const&指非易失性变量。变量改变。更改会使const&无效吗？

在C ++中， const& 的值可以更改吗？

In C++, can the value of a const & change?

当然，它不能改变，可以吗？这就是 const 的意思。此外，请听Stroustrup：

Well, of course it cannot change, can it? That's what const means. Moreover, listen to Stroustrup:

A const 左值引用是指常量，

A const lvalue reference refers to a constant, which is immutable from the point of view of the user of the reference.

但是这怎么办？

#include <iostream>

int main() {
    int           a = 0;
    const int&    r = a;
    const int old_r = r;
    ++a;
    const int new_r = r;
    std::cout
      <<      "old_r == " << old_r
      << " but new_r == " << new_r << std::endl;
    return 0;
}

在我的计算机上，此输出为 old_r == 0，但new_r == 1 。

On my machine, this outputs, old_r == 0 but new_r == 1.

这是我真正的问题。在上面的代码中，查看以下行

That gets to my real question. In the above code, look at the line

    const int new_r = r;

只要

• 地址& new_r 既不在此行也不在代码中的其他位置提取，并且


• 该代码没有 volatile ，

• the address &new_r is extracted neither on this line nor elsewhere in the code and
• the code has nothing volatile,

任何事情都会阻止优化编译器合并 $ _b
$将old_r 和 new_r 放入单个常量对象，将其视为如下所示？

does anything prevent an optimizing compiler from merging old_r and new_r into a single constant object, treating the line as though it read as follows?

    const int& new_r = old_r;

我之所以问是因为，据我所知，如果编译器进行了优化，那可能会改变行为。该程序可能会输出 old_r == 0但new_r == 0 。

I ask because, as far as I know, if the compiler did so optimize, that might alter the behavior. The program might output, old_r == 0 but new_r == 0.

相关问题

我发现的最相关的现有问题是：

The most nearly related existing question I find is this one:

• （C语言） 可以更改const吗？ （尤其请参阅链接的问题的要点。 1。）

以下内容也是相关的，但与当前问题不同，它涉及强制转换：

The following are also related but, unlike the present question, involve casts:

• 更改const的值


• 在同一内存地址有两个不同的值


• 修改常量对象


• 更改const变量的值[重复]


• （C语言）通过const声明访问非const


• （C语言）我们可以修改const变量的值吗？


• （C语言）使用指针更改常量变量值


• （C语言）关于使用指针修改const变量的困惑


• （C语言）使用const_cast的怪异行为[重复]

• Changing the value of a const
• Two different values at the same memory address
• Modifying constant object
• Changing the value of const variable [duplicate]
• (C language) Accessing a non-const through const declaration
• (C language) Can we modify the value of a const variable?
• (C language) Const variable value is changed by using a pointer
• (C language) Confusion regarding modification of const variable using pointers
• (C language) Weird Behaviour with const_cast [duplicate]

另请参见 N4659 （C ++ 17标准草案），第10.1.7.1节， cv限定词。

See also N4659 (draft C++17 standard), sect. 10.1.7.1, "The cv-qualifiers."

Stroustrup在问题顶部的引文来自第 C ++编程语言，第四版。当然，没有作者能在一千页的书中完美地写出每一个句子。但也许Stroustrup很清楚，我只是看错了他。不过，您可能会明白为什么这句话使我感到困惑。这就是我问的原因。

The quote of Stroustrup at the top of the question comes from sect. 7.7.2 of The C++ Programming Language, 4th ed. Of course, no author can write every sentence perfectly in a thousand-page book; yet perhaps Stroustrup is clear and I have merely read him wrong. Nevertheless, you might see why the sentence has confused me. This is why I have asked.