使用decltype的显式析构函数调用 [英] Explicit destructor call with decltype

问题描述

请考虑以下代码段:

struct Foo {};

int main()
{
   Foo f;
   f.~decltype(f)(); // fine with clang, error with gcc
   f.~decltype(auto)(); // error with both clang and gcc 
}

显式析构函数调用的规则由标准语法pseudo-destructor-name处理，其定义如下:

The rules for an explicit destructor call are handled by the standard grammar with pseudo-destructor-name which is defined as follows:

伪析构函数名称:
嵌套名称说明符opt类型名称::〜类型名称
嵌套名称说明符模板simple-template-id ::〜类型名称
〜类型名称
〜decltype-specifier

pseudo-destructor-name:
nested-name-specifier opt type-name :: ~ type-name
nested-name-specifier template simple-template-id :: ~type-name
~ type-name
~ decltype-specifier

并且:

decltype-specifier:
decltype(表达式)
decltype(auto)

decltype-specifier:
decltype ( expression )
decltype ( auto )

那么上面的代码片段是否应该按照标准格式编排? (不考虑在同一对象上两次调用析构函数，然后第三次调用析构函数的事实.)

Then shouldn't the above snippet be well-formed as per standard? (Not considering the fact that the destructor is called twice and then a third time on the same object.)

GCC直播
Clang Live

推荐答案

您的程序格式不正确.
§7.1.6.4/[dcl.spec.auto] 指出:

Your program is ill-formed.
§7.1.6.4/[dcl.spec.auto] states:

在本节未明确允许的上下文中使用auto或decltype(auto)的程序格式错误.

A program that uses auto or decltype(auto) in a context not explicitly allowed in this section is ill-formed.

在这里，我找不到任何应该允许您编写的内容.通常，decltype(auto)仅用于变量和函数声明.语法允许的事实并不意味着它的格式正确.

There, I cannot find anything that should allow you to write this. Generally, decltype(auto) is used in variable and function declarations only. The fact the grammar allows is doesn't mean it's well-formed, though.

因此，没有明确禁止编写类似f.~decltype(f)()的内容，并且可以按照语法中的说明进行编写. GCC无法编译的事实很可能是一个错误.

Therefore, writing something like f.~decltype(f)() hasn't been explicitely forbidden and is allowed as stated in the grammar. The fact that the GCC won't compile it is most likely a bug.

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