[C/C++]C++声明

【注】本文是Declarations的翻译和注解版。

https://msdn.microsoft.com/en-us/library/f432x8c6.aspx

1、声明：

我们通过声明往C++程序中引入（或重新引入）名字。Definitions are declarations that are sufficient to use the entity identified by the name（不知如何翻译，意思是定义也是一种声明）。不同种类的实体其声明也是不同的，具体如下所示：

• 函数声明(Function declaration)
• 模板声明(Template declaration)
• 显式模板实例化(Explicit template instantiation)
• 显式模板具体化(Explicit template specialization)
• 命名空间定义(Namespace definition)
• 链接具体化(Linkage specification)
• 属性声明(Attribute declaration)(C++11)
• 空声明(Empty declaration)
• 块声明(Block declaration)(【注】块声明指可以出现在块中的声明)。它们可以是以下的声明：

• asm definition
• type alias declaration
• namespace alias definition
• using declaration
• using directive
• static assert declaration
• opaque enum declaration
• 简单声明(simple declaration)

简单声明是一条引入、创建一个或多个标识符（一般是变量）的语句。其语法规则如下：

attr(optional) decl-specifier-seq(optional) init-declarator-list(optional) ; (1)

• attr(C++11)           任意数目的属性序列。
• decl-specifier-seq      类型修饰符序列。它仅在声明构造函数，析构函数和用户定义类型转换函数(type conversion functions)时是可选的。
• init-declarator-list     逗号分隔的声明符（每个声明符可以有初始化值）列表。当声明有名类/结构体/联合体或有名枚举变量时Init-declarator-list是可选的。

1.1、修饰符(Specifiers)

decl-specifier-seq是由一个或多个修饰符以空白符间隔组成。修饰符有以下六大类：

• typedef修饰符：如果出现这个修饰符，整个声明就是一个类型定义声明，每个声明符都引入了一个新的类型名，而不是一个对象或函数。
• 函数修饰符(inline、virtual、explicit)：这三个修饰符只能在函数声明中使用。
• friend修饰符：这个修饰符只能在函数或类类型声明中使用。
• constexpr修饰符：这个修饰符可以在变量定义，函数、函数模板声明，字面量类型静态数据成员的声明中使用。
• 存储类型修饰符(register、static、thread local(C++11)、extern、mutable)：一个声明中只能使用一种存储类型修饰符，但是thread local可以与extern或static一起出现。
• 类型修饰符，又可以细分为以下几种情况：

• class修饰符(class specifier)
• enum修饰符(enum specifier)
• 简单类型修饰符(simple type specifier)

• 基本类型(fundamental type、base type)关键字：char, char16_t, char32_t (C++11), wchar_t, bool, short, int, long, signed, unsigned, float, double, void
• auto(C++11)
• decltype修饰符(C++11)
• 先前声明过的类类型名(限定或未限定)
• 先前声明过的枚举类型名(限定或未限定)
• 先前声明过的typedef名或type alias(C++11)(限定或未限定)
• any keyword that names a fundamental type: char, char16_t, char32_t (since C++11), wchar_t, bool, short, int, long, signed, unsigned, float, double, void

auto decltype specifier

(since C++11)

• previously declared class name (optionally qualified)
• previously declared enum name (optionally qualified)
• previously declared typedef-name or type alias (since C++11) (optionally qualified)
• template name with template arguments (optionally qualified, optionally using template disambiguator)

• elaborated type specifier

• the keyword class, struct, or union, followed by the identifier (optionally qualified), previously defined as the name of a class, struct, or union.
• the keyword class, struct, or union, followed by template name with template arguments (optionally qualified, optionally using template disambiguator), previously defined as the name of a class template.
• the keyword enum followed by the identifier (optionally qualified), previously declared as the name of an enumeration.

• typename specifier
• cv qualifier

only one type specifier is allowed in a decl-specifier-seq, with the following exceptions:

- const can be combined with any type specifier except itself.

- volatile can be combined with any type specifier except itself.

- signed or unsigned can be combined with char, long, short, or int.

- short or long can be combined with int.

- long can be combined with double.

- long can be combined with long.

(since C++11)

Attributes may appear in decl-specifier-seq, in which case they apply to the type determined by the preceding specifiers.

The only specifier that is allowed to appear twice in a decl-specifier-seq is long (which can appear twice in a row. All other repeats, such as const static const, or virtual inline virtual are errors.

(since C++17)