有没有一种有效的方法来引用常量，实际上是const而不是只读？ [英] Is there an efficient way to make reference to constants actually const instead of read only?

问题描述

让我们看看下面的C ++代码：

Let's look at the following C++ code:

#include <iostream>

int main()
{
    int z = 2;

    class A {
        public:
        const int & x;
        A(const int & x) : x(x) {}
        void show(){
            std::cout << "x=" << this->x << std::endl ;
        }
    } a(z);

    a.show();
    z = 3;
    a.show();
}

程序打印：2和3

这清楚地表明，在A类内部不能被修改，它只是意味着它是只读的，因为我可以从外部改变它的值。

It clearly shows that while inside class A x can't be modified, it merely means it's read only, because I can change it's value from outside.

当然，我可以让它存储在类A中的副本，但我想知道是否有（或者有提案？）一个方法来说，类A，成员x将是真正的恒定，而不是只是只读，意味着外部代码不会改变它的承诺？

Of course I can make it a copy stored inside class A, but I'm wondering if there is (or if there is a proposal?) of a way to say to class A that the member x will be truly constant instead of merely read only, with the meaning of a promise that the external code won't change it ?

对我来说，它看起来像是与C restrict 关键字，但我还没有听说过任何这样的C ++功能。你是吗？

To my eyes it looks like something related to the meaning of the C restrict keyword, but I've not heard of any such C++ feature yet. Do you ?

推荐答案

Constness是实际变量的属性。

Constness is an attribute of the actual variable.

const int& x 只是意味着x是对int的引用，它不会被修改，当然编译器强制这样做。

The term const int& x simply means "x is a reference to an int which it will not modify" and of course the compiler enforces this.

x指向的实际变量是const，简单地声明为：

If you want the actual variable to which x refers to be const, simply declare it so:

#include <iostream>

int main()
{
    const int z = 2;    // declared const. Nothing may ever modify it

    class A {
    public:
        const int & x;
        A(const int & x) : x(x) {}
        void show(){
            std::cout << "x=" << this->x << std::endl ;
        }
    } a(z);

    a.show();
    z = 3;        // this is a logic error, caught by the compiler.
    a.show();
}

正确编译会产生错误：

./const.cpp:41:7: error: read-only variable is not assignable
    z = 3;
    ~ ^
1 error generated.

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