Should a using declaration be able to refer to a friend function
I have a class with a friend function defined in the class head. When I do a using declaration of this friend function it works in MSVC2017 but neither in GCC 8.2 or Clang 7.0. Who is right? Here is the godbolt link (https://godbolt.org/z/_7MVlh), and here is the code:
namespace vec {
class Vec {
friend Vec vec_max(const Vec& a, const Vec& b) { return a; }
};
}
void test() {
using vec::vec_max;
}
GCC gives the error on the using declaration: "error: no member named 'vec_max' in namespace 'vec'". Clang gave a similar message. MSVC compiled it as intended.
According to my understanding vec_max should reside in the vec namespace and MSVC should be correct. But there may be some subtle writing in the standard that makes the more restrictive interpretation of gcc and clang correct (although less intuitive).
c++ declaration using friend
asked Nov 21 '18 at 10:11
Bengt GustafssonBengt Gustafsson
19428
add a comment |
I have a class with a friend function defined in the class head. When I do a using declaration of this friend function it works in MSVC2017 but neither in GCC 8.2 or Clang 7.0. Who is right? Here is the godbolt link (https://godbolt.org/z/_7MVlh), and here is the code:
namespace vec {
class Vec {
friend Vec vec_max(const Vec& a, const Vec& b) { return a; }
};
}
void test() {
using vec::vec_max;
}
GCC gives the error on the using declaration: "error: no member named 'vec_max' in namespace 'vec'". Clang gave a similar message. MSVC compiled it as intended.
According to my understanding vec_max should reside in the vec namespace and MSVC should be correct. But there may be some subtle writing in the standard that makes the more restrictive interpretation of gcc and clang correct (although less intuitive).
c++ declaration using friend
asked Nov 21 '18 at 10:11
Bengt GustafssonBengt Gustafsson
19428
3
friendfunctions defined inline in a class can only be found through ADL for that class, so MSVC is wrong here.
– Quentin
Nov 21 '18 at 10:15
add a comment |
I have a class with a friend function defined in the class head. When I do a using declaration of this friend function it works in MSVC2017 but neither in GCC 8.2 or Clang 7.0. Who is right? Here is the godbolt link (https://godbolt.org/z/_7MVlh), and here is the code:
namespace vec {
class Vec {
friend Vec vec_max(const Vec& a, const Vec& b) { return a; }
};
}
void test() {
using vec::vec_max;
}
GCC gives the error on the using declaration: "error: no member named 'vec_max' in namespace 'vec'". Clang gave a similar message. MSVC compiled it as intended.
According to my understanding vec_max should reside in the vec namespace and MSVC should be correct. But there may be some subtle writing in the standard that makes the more restrictive interpretation of gcc and clang correct (although less intuitive).
c++ declaration using friend
asked Nov 21 '18 at 10:11
Bengt GustafssonBengt Gustafsson
19428
I have a class with a friend function defined in the class head. When I do a using declaration of this friend function it works in MSVC2017 but neither in GCC 8.2 or Clang 7.0. Who is right? Here is the godbolt link (https://godbolt.org/z/_7MVlh), and here is the code:
namespace vec {
class Vec {
friend Vec vec_max(const Vec& a, const Vec& b) { return a; }
};
}
void test() {
using vec::vec_max;
}
GCC gives the error on the using declaration: "error: no member named 'vec_max' in namespace 'vec'". Clang gave a similar message. MSVC compiled it as intended.
According to my understanding vec_max should reside in the vec namespace and MSVC should be correct. But there may be some subtle writing in the standard that makes the more restrictive interpretation of gcc and clang correct (although less intuitive).
c++ declaration using friend
c++ declaration using friend
asked Nov 21 '18 at 10:11
Bengt GustafssonBengt Gustafsson
19428
asked Nov 21 '18 at 10:11
Bengt GustafssonBengt Gustafsson
19428
asked Nov 21 '18 at 10:11
Bengt GustafssonBengt Gustafsson
19428
asked Nov 21 '18 at 10:11
Bengt GustafssonBengt Gustafsson
19428
asked Nov 21 '18 at 10:11
Bengt GustafssonBengt Gustafsson
19428
19428
3
friendfunctions defined inline in a class can only be found through ADL for that class, so MSVC is wrong here.
– Quentin
Nov 21 '18 at 10:15
add a comment |
3
friendfunctions defined inline in a class can only be found through ADL for that class, so MSVC is wrong here.
– Quentin
Nov 21 '18 at 10:15
3
3
friend functions defined inline in a class can only be found through ADL for that class, so MSVC is wrong here.– Quentin
Nov 21 '18 at 10:15
friend functions defined inline in a class can only be found through ADL for that class, so MSVC is wrong here.– Quentin
Nov 21 '18 at 10:15
add a comment |
1 Answer
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MSVC is wrong here. When a friend function is defined inline within a class definition and not declared outside, it should not be found by normal name lookup and only Argument-Dependent Lookup should be able to find it.
Quoting C++17 (n4659):
14.3 [class.friend]
6 A function can be defined in a friend declaration of a class if and only if the class is a non-local class (12.4),
the function name is unqualified, and the function has namespace scope. ...
