关于静态常量数据成员的声明和定义的混淆 [英] Confusion about declaration and definition of static const data memebers

问题描述

Scott Meyers 在 Effective Modern C++，Item 30 page 210 中写道，有

<块引用>

不需要在类中定义完整的static const数据成员；仅声明就足够了，

那么示例代码是

class Widget {民众:静态常量 std::size_t MinVals = 28;//MinVals 的声明；...};...//没有定义.对于 MinValsstd::vector小部件数据；widgetData.reserve(Widget::MinVals);//使用 MinVals

我确信 static const std::size_t MinVals = 28; 是声明 也是 一个定义，因为它给出了MinVals 的值，但评论似乎声称这只是一个声明；第二条评论实际上声称没有定义.代码后面的文字，确实读起来

<块引用>

MinVals 缺少定义.

这证实了 static const std::size_t MinVals = 28; 不是定义，所以我有点困惑.

cppreference 对我帮助不大(我的粗斜体):

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如果一个 static 整数或枚举类型的数据成员被声明 const(而不是 volatile)，它可以用一个初始化器初始化，其中每个表达式都是一个常量表达式，就在类定义中:

struct X{const static int n = 1;const static int m{2};//从 C++11 开始const static int k;};const int X::k = 3;

但类中的前两行在我看来是定义.

以下 cppreference 示例也是如此:

<块引用>

struct X {静态常量 int n = 1;静态 constexpr int m = 4;};const int *p = &X::n, *q = &X::m;//X::n 和 X::m 是 odr 使用的const int X::n;//... 所以定义是必要的constexpr int X::m;//...(C++17 中的 X::m 除外)

我会说 static const int n = 1; 是一个定义，但它不是，基于倒数第二个评论.

解决方案

不需要在类中定义完整的静态常量数据成员；仅声明就足够了，

仅当该对象未使用 ODR(即未使用数据成员)时，仅声明就足够了在需要其地址存在的上下文中(例如绑定到引用或应用运算符 &).初始化器的存在不等于定义.

在书中的例子中，很明显 MinVals 不是 ODR 使用的，即编译器可以直接使用它的值，而不必在内存中创建一个对象，所以语句:

widgetData.reserve(Widget::MinVals);

变成:

widgetData.reserve(28);

但是，如果在任何其他地方，MinVals 被 ODR 使用，那将使程序格式错误.

cppreference 中的所有其他示例都清楚地说明了何时使用 ODR 且需要定义，何时不需要:

struct X{const static int n = 1;const static int m{2};//从 C++11 开始const static int k;};const int X::k = 3;

n 和 m 是带有初始化器的声明.尝试获取 n 或 m 的地址应该失败.

struct X {静态常量 int n = 1;静态 constexpr int m = 4;};const int *p = &X::n, *q = &X::m;const int X::n;constexpr int X::m;

表达式 &X::n 和 &X::m 算作 n 和 m 的 ODR 使用，分别(即请求一个地址).对于 constexpr 静态数据成员，在 C++17 之前需要定义.从 C++17 开始，static constexpr 数据成员隐式地inline，这意味着不需要类外定义，因为它们本身就是定义.

Scott Meyers writes in Effective Modern C++, Item 30 page 210, that there's

no need to define integral static const data members in classes; declarations alone suffice,

then the sample code is

class Widget {
  public:
    static const std::size_t MinVals = 28; // MinVals' declaration;
    ...
};
...                                        // no defn. for MinVals
std::vector<int> widgetData;
widgetData.reserve(Widget::MinVals);       // use of MinVals

I was convinced that static const std::size_t MinVals = 28; is declaration and also a definition, as it is giving a value to MinVals, but the comment seems to claim that's only a declaration; the second comment actually claims there's no definition. The text after the code, indeed reads

MinVals lacks a definition.

Which confirms that static const std::size_t MinVals = 28; is not a definition, so I'm a bit confused.

cppreference doesn't help me much (my bold-italic):

If a static data member of integral or enumeration type is declared const (and not volatile), it can be initialized with an initializer in which every expression is a constant expression, right inside the class definition:

struct X
{
   const static int n = 1;
   const static int m{2}; // since C++11
   const static int k;
};
const int X::k = 3;

but first two lines in the class look definitions to me.

The same goes for a following example on cppreference:

struct X {
    static const int n = 1;
    static constexpr int m = 4;
};
const int *p = &X::n, *q = &X::m; // X::n and X::m are odr-used
const int X::n;             // … so a definition is necessary
constexpr int X::m;         // … (except for X::m in C++17)

where I'd have said static const int n = 1; is a definition, but it is not, based on the second to last comment.

解决方案

no need to define integral static const data members in classes; declarations alone suffice,

Declarations alone suffice only if that object is not ODR-used, that is, if a data member is not used in a context that would require its address to exist (like binding to a reference or applying operator &). The presence of an initializer does not equal a definition.

In the example from the book, it's clear that MinVals is not ODR-used, i.e., the compiler can use its value directly, without having to create an object in memory, and so the statement:

widgetData.reserve(Widget::MinVals);

becomes:

widgetData.reserve(28);

If, however, in any other place, MinVals were ODR-used, that would make the program ill-formed.

All other examples from cppreference clearly indicate when a value is ODR-used and a definition is required, and when not:

struct X
{
    const static int n = 1;
    const static int m{2}; // since C++11
    const static int k;
};
const int X::k = 3;

n and m are declarations with initializers. An attempt to obtain the address of either n or m should fail.

struct X {
    static const int n = 1;
    static constexpr int m = 4;
};
const int *p = &X::n, *q = &X::m;
const int X::n;
constexpr int X::m;

Expressions &X::n and &X::m count as ODR-use of n and m, respectively (that is, an address is requested). For a constexpr static data members, a definition was required prior to C++17. From C++17, static constexpr data members are implicitly inline, which means, no out-of-class definition is needed, as they are definitions themselves.

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