Consider the following:
namespace N {
struct A { };
struct B {
B() { }
B(A const&) { }
friend void f(B const& ) { }
};
}
int main() {
f(N::B{}); // ok
f(N::A{}); // error
}
In the first case, the case succeeds - we consider the associated namespaces of N::B and find N::f(B const&). Great.
The second case fails. Why? According to [namespace.memdef]:
If a friend declaration in a non-local class first declares a class, function, class template or function template the friend is a member of the innermost enclosing namespace. [...] If a friend function or function template
is called, its name may be found by the name lookup that considers functions from namespaces and classes associated with the types of the function arguments (3.4.2).
The associated namespace of N::A is N, of which f is a member, so why is it not found by lookup?
It's because f is not declared in an associated class. B is an associated class when the argument is of type B, but not when the argument is of type A.
I quote from [basic.lookup.argdep]/4, emphasis mine:
When considering an associated namespace, the lookup is the same as the lookup performed when the
associated namespace is used as a qualifier (3.4.3.2) except that:
— Any using-directives in the associated namespace are ignored.
— 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 (11.3).
— All names except those of (possibly overloaded) functions and function templates are ignored.
Related
[namespace.memdef]/3 (emphases are mine):
If a friend declaration in a non-local class first declares a class,
function, class template or function template106 the friend is a member
of the innermost enclosing namespace. The friend declaration does not
by itself make the name visible to unqualified lookup (6.4.1) or
qualified lookup (6.4.3). [ Note: The name of the friend will be
visible in its namespace if a matching declaration is provided at
namespace scope (either before or after the class definition granting
friendship). —end note ] If a friend function or function template is
called, its name may be found by the name lookup that considers
functions from namespaces and classes associated with the types of the
function arguments (6.4.2). If the name in a friend declaration is
neither qualified nor a template-id and the declaration is a function
or an elaborated-type-specifier, the lookup to determine whether the
entity has been previously declared shall not consider any scopes
outside the innermost enclosing namespace. [ Note: The other forms of
friend declarations cannot declare a new member of the innermost
enclosing namespace and thus follow the usual lookup rules. —end note
] [ Example: ...
The code below executes correctly irrespective of the fact whether the friend declaration is or is not the first declaration in its namespace.
#include<iostream>
namespace N{
struct A;
void f(A&); // If you comment out this declaration, i.e., if the
// friend declaration turns out to be the first
// declaration in namespace N, the code will still
// execute correctly, i.e., printing the same result
// below.
struct A {
friend void f(A& ra) { std::cout << "friend void f(A&)\n" << ra.i; }
private:
int i = 100;
};
}
N::A a;
int main(){
f(a);
}
This snippet prints out the following:
friend void f(A&)
100
It is found with ADL because you pass a single parameter from namespace N. Example can be changed to this:
namespace N{
void f(int); // If you comment out this declaration, i.e., if the
// friend declaration turns out to be the first
// declaration in namespace N, the code will fail to compile.
struct A {
friend void f(int) { std::cout << "friend void f()\n"; }
private:
int i = 100;
};
}
int main(){
N::f(1);
}
online compiler
Let us define f, as a friend function of S, inside the declaration of S:
struct S
{
friend void f() {}
};
I cannot find a way to call f.
Is it true, then, that such an inline friend function can only be called with argument-dependant lookup?
struct S
{
friend void f() {}
friend void g(S const&) {}
} const s;
int main()
{
// f(); // error: 'f' was not declared in this scope
// S::f(); // error: 'f' is not a member of 'S'
g(s);
// S::g(s); // error: 'g' is not a member of 'S'
}
Bonus: what if I want to get a function-pointer/std::function/lambda to g?
Is it true, then, that such an inline friend function can only be called with argument-dependant lookup?
Yes. As specified in [namespace.memdef]/3:
If a friend declaration in a non-local class first declares a class,
function, class template or function template. the friend is a member
of the innermost enclosing namespace. The friend declaration does not
by itself make the name visible to unqualified lookup
([basic.lookup.unqual]) or qualified lookup ([basic.lookup.qual]).
Since the only declaration of f is its inline definition, it's not made visible to qualified or unqualified lookup. ADL however, has a special provision for such friend functions, [basic.lookup.argdep]/4:
When considering an associated namespace, the lookup is the same as
the lookup performed when the associated namespace is used as a
qualifier ([namespace.qual]) except that:
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
([class.friend]).
As for your bonus question, a lambda should do it:
auto exposed_g = [](S const& s){ g(s); };
It wraps the ADL into its body. Though the usual caveats about return type deduction apply. It will be a value (assuming you don't return void).
