appears in the code. But when does this happen? Compilation time? Boot up time? Preprocessing time?

PS：我讲的是嵌入式C我的桌面上，而不是C运行（在微控制器等运行）。因此，嵌入式系统必然有一个ROM（闪存，EEPROM ...）。然后会发生什么？

PS: I am talking about Embedded C (running on a Microcontroller etc), not C running on my desktop. So the embedded system is bound to have a ROM (Flash, EEPROM...). What would happen then?

推荐答案

它只是语言的限制。静态界数组的大小必须的恒前pressions 的，不幸的是在C，它只是像一个常量或的sizeof 前pression或诸如此类，但的不的一个常量 -typed变量。

It's simply a limitation of the language. The sizes of statically-bounded arrays need to be constant expressions, and unfortunately in C that's only something like a literal constant or a sizeof expression or such like, but not a const-typed variable.

（西蒙指出，由于C99还有的运行时有界的阵列，或变长数组，其大小可以通过任何变量的值给出。但是这是一个不同动物。）

(As Simon pointed out, since C99 there are also runtime-bounded arrays, or "variable-length arrays", whose size can be given by the value of any variable. But that's a different animal.)

您可能会感兴趣地听到，这些规则在C ++中，其中静态const int的的确是一个恒定的前pression是不同的，C ++ 11即使增加了一个新的关键字， constexpr ，让更多的一般使用恒定的前pressions其中包括更多的东西，其价值可以合理地在编译时确定的。

You may be interested to hear that the rules are different in C++, where a static const int is indeed a constant expression, and C++11 even adds a new keyword, constexpr, to allow even more general use of constant expressions which encompass more things whose value "could reasonably be determined at compile time".

这篇关于可一个const变量用来声明C中的数组的大小？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！