[英]g++ and clang++ different behaviour with friend template function defined inside a template class
Another question of type "who's right between g++ and clang++?" 另一个问题是“g ++和clang ++之间谁是对的?” for C++ standard gurus. 对于C ++标准大师。
The following code 以下代码
template <int>
struct foo
{
template <typename>
friend void bar ()
{ }
};
int main ()
{
foo<0> f0;
foo<1> f1;
}
compile without problem with clang++ (only two "unused variable" warnings) but give a the following error 使用clang ++编译没有问题(只有两个“未使用的变量”警告)但是给出以下错误
tmp_002-11,14,gcc,clang.cpp: In instantiation of ‘struct foo<1>’:
tmp_002-11,14,gcc,clang.cpp:27:12: required from here
tmp_002-11,14,gcc,clang.cpp:20:16: error: redefinition of ‘template<class> void bar()’
friend void bar ()
^~~
tmp_002-11,14,gcc,clang.cpp:20:16: note: ‘template<class> void bar()’ previously defined here
compiling with g++. 用g ++编译。
The question, as usual, is: who's right ? 像往常一样,问题是:谁是对的? g++ or clang++ ? g ++或clang ++?
Checked with clang++ 3.9.1 and g++ 6.3.0 in my Debian platform. 在我的Debian平台上查看了clang ++ 3.9.1和g ++ 6.3.0。 But, trying in Wandbox, seems equals with more recent versions. 但是,尝试使用Wandbox,似乎与更新的版本相同。
GCC is right in this case. GCC在这种情况下是对的。
The relevant standard wording is in [temp.inst]/2 : 相关的标准措辞在[temp.inst] / 2中 :
The implicit instantiation of a class template specialization causes 类模板特化的隐式实例化导致
— the implicit instantiation of the declarations, but not of the definitions, of the non-deleted class member functions, member classes, scoped member enumerations, static data members, member templates, and friends; - 未删除的类成员函数,成员类,作用域成员枚举,静态数据成员,成员模板和朋友的声明的隐式实例化,但不是定义的隐式实例化; and 和
[...] [...]
However, for the purpose of determining whether an instantiated redeclaration is valid according to 6.2 and 12.2, a declaration that corresponds to a definition in the template is considered to be a definition. 但是,为了根据6.2和12.2确定实例化的重新声明是否有效,与模板中的定义相对应的声明被认为是定义。 [ Example: [...] [ 例如: [...]template<typename T> struct Friendly { template<typename U> friend int f(U) { return sizeof(T); } }; Friendly<char> fc; Friendly<float> ff; // ill-formed: produces second definition of f(U)
— end example ] - 结束例子 ]
The parts related to friends were added to this paragraph by DR2174 and published in C++17 (it's a defect report, so compilers should apply it to previous standard versions as well). 与朋友相关的部分由DR2174添加到本段并在C ++ 17中发布(这是一个缺陷报告,因此编译器也应该将它应用于以前的标准版本)。
Recent versions of MSVC and EDG in strict mode also reject the code, complaining about a redefinition. 最新版本的MSVC和EDG在严格模式下也拒绝代码,抱怨重新定义。
[temp.inject]/1 is somewhat related, but it only talks about friend functions, not friend function templates: [temp.inject] / 1有点相关,但它只谈到朋友的功能,而不是朋友的功能模板:
Friend classes or functions can be declared within a class template. 可以在类模板中声明好友类或函数。 When a template is instantiated, the names of its friends are treated as if the specialization had been explicitly declared at its point of instantiation. 实例化模板时,会将其朋友的名称视为在实例化时明确声明了特化。
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.