`type_alias<char[N]>{}` VS `char[N]{}` 在函数的参数中 [英] `type_alias&lt;char[N]&gt;{}` VS `char[N]{}` in function&#39;s argument

问题描述

• 环境:x86-64 Clang 6.0.0

• Environment: x86-64 Clang 6.0.0

函数定义:

void foo(const char*) {}

1. foo(char[16]{});//休斯顿，有问题！
2. foo(type_alias{});//编译愉快

type_alias 很简单:

template<typename T>
using type_alias = T;

活的恶魔

作为注释，case 1 不能编译，而 case 2 可以.

As comment notes, case 1 cannot compile while case 2 can.

我知道带有 using 的 alias declarations 不是文本替换(如 #define)，它是类型的同义词.

I know that alias declarations with using is not text substitution(like #define) and it is a synonym for the type.

但我仍然不知道如何解释这种情况.然后我给 GCC 一个 try>:

But I still cannot figure out how to account for this situation. Then I give GCC a try:

prog.cc: In function 'int main()':
prog.cc:11:7: error: expected primary-expression before 'char'
   foo(char[16]{});
       ^~~~
prog.cc:12:7: error: taking address of temporary array
   foo(type_alias<char[16]>{});
       ^~~~~~~~~~~~~~~~~~~~~~

啊，GCC反而给了我一个错误！然后我用两个编译器的不同版本编译它:

Ah, GCC gave me an error instead! Then I compile it with different versions of the two compilers:

• Clang 的 case 1 错误消息是:

• Clang's error message for case 1 is:

prog.cc:11:11: 错误:函数式强制转换或类型构造的预期为 '('

prog.cc:11:11: error: expected '(' for function-style cast or type construction

foo(char[16]{});
      ~~~~^

• Clang 让 case 2 通过.

  GCC 禁止这两种情况都通过竞争.case 1 和 case 2 的错误消息已在上面列出.

  GCC's forbid both the two cases to pass competition. Error messages for case 1 and case 2 have been listed above.

  顺便说一句，对于Clang，我也用pedantic-errors 进行了测试，但没有任何改变.

  BTW, for Clang, I have also tested with pedantic-errors, but nothing changed.

  问题:

  • 对于case 2:Clang、GCC，谁符合标准?标准(语言律师)中有任何规范吗?
  • 对于case 1:谁的错误信息更正确(IOW，符合标准)?
  • For case 2: Clang, GCC, who conforms the standard? Any spec in standard(language lawyer)?
  • For case 1: Whose error message is more correct(IOW, conforms the standard)?

  正如 VTT 评论的那样，对于 case 1，它应该是 foo(const char[16]{});.为这个错误道歉.

  As VTT comments, for case 1, it should be foo(const char[16]{});. Apology for this mistake.

  但是Clang可以编译foo(type_alias<char[16]>{});.这似乎是一个错误?

  But Clang can compile foo(type_alias<char[16]>{});. It seems to be a bug?

  推荐答案

  嗯，type_alias{} 等价于 (cv T){}，不要cv T{}.当 T 是一个数组时，这种区别很重要:

  Well, type_alias<cv T>{} is equivalent to (cv T){}, not to cv T{}. This distinction matters when T is an array:

  foo((const char[16]){});              // OK
  foo(type_alias<const char[16]>{});    // OK
  
  foo(const type_alias<char>[16]{});    // KO
  foo(const char[16]{});                // KO
  

  演示:https://wandbox.org/permlink/KGf3HVqN3USq6yy8

  对于case 2:Clang，GCC，谁符合标准?标准(语言律师)中的任何规范?

  For case 2: Clang, GCC, who conforms the standard? Any spec in standard(language lawyer)?

  两者都可以，都接受 foo(type_alias<char>[16]{})，但是 gcc 警告你(并且因为你用 -Werror，这个警告变成了错误;).

  Both does, both accept foo(type_alias<char>[16]{}), but gcc warns you about it (and since you compiled with -Werror, this warning is turned into an error ;).

  这篇关于`type_alias<char[N]>{}` VS `char[N]{}` 在函数的参数中的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！