c++ - 在定义(.cpp 文件)中初始化 static float constexpr 成员是否可能

Question

我想在类的定义中初始化我的私有(private)静态成员“pi”，以便更好地整理我的代码，以便在 .cpp 文件中进行初始化。

当我尝试时，出现此错误:“声明 constexpr 静态数据成员 'pi' 需要初始化程序”

我使用的是 CLion 2018.3.4 和 C++ 11。

我尝试解决，唯一的解决方案是在声明中初始化成员。

Stack Overflow 上的其他答案为我提供了更好的知识，但没有回答我的问题。

//.h文件

class Shape {
public:
    virtual void getArea();

private:
    static constexpr float pi; // the error shows up here
};

//.cpp 文件

#include "Shape.h"

const float Shape::pi = 3.14; //here I don't exactly know why it does not require constexpr, but it's fine even with and without const 

// what I think this is equivalent to (in .h file)
static constexpr float pi = 3.14;

我期望它能像我在声明中分配“3.14”一样工作。

我没有定义构造函数，在这种情况下我知道它不会工作，因为 c'tor 旨在初始化类的实例，而静态成员应该已经初始化为全局元素“Shape”命名空间。

我认为发生的情况是链接器尝试初始化 header 中的成员，因为它必须在预处理阶段完成，而稍后使用 .cpp 文件。

Answer 1

对于static constexpr 成员，您不能在类定义中省略初始值设定项。 constexpr 变量必须在声明时初始化，因为它可以在常量表达式中声明后使用。这在[class.static.data]/3中有详细介绍。 C++11 标准

If a non-volatile const static data member is of integral or enumeration type, its declaration in the class definition can specify a brace-or-equal-initializer in which every initializer-clause that is an assignment-expression is a constant expression ([expr.const]). A static data member of literal type can be declared in the class definition with the constexpr specifier; if so, its declaration shall specify a brace-or-equal-initializer in which every initializer-clause that is an assignment-expression is a constant expression. [ Note: In both these cases, the member may appear in constant expressions. — end note ] The member shall still be defined in a namespace scope if it is odr-used ([basic.def.odr]) in the program and the namespace scope definition shall not contain an initializer.

^强调我的

因此，您的代码需要是

// .h file
class Shape {
public:
    virtual void getArea();

private:
    static constexpr float pi = 3.14; // we initialize here so it can be used.
};

// .cpp file
constexpr float Shape::pi; // we define here so it can be odr-used

请注意，这在 C++17 中已发生变化。随着介绍inline variables static constexpr 成员变量不再需要在类外部定义。编译器将为您处理它并确保仅存在该对象的单个定义。如果需要，您仍然可以定义成员，但该功能已弃用，并且很可能会在未来的标准修订版中删除。 [class.static.data]/3 的新文本是

If a non-volatile non-inline const static data member is of integral or enumeration type, its declaration in the class definition can specify a brace-or-equal-initializer in which every initializer-clause that is an assignment-expression is a constant expression ([expr.const]). The member shall still be defined in a namespace scope if it is odr-used ([basic.def.odr]) in the program and the namespace scope definition shall not contain an initializer. An inline static data member may be defined in the class definition and may specify a brace-or-equal-initializer. If the member is declared with the constexpr specifier, it may be redeclared in namespace scope with no initializer (this usage is deprecated; see [depr.static.constexpr]). Declarations of other static data members shall not specify a brace-or-equal-initializer.

^强调我的

和[dcl.constexpr]/1表示 static constexpr 变量是隐式内联的

The constexpr specifier shall be applied only to the definition of a variable or variable template or the declaration of a function or function template. The consteval specifier shall be applied only to the declaration of a function or function template. A function or static data member declared with the constexpr or consteval specifier is implicitly an inline function or variable ([dcl.inline]). If any declaration of a function or function template has a constexpr or consteval specifier, then all its declarations shall contain the same specifier.

^强调我的