没有初始化程序的constexpr静态数据成员 [英] constexpr static data member without initializer

问题描述

#include <complex>

struct S
{
  static std::complex<double> constexpr c;
};

gcc生成错误，因为缺少初始化程序. Clang和MSVC不会产生错误.

gcc generates an error because an initializer is missing. Clang and MSVC do not generate an error.

据我所知，constexpr静态数据成员必须具有初始化程序，即使它是具有可不带参数调用的构造函数的类类型(在这种情况下).不幸的是，我没有最新的C ++标准来支持我的假设.

As far as I know a constexpr static data member must have an initializer, even if it is of class type that has a constructor that can be called without arguments (as in this case). Unfortunately I don't have the latest C++ standard to back up my assumption.

因此正确的代码应使用构造函数进行初始化，例如:

So the correct code should initialize with a constructor, for instance:

struct S
{
  static std::complex<double> constexpr c {};
};

谁能证明哪个编译器是正确的，哪个是错误的?

Can anyone prove which compiler is right and which is wrong?

推荐答案

GCC错误.

GCC对constexpr变量使用C ++ 14规则，这需要提供一个初始化程序.每个 P0386 (粗体是新添加的文本):

GCC uses the C++14 rules for constexpr variables, which requires an initializer to be provided. This is changed per P0386 (bold text is newly added text):

在9.2.3.2p3中，更改:

如果非易失性 n-in-inline const静态数据成员是整数或枚举类型，则其在类定义中的声明可以指定ab race-or-equal -initializer ，其中每个 assignment-expression 的 initializer-clause 都是一个常数表达式(5.20). 可以使用constexpr说明符在类定义中声明文字类型的静态数据成员；如果是这样，则其声明应指定一个括号等于相等的初始化程序，其中作为赋值表达式的每个初始化程序子句都是一个常量表达式. [注意:在这两种情况下，该成员都可能出现在常量表达式中. -end note] 如果在程序中使用了成员(3.2)，则该成员仍应在名称空间范围中定义，并且该名称空间范围定义不应包含初始化程序.静态数据成员可以在类定义中定义，并且可以指定 b种族或相等初始化器.如果使用constexpr说明符声明了该成员，则可以在命名空间范围内使用以下内容重新声明该成员:没有初始化程序(不建议使用此用法；请参见DX).其他静态数据成员的声明中不应指定 brace-or-equal-initializer .

If a non­-volatile n​on-­inline ​const static data member is of integral or enumeration type, its declaration in the class definition can specify a b​race­-or-­equal-­initializer​ in which every initializer-­clause​ that is an ​assignment-­expression​ is a constant expression (5.20). 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 (3.2) in the program and the namespace scope definition shall not contain an initializer.​ A​n inline static data member can be defined in the class definition and may specify a b​race­-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 D.X). Declarations of other static data members shall not specify a b​race­-or-­equal-initializer.​

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