The name f is declared in a friend declaration, even it becomes the member of the namespace which contains S, but it's not visible for name lookup, unless it's redeclared at namespace scope. If not, it could be found only by ADL.
Names introduced by friend declarations within a non-local class X become members of the innermost enclosing namespace of X, but they do not become visible to ordinary name lookup (neither unqualified nor qualified) unless a matching declaration is provided at namespace scope, either before or after the class definition. Such name may be found through ADL which considers both namespaces and classes.
No. You should just declare function properly.
struct S;
inline void f();
inline void g(S const&);
struct S
{
friend void f() {}
friend void g(S const&) {}
} const s;
int main()
{
f(); // Ok
// S::f(); // error: 'f' is not a member of 'S'
g(s);
// S::g(s); // error: 'g' is not a member of 'S'
}
online compiler
According to clang, gcc and vs2013, the function Outer::f is not a friend of the class Outer::Inner.
struct Outer {
void f() {}
class Inner {
friend void f();
static const int i = 0;
};
};
void f() { int i = Outer::Inner::i; }
From [namespace.memdef]/3 I would expect the function Outer::f to be a friend of Outer::Inner, instead of ::f, because the friend declaration is not the first in its namespace containing the name f.
[namespace,memdef]/3 (emphasis is mine):
Every name first declared in a namespace is a member of that
namespace. If a friend declaration in a non-local class first
declares a class, function, class template or function
template97 the friend is a member of the innermost
enclosing namespace. The friend declaration does not by itself make
the name visible to unqualified lookup (3.4.1) or qualified lookup
(3.4.3). [ Note: The name of the friend will be visible in its
namespace if a matching declaration is provided at namespace scope
(either before or after the class definition granting friendship). —
end note ] If a friend function or function template is called, its
name may be found by the name lookup that considers functions from
namespaces and classes associated with the types of the function
arguments (3.4.2). If the name in a friend declaration is neither
qualified nor a template-id and the declaration is a function or an
elaborated-type-specifier, the lookup to determine whether the entity
has been previously declared shall not consider any scopes outside the
innermost enclosing namespace.
The first part of the standard that you quoted says (emphasis mine):
Every name first declared in a namespace is a member of that namespace. If a friend declaration in a nonlocal class first declares a class or function the friend class or function is a member of the innermost enclosing namespace.
You are assuming a class is the same as a namespace, which is not correct.
namespace Outer {
void f();
class Inner {
friend void f();
static const int i = 0;
};
}
void Outer::f() { int i = Outer::Inner::i; }
should work. To use the class member function as the friend, you'll have to use:
struct Outer {
void f();
class Inner {
friend void Outer::f();
static const int i = 0;
};
};
void Outer::f() { int i = Outer::Inner::i; }
According to [namespace.memdef]:
If the name in a friend declaration is neither
qualified nor a template-id and the declaration is a function or an elaborated-type-specifier, the lookup to determine whether the entity has been previously declared shall not consider any scopes outside the innermost
enclosing namespace.
What does "outside" mean? It could mean (1) external of (as in, all scopes within the innermost enclosing namespace are permitted, but no others) or it could mean (2) exclusive of (as in, only the innermost enclosing namespace is considered). The wording is potentially ambiguous. However, consider this example which is merged from OP's original question and OP's comments:
struct Outer {
void f() { }
class C { void foo(); };
class Inner {
friend class C;
friend void f();
static const int i = 0;
};
};
void f() { (void)Outer::Inner::i; } // compiles on GCC,Clang
void Outer::C::foo() { (void)Outer::Inner::i; } // compiles on GCC,Clang
int main() { }
Based on wording (1), Outer::f and Outer::C should be friends of Inner. Based on wording (2), ::f and ::C should be the friends. One or the other interpretation could make sense, however both GCC and Clang end up with ::f and Outer::C as the friends, which clearly doesn't make any sense. I have filed GCC Bug 66836 and Clang Bug 24088. So either both compilers are wrong in one direction or another, or there's some part of the standard that explains this logic that definitely escapes me. I wouldn't bet against the latter.
The friend function below is not found by ordinary lookup (§7.3.1.2/3), but is found by ADL (§3.4.2/4 second bullet point), so the code compiles and executes normally (live example). But the function f isn't declared in any namespace. For example, if you try replacing the call f(x); by any of these calls ::f(x);, A::f(x); or A::X::f(x);, the code will not compile. Which namespace does contain the declaration of this friend function? Does the Standard say anything about this?
