是一个声明void();法律，这实际上是什么? [英] Is a statement void(); legal and what is it actually?

问题描述

我有一小段代码，其中有一条语句void();

I have a little piece of code which has a statement void();

int main() 
{
   void( ); // 1: parses fine in GCC 5.4.0 -Wpedantic 
   // void;    // 2: error declaration does not declare anything
} 

1 void()到底是什么?

• 匿名函数声明?
• 类型声明?
• 一个空表达式?

什么使1 void()与2 void;不同?

What makes 1 void() different from 2 void;?

我已经阅读:

1. sizeof(void())是合法表达式吗?，但void()被认为是sizeof
2. decltype(void())中的void()到底是什么意思?在declspec中被认为是什么?
3. 我读到 void {}是否合法?

1. Is sizeof(void()) a legal expression? but there void() is considered a type in sizeof
2. What does the void() in decltype(void()) mean exactly? where it is considered in declspec.
3. And I read Is void{} legal or not?

但是我很好奇这个松散的声明是否为void();与其中之一不同(当然还有原因)

But I am curious if the loose statement void(); is different from one of those (and why of course)

推荐答案

void;是错误的，因为语言语法中没有与该代码匹配的规则.特别是，没有规则 type-id ;，

void; is an error because there is no rule in the language grammar which matches that code. In particular, there is no rule type-id ;,

但是，代码void()匹配两个语法规则:

However, the code void() matches two grammar rules:

1. type-id .
2. postfix-expression ，子情况为 simple-type-specifier ( expression-list-opt ).

1. type-id .
2. postfix-expression, with the sub-case being simple-type-specifier ( expression-list-opt ).

现在，解析器需要将void();与语法规则匹配.即使void()匹配 type-id ，如前所述，也没有规则匹配 type-id ;.因此，在这种情况下，解析器拒绝将void()解析为 type-id ，并尝试其他可能性.

Now, the parser needs to match void(); to a grammar rule. Even though void() matches type-id, as mentioned earlier there is no rule that would match type-id ;. So the parser rejects the possible parsing of void() as type-id in this context, and tries the other possibility.

有 一系列规则，这些规则定义 postfix-expression ;进行声明.因此，在这种情况下，void()被明确地解析为 postfix-expression .

There is a series of rules defining that postfix-expression ; makes a statement. So void() is unambiguously parsed as postfix-expression in this context.

正如您已链接的其他答案所描述的那样，此代码作为 postfix-expression 的语义是void类型的prvalue.

As described by the other answers you linked already, the semantic meaning of this code as a postfix-expression is a prvalue of type void.

相关链接: sizeof(int())是合法表达吗?

Related link: Is sizeof(int()) a legal expression?

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