在 C 中，可以通过指针修改 const 变量吗? [英] In C, can a const variable be modified via a pointer?

问题描述

我在我的代码中写了一些类似的东西

I wrote some thing similar to this in my code

const int x=1;
int *ptr;
ptr = &x;
*ptr = 2;

这是否适用于所有编译器?为什么 GCC 编译器没有注意到我们正在改变一个常量变量?

Does this work on all compilers? Why doesn't the GCC compiler notice that we are changing a constant variable?

推荐答案

const 实际上并不意味着恒定".C 中的常量"具有在编译时确定的值；文字 42 就是一个例子.const 关键字真正意味着只读.考虑，例如:

const actually doesn't mean "constant". Something that's "constant" in C has a value that's determined at compile time; a literal 42 is an example. The const keyword really means read-only. Consider, for example:

const int r = rand();

r 的值直到程序执行时才确定，但是 const 关键字意味着您不能修改 r初始化之后.

The value of r is not determined until program execution time, but the const keyword means that you're not permitted to modify r after it's been initialized.

在您的代码中:

const int x=1;
int *ptr;
ptr = &x;
*ptr = 2;

赋值 ptr = &x; 是一个约束违规，这意味着需要一个符合标准的编译器来抱怨它；您不能合法地将 const int*(指向 const int 的指针)值分配给非常量 int* 对象.如果编译器生成一个可执行文件(它不需要这样做；它可以拒绝它)，那么该行为不是由 C 标准定义的.

the assignment ptr = &x; is a constraint violation, meaning that a conforming compiler is required to complain about it; you can't legally assign a const int* (pointer to const int) value to a non-const int* object. If the compiler generates an executable (which it needn't do; it could just reject it), then the behavior is not defined by the C standard.

例如，生成的代码实际上可能将值 2 存储在 x 中——但随后对 x 的引用可能会产生值1，因为编译器知道x在初始化后不能被修改.它知道，因为你告诉它，通过将 x 定义为 const.如果你对编译器撒谎，后果可能会很糟糕.

For example, the generated code might actually store the value 2 in x -- but then a later reference to x might yield the value 1, because the compiler knows that x can't have been modified after its initialization. And it knows that because you told it so, by defining x as const. If you lie to the compiler, the consequences can be arbitrarily bad.

实际上，可能发生的最糟糕的事情是程序按照您的预期运行；这意味着您有一个很难检测到的错误.(但你应该得到的诊断结果将是一个很大的线索.)

Actually, the worst thing that can happen is that the program behaves as you expect it to; that means you have a bug that's very difficult to detect. (But the diagnostic you should have gotten will have been a large clue.)

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