7 Such a function is implicitly an inline function (10.1.6). A
friendfunction defined in a class is in the (lexical)
scope of the class in which it is defined. A friend function defined outside the class is not (6.4.1).
and 6.4.2/4 [basic.lookup.argdep]
When considering an associated namespace, the lookup is the same as the lookup performed when the
associated namespace is used as a qualifier (6.4.3.2) except that:
(4.1) ...
(4.2) Any namespace-scope friend functions or friend function templates declared in associated classes are
visible within their respective namespaces even if they are not visible during an ordinary lookup (14.3).
- (4.3) ...
answered Nov 21 '18 at 10:16
AngewAngew
133k11258350
add a comment |
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1 Answer
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1 Answer
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active
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MSVC is wrong here. When a friend function is defined inline within a class definition and not declared outside, it should not be found by normal name lookup and only Argument-Dependent Lookup should be able to find it.
Quoting C++17 (n4659):
14.3 [class.friend]
6 A function can be defined in a friend declaration of a class if and only if the class is a non-local class (12.4),
the function name is unqualified, and the function has namespace scope. ...
7 Such a function is implicitly an inline function (10.1.6). A
friendfunction defined in a class is in the (lexical)
scope of the class in which it is defined. A friend function defined outside the class is not (6.4.1).
and 6.4.2/4 [basic.lookup.argdep]
When considering an associated namespace, the lookup is the same as the lookup performed when the
associated namespace is used as a qualifier (6.4.3.2) except that:
(4.1) ...
(4.2) Any namespace-scope friend functions or friend function templates declared in associated classes are
visible within their respective namespaces even if they are not visible during an ordinary lookup (14.3).
- (4.3) ...
answered Nov 21 '18 at 10:16
AngewAngew
133k11258350
add a comment |
MSVC is wrong here. When a friend function is defined inline within a class definition and not declared outside, it should not be found by normal name lookup and only Argument-Dependent Lookup should be able to find it.
Quoting C++17 (n4659):
14.3 [class.friend]
6 A function can be defined in a friend declaration of a class if and only if the class is a non-local class (12.4),
the function name is unqualified, and the function has namespace scope. ...
7 Such a function is implicitly an inline function (10.1.6). A
friendfunction defined in a class is in the (lexical)
scope of the class in which it is defined. A friend function defined outside the class is not (6.4.1).
and 6.4.2/4 [basic.lookup.argdep]
When considering an associated namespace, the lookup is the same as the lookup performed when the
associated namespace is used as a qualifier (6.4.3.2) except that:
(4.1) ...
(4.2) Any namespace-scope friend functions or friend function templates declared in associated classes are
visible within their respective namespaces even if they are not visible during an ordinary lookup (14.3).
- (4.3) ...
answered Nov 21 '18 at 10:16
AngewAngew
133k11258350
add a comment |
MSVC is wrong here. When a friend function is defined inline within a class definition and not declared outside, it should not be found by normal name lookup and only Argument-Dependent Lookup should be able to find it.
Quoting C++17 (n4659):
14.3 [class.friend]
6 A function can be defined in a friend declaration of a class if and only if the class is a non-local class (12.4),
the function name is unqualified, and the function has namespace scope. ...
7 Such a function is implicitly an inline function (10.1.6). A
friendfunction defined in a class is in the (lexical)
scope of the class in which it is defined. A friend function defined outside the class is not (6.4.1).
and 6.4.2/4 [basic.lookup.argdep]
When considering an associated namespace, the lookup is the same as the lookup performed when the
associated namespace is used as a qualifier (6.4.3.2) except that:
(4.1) ...
(4.2) Any namespace-scope friend functions or friend function templates declared in associated classes are
visible within their respective namespaces even if they are not visible during an ordinary lookup (14.3).
- (4.3) ...
answered Nov 21 '18 at 10:16
AngewAngew
133k11258350
MSVC is wrong here. When a friend function is defined inline within a class definition and not declared outside, it should not be found by normal name lookup and only Argument-Dependent Lookup should be able to find it.
Quoting C++17 (n4659):
14.3 [class.friend]
6 A function can be defined in a friend declaration of a class if and only if the class is a non-local class (12.4),
the function name is unqualified, and the function has namespace scope. ...
7 Such a function is implicitly an inline function (10.1.6). A
friendfunction defined in a class is in the (lexical)
scope of the class in which it is defined. A friend function defined outside the class is not (6.4.1).
and 6.4.2/4 [basic.lookup.argdep]
When considering an associated namespace, the lookup is the same as the lookup performed when the
associated namespace is used as a qualifier (6.4.3.2) except that:
(4.1) ...
(4.2) Any namespace-scope friend functions or friend function templates declared in associated classes are
visible within their respective namespaces even if they are not visible during an ordinary lookup (14.3).
- (4.3) ...
answered Nov 21 '18 at 10:16
AngewAngew
133k11258350
edited Nov 21 '18 at 10:22
answered Nov 21 '18 at 10:16
AngewAngew
133k11258350
answered Nov 21 '18 at 10:16
AngewAngew
133k11258350
answered Nov 21 '18 at 10:16
AngewAngew
133k11258350
133k11258350
add a comment |
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3
friendfunctions defined inline in a class can only be found through ADL for that class, so MSVC is wrong here.– Quentin
Nov 21 '18 at 10:15