如何初始化 constexpr 引用

我试图初始化一个 constexpr 引用，但没有成功.我试过了

#include constexpr int&f(int& x)//可以定义返回 constexpr 引用的函数{返回 x;}int main(){constexpr int x{20};constexpr const int&z = x;//此处出错}

但我收到编译时错误

<块引用>

错误:constexpr 变量 'z' 必须由常量表达式初始化

删除 const 导致

<块引用>

错误:将int"类型的引用绑定到const int"类型的值会丢弃限定符

尽管我感觉 constexpr 自动暗示了变量声明的 const.

所以我的问题是:

1. constexpr 引用有用吗?(即，比 const 引用更好")
2. 如果是，我如何有效地定义它们?

PS:我看到了一些与我的相关的问题，例如 哪些值可以分配给 `constexpr` 引用? ，但我认为他们没有解决我的问题.

解决方案

1. constexpr 引用有用吗?(即，比 const 引用更好")

它们保证在程序启动之前被初始化，而对 const 的引用可以在程序开始运行后的动态初始化期间初始化.

<块引用>

2. 如果是，我如何有效地定义它们?

constexpr 引用必须绑定到全局变量，而不是局部变量(或者更正式地说，它必须绑定到具有静态存储持续时间的东西).

引用在概念上等同于获取变量的地址，而局部变量的地址不是常量(即使在只能调用一次的main中，因此它的局部变量是只初始化一次).

I am trying to initialize a constexpr reference with no success. I tried

#include <iostream>

constexpr int& f(int& x) // can define functions returning constexpr references
{
    return x;
}

int main()
{
    constexpr int x{20};
    constexpr const int& z = x; // error here
}

but I'm getting a compile time error

error: constexpr variable 'z' must be initialized by a constant expression

Dropping the const results in

error: binding of reference to type 'int' to a value of type 'const int' drops qualifiers

even though I had the feeling that constexpr automatically implies const for variable declarations.

So my questions are:

1. Are constexpr references ever useful? (i.e., "better" than const references)
2. If yes, how can I effectively define them?

PS: I've seen a couple of questions related to mine, such as Which values can be assigned to a `constexpr` reference? , but I don't think they address my questions.

解决方案

1. Are constexpr references ever useful? (i.e., "better" than const references)

They are guaranteed to be initiailized before the program starts, whereas a reference to const can be initialized during dynamic initialization, after the program starts running.

2. If yes, how can I effectively define them?

A constexpr reference has to bind to a global, not a local variable (or more formally, it has to bind to something with static storage duration).

A reference is conceptually equivalent to taking the address of the variable, and the address of a local variable is not a constant (even in main which can only be called once and so its local variables are only initialized once).