Declarations

在basic concept中，我们已经介绍了declaration的概念，关于各种entity的declaration，将分散到在介绍各个entity的时候进行说明，本章主要介绍的是c++ declaration的一般形式，掌握此，进而掌握c++ declaration非常重要。熟练掌握declaration非常重要。

cppreference Declarations

NOTE: 原文的行文思路是这样的：首先介绍了c++ language中的各种declaration，粗略地数了一下，一共有10种大类，然后原文开始着重介绍Simple declaration，原文给出了Simple declaration的grammar，简单而言，它是由decl-specifier-seq和 init-declarator-list组成。本文，对原文的simple declaration进行详细阅读。其实Simple declaration，就是最最简单的形如int a = 0的这种declaration，而对于其他entity的declaration，在原文中仅仅给出了link。

Simple declaration

A simple declaration is a statement that introduces, creates, and optionally initializes one or several identifiers, typically variables.

NOTE: 简单地说，simple declaration就是形如int a = 0的这种declaration。

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

NOTE: 这就是c++中，simple declaration的grammar，后面会对它进行详细分析。

Specifiers

参见Specifiers。

Declarators

init-declarator-list is a comma-separated sequence of one or more init-declarators, which have the following syntax:

NOTE: init-declarator-list中的list表示它可能包含多个init-declarator。

declarator initializer(optional)    
declarator requires-clause

declarator	-	the declarator
initializer	-	optional initializer (except where required, such as when initializing references or const objects). See Initialization for details.
requires-clause(C++20)	-	a requires-clause, which adds a constraint to a function declaration

Each declarator introduces exactly one object, reference, function, or (for typedef declarations) type alias, whose type is provided by decl-specifier-seq and optionally modified by operators such as & (reference to) or [] (array of) or () (function returning) in the declarator. These operators can be applied recursively, as shown below.

NOTE: 上面这段话非常重要，它指出了，在c++中，type一般是由decl-specifier-seq来指定的。

上面这段话中的function returning的含义是函数返回类型。

NOTE: 原文的后面总结了各种simple declaration的declarator，本文略去。

Notes

If a declaration introduces a variable with automatic storage duration, it is initialized when its declaration statement is executed. All automatic variables declared in a block are destroyed on exit from the block (regardless how the block is exited: via exception, goto, or by reaching its end), in order opposite to their order of initialization.

NOTE: 这一段非常重要，它是我们所谓的exception safety，RAII的理论基础。