C++语法详解

### C++语法详解 #### 1. 引言 C++语法总结旨在辅助理解，并非语言的精确表述。这里描述的语法接受有效C++构造的超集，需应用消歧规则区分表达式和声明，同时要借助访问控制、歧义性和类型规则剔除语法有效但无意义的构造。C和C++标准语法通过语法结构而非约束来表达细微差别，这虽带来了精确性，但并非总能提高可读性。 #### 2. 关键字 新的依赖上下文的关键字通过`typedef`、`namespace`、`class`、`enumeration`和`template`声明引入程序。以下是一些关键字相关的定义： - `typedef-name`：标识符 - `namespace-name`：`original-namespace-name` 或 `namespace-alias` - `original-namespace-name`：标识符 - `namespace-alias`：标识符 - `class-name`：标识符或 `template-id` - `enum-name`：标识符 - `template-name`：标识符 除非标识符被显式声明为类型名，否则假定它命名的不是类型。C++的关键字如下表所示： | 关键字 | 关键字 | 关键字 | 关键字 | | --- | --- | --- | --- | | `and` | `and_eq` | `asm` | `auto` | | `bitand` | `bitor` | `bool` | `break` | | `case` | `catch` | `char` | `class` | | `compl` | `const` | `const_cast` | `continue` | | `default` | `delete` | `do` | `double` | | `dynamic_cast` | `else` | `enum` | `explicit` | | `export` | `extern` | `false` | `float` | | `for` | `friend` | `goto` | `if` | | `inline` | `int` | `long` | `mutable` | | `namespace` | `new` | `not` | `not_eq` | | `operator` | `or` | `or_eq` | `private` | | `protected` | `public` | `register` | `reinterpret_cast` | | `return` | `short` | `signed` | `sizeof` | | `static` | `static_cast` | `struct` | `switch` | | `template` | `this` | `throw` | `true` | | `try` | `typedef` | `typeid` | `typename` | | `union` | `unsigned` | `using` | `virtual` | | `void` | `volatile` | `wchar_t` | `while` | | `xor` | `xor_eq` | | | #### 3. 词法约定 标准C和C++语法将词法约定表示为语法产生式，这增加了精确性，但也使语法变得庞大，且并非总能提高可读性。以下是一些词法约定的定义： - `hex-quad`：`hexadecimal-digit hexadecimal-digit hexadecimal-digit hexadecimal-digit` - `universal-character-name`：`\u hex-quad` 或 `\U hex-quad hex-quad` - `preprocessing-token`：`header-name`、`identifier`、`pp-number`、`character-literal`、`string-literal`、`preprocessing-op-or-punc` 或 不能属于上述类型的非空白字符 - `token`：`identifier`、`keyword`、`literal`、`operator` 或 `punctuator` 在组成词法标记时，选择最长的可能字符序列。例如，`double` 是单个标记，而不是 `do` 后跟 `uble`，`+++` 是 `++` 后跟 `+`。 #### 4. 程序 程序是通过链接组合的翻译单元的集合。翻译单元（通常称为源文件）是声明的序列： ```plaintext translation-unit: declaration-seqopt ``` #### 5. 表达式 表达式在相关章节有描述并在§6.2中总结。`expression-list` 的定义与 `expression` 相同。有两条规则用于区分函数参数逗号分隔符和逗号（顺序）运算符。以下是表达式相关的定义： ```plaintext primary-expression: literal this :: identifier :: operator-function-id :: qualified-id ( expression ) id-expression id-expression: unqualified-id qualified-id unqualified-id: identifier operator-function-id conversion-function-id ~ class-name template-id qualified-id: nested-name-specifier templateopt unqualified-id nested-name-specifier: class-or-namespace-name :: nested-name-specifieropt class-or-namespace-name :: template nested-name-specifier class-or-namespace-name: class-name namespace-name postfix-expression: primary-expression postfix-expression [ expression ] postfix-expression ( expression-listopt ) simple-type-specifier ( expression-listopt ) typename ::opt nested-name-specifier identifier ( expression-listopt ) typename ::opt nested-name-specifier templateopt template-id ( expression-listopt ) postfix-expression . templateopt ::opt id-expression postfix-expression -> templateopt ::opt id-expression postfix-expression . pseudo-destructor-name postfix-expression -> pseudo-destructor-name postfix-expression ++ postfix-expression -- dynamic_cast < type-id > ( expression ) static_cast < type-id > ( expression ) reinterpret_cast < type-id > ( expression ) const_cast < type-id > ( expression ) typeid ( expression ) typeid ( type-id ) expression-list: assignment-expression expression-list , assignment-expression pseudo-destructor-name: ::opt nested-name-specifieropt type-name :: ~ type-name ::opt nested-name-specifier template template-id :: ~ type-name ::opt nested-name-specifieropt ~ type-name unary-expression: postfix-expression ++ cast-expression -- cast-expression unary-operator cast-expression sizeof unary-expression sizeof ( type-id ) new-expression delete-expression unary-operator: one of * & + - ! ~ new-expression: ::opt new new-placementopt new-type-id new-initializeropt ::opt new new-placementopt ( type-id ) new-initializeropt new-placement: ( expression-list ) new-type-id: type-specifier-seq new-declaratoropt new-declarator: ptr-operator new-declaratoropt direct-new-declarator direct-new-declarator: [ expression ] direct-new-declarator [ constant-expression ] new-initializer: ( expression-listopt ) delete-expression: ::opt delete cast-expression ::opt delete [ ] cast-expression cast-expression: unary-expression ( type-id ) cast-expression pm-expression: cast-expression pm-expression .* cast-expression pm-expression ->* cast-expression multiplicative-expression: pm-expression multiplicative-expression * pm-expression multiplicative-expression / pm-expression multiplicative-expression % pm-expression additive-expression: multiplicative-expression additive-expression + multiplicative-expression additive-expression - multiplicative-expression shift-expression: additive-expression shift-expression << additive-expression shift-expression >> additive-expression relational-expression: shift-expression relational-expression < shift-expression relational-expression > shift-expression relational-expression <= shift-expression relational-expression >= shift-expression equality-expression: relational-expression equality-expression == relational-expression equality-expression != relational-expression and-expression: equality-expression and-expression & equality-expression exclusive-or-expression: and-expression exclusive-or-expression ^ and-expression inclusive-or-expression: exclusive-or-expression inclusive-or-expression | exclusive-or-expression logical-and-expression: inclusive-or-expression logical-and-expression && inclusive-or-expression logical-or-expression: logical-and-expression logical-or-expression || logical-and-expression conditional-expression: logical-or-expression logical-or-expression ? expression : assignment-expression assignment-expression: conditional-expression logical-or-expression assignment-operator assignment-expression throw-expression assignment-operator: one of = *= /= %= += -= >>= <<= &= ^= |= expression: assignment-expression expression , assignment-expression constant-expression: conditional-expression ``` 语法歧义源于函数风格强制类型转换和声明的相似性。例如： ```cpp int x; void f( ```