C ++ 17中的静态constexpr和静态的内联变量有什么区别？ [英] What's the difference between static constexpr and static inline variables in C++17?

问题描述

使用C ++ 17，我们可以获得内联变量。

With C++17 we get inline variables.

它们的用途之一是在类中定义常量字段。

One of the uses for them is to define constant fields in classes.

那么这两个常量定义之间有什么区别：

So what's the difference between these two constant definitions:

class MyClass {
    static constexpr int myFirstVar = 10;
    static const inline int mySecondVar = 100;
};

当然， constexpr 使 myFirstVar 隐式内联。

在这里更好的选择是使用 constexpr 还是 inline ？

What's the better choice here, to use constexpr or inline?

注意：当您不需要常量性时，请 inline 使其更容易。使用 constexpr ，您没有选择。

Note: when you don't need constness, then inline makes it easier. With constexpr you don't have that choice.

推荐答案

您没有在声明时不必为 mySecondVar 指定一个初始化程序。初始化器本身也不必是 constexpr 。

You don't have to specify an initializer for mySecondVar at the point of declaration. Nor is the initializer required to be constexpr itself.

这意味着，如果我们尝试定义 myFirstVar ，例如：

This means that if we attempt to define myFirstVar like this:

class MyClass {
    static constexpr int myFirstVar;
};

int MyClass::myFirstVar = 1;

或者像这样：

#include <cstdlib>

class MyClass {
    static constexpr int myFirstVar = rand();
};

这两种格式都是错误的。 constexpr 语义需要它，并且有充分的理由。

It's ill-formed either way. constexpr semantics demand it and for a good reason.

内联说明符方法使我们可以在标头本身中包含静态变量定义，而无需将初始化程序设为 constexpr ；或者，如果初始化程序非常复杂，则不必在类定义本身中。

The inline specifier approach allows us to include a static variable definition in the header itself, without the initializer being constexpr; or if the initializer is fairly complex it doesn't have to be in the class definition itself.

因此，这在C ++ 17中是一个非常有效的标头：

So this is a perfectly valid header in C++17:

#include <cstdlib>

class MyClass {
    static const int mySecondVar;
};

inline const int MyClass::mySecondVar = rand();

标准向我们保证，所有包含标头的翻译单元都将看到相同的变量值，即使直到运行时我们也不知道它是什么。

The standard promises us that all translation units that include the header will see the same value for the variable, even though we won't know what it is until run-time.

它主要是一个库编写器工具。假设您的库仅是标题。然后在过去，如果需要这样定义的静态常量，您有什么选择？

It's mostly a library writers tool. Assume your library is header only. Then in the olden days, what were your options if you needed a static constant defined like this?

那么，库中可能附带了一个目标文件。它将从仅包含常量定义的转换单元进行编译。现在，该库已不再是仅标头。

Well, you could have an object file shipped with your library. It will be compiled from a translation unit that contains just the constant definition. Now the library isn't header-only.

或者您也可以依赖于内联函数。内联变量效果可以通过以下方式实现：

Or you could rely on inline functions instead. The inline variable effect can be achieved with the following:

class MyClass {
    static inline int mySecondVar();
};

inline int MyClass::mySecondVar() {
  static const int value = rand();
  return value;
}

但是它隐藏在语法墙的后面，掩盖了本质上是一个常量，带有函数调用运算符。

But it's hidden behind a wall of syntax, and masks what is essentially a constant, with a function call operator.

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