为什么 C++ 的类/结构体/联合体/枚举 的}后面要写;，而函数定义不用？

引言

相信大多数人都遇到以下的编译错误:

error: expected ';' after class definition

error: expected ';' after struct definition

error: expected ';' after union definition

error: expected unqualified-id at end of input

在Stack Overflow等社区也有相关的不少讨论:

• Semicolon after class declaration braces
• Semicolon after Function
• When do we need to put semicolon after curly brackets?
• Shall a C++ class function end with a }; or only a }?

some people put Semicolon always, because they don't want to remember when it is necessary

btw:在Java和C#中,类声明语法是不需要“尾随分号”的。

原因

显然，这种规则是从C继承过来的，在C语言中，可以在定义结构体之后立即声明对象，和int a = 1;是同样的简单声明:

void Example() {
  struct { int x; } s1;
  s1.x = 42;

  struct ADifferentType { int x; };
}

立即声明对象的一个好处是可以实现匿名类的对象。

在早期的C版本中，除非另有声明，函数默认隐式返回int类型。若在结构体定义末尾省略分号，例如：

struct fred { int x; long y; } 
main()
{ 
  return 0; /* invalid return type, expected fred type */
} 

这里我们不仅定义了一个新类型fred，还声明了将返回一个fred类型的实例的main函数。

语法

class|struct|union|enum 这类属于是Simple declaration:

Simple declaration
A simple declaration introduces, creates, and optionally initializes identifiers (typically variables).

decl-specifier-seq init-declarator-list /* optional */ ;
attr decl-specifier-seq init-declarator-list ; // since C++11

• decl-specifier-seq — sequence of specifiers
• init-declarator-list — comma-separated list of init-declarator
• attr — sequence of attributes

函数定义以function-body结尾，可以是:

ctor-initializer (optional) compound-statement
: Regular function body.

function-try-block
: Function try block.

= default ;
: Explicitly defaulted function definition(since C++11).

= delete ;
: Explicitly deleted function definition(since C++11).

= delete ( string-literal );
: Explicitly deleted function definition with error message (since C++26).

• ctor-initializer - member initializer list, only allowed in constructors

• compound-statement - the brace-enclosed sequence of statements that constitutes the body of a function

• function-try-block - a function try block

• string-literal - an unevaluated string literal that could be used to explain the rationale for why the function is deleted