C++11统一初始化：字段初始值设定项不是常量

Question

我正在尝试像这样实例化一组字符串：

class POI {
public:
...
  static const std::set<std::string> TYPES { "restaurant", "education", "financial", "health", "culture", "other" };
...
}

现在，当我这样做时，我得到了这些错误（全部在这一行）：

error: field initializer is not constant
 static const std::set<std::string> TYPES { "restaurant", "education", "financial", "health", "culture", "other" };
error: in-class initialization of static data member 'const std::set<std::basic_string<char> > POI::TYPES' of non-literal type
error: non-constant in-class initialization invalid for static member 'POI::TYPES'                                                              
error: (an out of class initialization is required)
error: 'POI::TYPES' cannot be initialized by a non-constant expression when being declared

如果我假设集合中的字符串不被视为常量，那对我来说是有意义的。这真的是这里的问题吗？不幸的是，我找不到将初始化程序中的那些字符串声明为 const 的方法。这可能吗？

Answer 1

您必须离线初始化您的静态变量，如：

#include <set>
#include <string>

class POI {
public:
  static const std::set<std::string> TYPES;
};
const std::set<std::string> POI::TYPES { "restaurant", "education", "financial", "health", "culture", "other" };

这适用于整数/枚举类型，如标准（第 9.4.2 节:)所指定

If a static data member is of const integral or const enumeration type, its declaration in the class definition can specify a constant-initializer which shall be an integral constant expression. In that case, the member can appear in integral constant expressions within its scope.

Answer 2

C++ 中的初始化器是定义的一部分，而不是声明的一部分。 const 整数和 enum 类型放宽了这一点。在 C++11 中，为 文字类型

的 constexpr 成员添加了进一步的指令

[class.static.data]/p3

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 (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. [ ... ] 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

由于它不适用于您的情况，因此您应该离线初始化您的静态变量as shown in this example

class POI {
public:
  static const std::set<std::string> TYPES;
};

const std::set<std::string> POI::TYPES = {
   "restaurant", "education", "financial", "health", "culture", "other" 
};

C++11统一初始化：字段初始值设定项不是常量

C++11 uniform initialization: Field initializer is not constant

c++

c++11

uniform-initialization