[英]Well-formed program containing an ill-formed template member function?
In the snippet below, I'm puzzled about why the definition of Wrapper::f() const
does not make my program ill-formed 1 although it calls a non-const member function of a non mutable member variable: 在下面的片段中,我很困惑为什么Wrapper::f() const
的定义不会使我的程序格式错误1尽管它调用非可变成员变量的非const成员函数:
// well-formed program (???)
// build with: g++ -std=c++17 -Wall -Wextra -Werror -pedantic
template<class T> struct Data { void f() {} };
template<class T> struct Wrapper
{
Data<T> _data;
void f() const { _data.f(); } // _data.f(): non-const!
};
int main()
{
Wrapper<void> w; // no error in instantiation point?
(void) w;
}
On the other hand, if Data
is a non template class 3 , a diagnostic is issued by my compiler: 另一方面,如果Data
是非模板类3 ,则我的编译器会发出诊断信息:
// ill-formed program (as expected)
// build with: g++ -std=c++17 -Wall -Wextra -Werror -pedantic
struct Data { void f() {} };
template<class T> struct Wrapper
{
Data _data;
void f() const { _data.f(); } //error: no matching function for call to 'Data::f() const'
};
int main()
{
Wrapper<void> w;
(void) w;
}
I feel like the answer will contain expressions such as "deduced context" ... but I really cannot pin down the exact part of the standard scecifying this behaviour. 我觉得答案将包含诸如“演绎的上下文”之类的表达...但我真的无法确定标准的确切部分,以此行为。
Is there a language lawyer to enlighten me on the matter? 是否有语言律师在此事上启发我?
Notes: 笔记:
1) But I get an error if I try and effectively call Wrapper<T>::f() const
. 1)但是如果我尝试有效地调用 Wrapper<T>::f() const
我会收到错误。
2) I've compiled with -std=c++17
but this is not specific to C++17, hence no specific tag. 2)我用-std=c++17
编译,但这不是特定于C ++ 17,因此没有特定的标记。
3) In this answer , @Baum mit Augen quotes [N4140, 14.7.1(2)]
: 3)在这个答案中 ,@ Baum mit Augen引用[N4140, 14.7.1(2)]
:
the specialization of the member is implicitly instantiated when the specialization is referenced in a context that requires the member definition to exist 当在需要成员定义存在的上下文中引用特化时,隐式实例化成员的特化
but here in the compiling snippet (#2) void f() const { _data.f(); }
但是在编译片段(#2)中void f() const { _data.f(); }
void f() const { _data.f(); }
fails although its "specialization is never referenced in a context that requires the member definition to exist" . void f() const { _data.f(); }
失败,虽然它的“专业化是从来没有在任何需要成员定义存在的上下文中引用”。
Snippet #2 is ill-formed . 片段#2格式不正确 。
As already stated in this answer , the template definition of Wrapper::f
is well-formed (thus no diagonstics are issued) as long as a valid specialization can be generated . 正如在这个答案中已经说明的那样,只要可以生成有效的专业化, Wrapper::f
的模板定义就是格式良好的(因此不会发出任何描述)。
§17.7/8 [temp.res] states: §17.7/ 8 [temp.res]指出:
Knowing which names are type names allows the syntax of every template to be checked. 知道哪些名称是类型名称允许检查每个模板的语法。 The program is ill-formed, no diagnostic required, if: 如果出现以下情况,该计划格式错误,无需诊断:
- no valid specialization can be generated for a template or a substatement of a constexpr if statement within a template and the template is not instantiated, or [...] 没有为模板或模板中的constexpr if语句的子语句生成有效的专业化,并且模板未实例化,或者[...]
In neither of the two code snippets, Wrapper<void>::f
is getting instantiated, because of the rules in §17.7.1/2 [temp.inst] : 在两个代码片段中, Wrapper<void>::f
都没有实例化,因为§17.7.1/ 2 [temp.inst]中的规则:
The implicit instantiation of a class template specialization causes the implicit instantiation of the declarations , but not of the definitions, [...]. 类模板特化的隐式实例化会导致声明的隐式实例化,而不是定义的实例化,[...]。
(emphasizing done by me) (强调由我完成)
But now §17.7/8 kicks in: if there is no instantiation and there can be no generated specialization for which the template definition of Wrapper::f
is valid (which is the case for snippet #2, as for every generated specialization Wrapper<T>::f
, a non-const
call inside a const
function on a member is would be performed), the program is ill-formed and diagnostics are issued. 但是现在§17.7/ 8开始了:如果没有实例化,并且没有生成的特殊化 , Wrapper::f
的模板定义是有效的(对于片段#2就是这种情况,对于每个生成的特化Wrapper<T>::f
,将执行成员上的const
函数内的non-const
调用),程序格式错误并发出诊断。
But because the diagnostics are not mandatory (see §17.7/8 above), the GCC can deny snippet #2 while both VS and clang compile the same code flawlessly. 但由于诊断不是强制性的(参见上面的§17.7/ 8),GCC可以拒绝代码片段#2,而VS和clang都可以完美地编译相同的代码。
For snippet #1 however you could provide a user-defined specialization for Data
where Data::f
is const
(say Data<void>::f
). 对于代码片段#1,您可以为Data
提供用户定义的特殊化,其中Data::f
是const
(比如Data<void>::f
)。 Therefore, a valid, generated specialization of Wrapper::f
is possible, ie Wrapper<void>::f
. 因此, Wrapper::f
的有效生成特化是可能的,即Wrapper<void>::f
。 So in conclusion, snippet #1 is well-formed and snippet #2 is invalid; 总而言之,代码片段#1格式正确,代码片段#2无效; all compilers work in a standard-conforming manner. 所有编译器都以符合标准的方式工作。
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