为什么在C中const对象不是编译时常量表达式？ [英] Why in C a const object is not a compile-time constant expression?
问题描述
在C中， const 限定符使对象成为只读对象，而不是常量表达式。例如，不可能使用 const int 变量来标注数组的尺寸：
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？
我不知道该对象实际上是一个常量值吗？认为我的问题是的精确重复项，是否可以使用const变量来声明
中的数组？因为我不是问这是否可能（我的问题中明确指出了不可能），而是原因的技术原因。
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.
在以下Olaf评论后，这个答案和一些沉思，我将尝试以这种方式总结和回答我的问题：
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 a const中对象不是 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;
所以在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，如Olaf所指出的， const 限定词表示程序本身不会修改声明初始化后的对象。但是内存中对象的值仍然可以由程序本身之外的其他实体修改，因此在这里我们不能保证实际的恒定性。
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.
请批评如果此答案不正确或不完整。
Please criticize if this answer is incorrect or incomplete.
推荐答案
一个技术原因可能是这样的声明甚至在初始化时也有效不是常数表达式，因此这是从初始化程序推导出来的语义属性。
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.
然后，按照当前规则，没有办法将头文件中要在文件作用域中声明和定义的东西。任何对象只能有一个定义，并且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.
有一些想法可以改善下一个C版本的情况。
There are ideas to improve that situation for the next version of C.
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