问题描述
在 C 中，const 限定符使对象只读，但不是常量表达式.例如，不可能使用 const int 变量来对数组进行标注:
In C the const qualifier makes an object read-only but not a constant expression. For example, it is not possible to use a const int variable to dimension an array:
const int n = 10;
int arr [n]; /* Compile-time error */
这是什么技术原因?编译器在编译时不可能知道对象实际上有一个常量值吗?
Which is the technical reason for this? Is it not possible for the compiler at compile-time to know that the object has actually a constant value?
我不认为我的问题是 Can a const 变量用于在 C 中声明数组的大小?因为我不是在问这是否可能(在我的问题中明确说明不是)，而是技术原因这是不可能的.
I don't think that my question is an exact duplicate of Can a const variable be used to declare the size of an array in C? because I'm not asking if that's possible (it is clearly stated in my question that it is not) but the technical reason why it's not possible.
在下面 Olaf 的评论之后，这个答案和一些思考我会尝试在这个总结和回答我的问题方式:
After the comment of Olaf below, this answer and some musings I would try to summarize and answer my question in this way:
在 C 中，const 对象不是 编译时常量，因为它可能违反这两个要求:
In C a const object is not a compile-time constant because it can violate both requirements:
首先，可以在运行时初始化 const 对象，如下所示:
First, it is possible to initialize the const object at runtime as in:
int i;
scanf ("%d", & i);
const int n = i;
所以这里我们违反了在编译时已知"的要求.
so here we violate the requirement of "known at compile-time".
其次，正如Olaf所指出的，const限定符意味着程序本身在声明初始化后不会修改对象的值.但是内存中对象的值仍然可以被程序本身之外的一些其他实体修改，所以这里我们不保证实际常量的要求.
Secondly, as Olaf pointed out, the const qualifier means that the program itself will not modify the value of the object after the declaration-initialization. But the value of the object in memory could still be modified by some other entity outside the program itself, so here we are not guaranteeing the requirement of actual constness.
如果此答案不正确或不完整，请批评.
Please criticize if this answer is incorrect or incomplete.
推荐答案
这样做的一个技术原因可能是这样的声明甚至在初始化器不是常量表达式时也是有效的，所以这是一个语义属性，从查看初始化程序.
One technical reason for this is probably that such a declaration is even valid when the initializer is not a constant expression, so this is a semantic property that is deduced from looking at the initializer.
那么，按照目前的规则，没有办法在头文件中包含这样的东西来在文件范围内声明和定义.任何对象只能有一个定义，多个对象文件不能链接在一起.
Then, with the current rules there is no way to have such a thing in a header file to be declared and defined in file scope. Any object can only have one definition, and several object files could not be linked together.
有一些想法可以在下一个 C 版本中改善这种情况.
There are ideas to improve that situation for the next version of C.
这篇关于为什么在 C 中 const 对象不是编译时常量表达式?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，WP2