#include <iostream>
namespace A {
class X {
int i;
friend void f(X x) { std::cout << x.i << '\n'; }
public:
X():i(101){}
};
}
int main()
{
A::X x;
f(x);
}
From the C++ Standard
11.3 Friends
6 A function can be defined in a friend declaration of a class if and
only if the class is a non-local class (9.8), the function name is
unqualified, and the function has namespace scope. [ Example:
class M { friend void f() { } // definition of global f, a friend of M,
// not the definition of a member function
};
—end example ]
Amd the other quote (7.3.1 Namespace definition)
3 Every name first declared in a namespace is a member of that
namespace. If a friend declaration in a non-local class first declares
a class, function, class template or function template98 the friend is
a member of the innermost enclosing namespace. The friend declaration
does not by itself make the name visible to unqualified lookup (3.4.1)
or qualified lookup (3.4.3). [ Note: The name of the friend will be
visible in its namespace if a matching declaration is provided at
namespace scope (either before or after the class definition granting
friendship). —end note ] If a friend function or function template is
called, its name may be found by the name lookup that considers
functions from namespaces and classes associated with the types of the
function arguments (3.4.2). If the name in a friend declaration is
neither qualified nor a template-id and the declaration is a function
or an elaborated-type-specifier, the lookup to determine whether the
entity has been previously declared shall not consider any scopes
outside the innermost enclosing namespace. [ Note: The other forms of
friend declarations cannot declare a new member of the innermost
enclosing namespace and thus follow the usual lookup rules. —end note
]
I would like to mention that enclosing a function name in parentheses suppresses the argument-dependent lookup.
For this code
int main()
{
A::X x;
( f )(x);
}
function f will not be found.
After searching aroung SO, one question taught me that the lexical scope of an inline friend function is the class it's defined in, meaning it can access e.g. the typedefs in the class without qualifying them. But then I wondered what is the actual scope of such a function? GCC at least rejects all my attempts to call it. Can a function such as in the example ever be called through means other than ADL, which is not possible here thanks to no arguments?
Standard quotations are appreciated, as I currently can't access my copy of it.
The following code
namespace foo{
struct bar{
friend void baz(){}
void call_friend();
};
}
int main(){
foo::baz(); // can't access through enclosing scope of the class
foo::bar::baz(); // can't access through class scope
}
namespace foo{
void bar::call_friend(){
baz(); // can't access through member function
}
}
results in these errors:
prog.cpp: In function ‘int main()’:
prog.cpp:9: error: ‘baz’ is not a member of ‘foo’
prog.cpp:10: error: ‘baz’ is not a member of ‘foo::bar’
prog.cpp: In member function ‘void foo::bar::call_friend()’:
prog.cpp:15: error: ‘baz’ was not declared in this scope
When you declare a friend function with an unqualified id in a class it names a function in the nearest enclosing namespace scope.
If that function hasn't previously been declared then the friend declaration doesn't make that function visible in that scope for normal lookup. It does make the declared function visible to argument-dependent lookup.
This is emphasised in many notes, but the definitive statement is in 7.3.1.2/3 (of ISO/IEC 14882:2011):
Every name first declared in a namespace is a member of that namespace. If a friend declaration in a non-local class first declares a class or function the friend class or function is a member of the innermost enclosing namespace. The name of the friend is not found by unqualified lookup (3.4.1) or by qualified lookup (3.4.3) until a matching declaration is provided in that namespace scope (either before or after the class definition granting friendship). If a friend function is called, its name may be found by the name lookup that considers functions from namespaces and classes associated with the types of the function arguments (3.4.2). If the name in a friend declaration is neither qualified nor a template-id and the declaration is a function or an elaborated-type-specifier, the lookup to determine whether the entity has been previously declared shall not consider any scopes outside the innermost enclosing namespace.
"The C++ Programming Language 3rd Edition (Stroustrap)" : p279:
I. "Like a member declaration, a friend declaration does not introduce a name into an enclosing scope"
II. "A friend class must be previously declared in an enclosing scope or defined in the nonclass
scope immediately enclosing the class that is declaring it a friend"
III. "A friend function can be explicitly declared just like friend classes, or it can be found through its argument types (§8.2.6) as if it was declared in the nonclass
scope immediately enclosing its class."
IV. "It follows that a friend function should either be explicitly declared in an enclosing scope or take an argument of its class. If not, the friend cannot be called. For example:"
//no f() here
void g();
class X{
friend void f(); //useless
friend void g(); //can be found because it is declared outside of class scope
friend void h(const X&); //can be found because the arguments access class members
};
void f() { } //enemy of X :)
But in your case there is more to it that has to do with the namespace, because if you put the proper declaration in foo e.g.:
namespace foo{
struct bar{
friend void baz(const &bar){};
void call_friend();
}
}
does not compile. However, if you declare it outside foo is works like a charm. Now consider that in fact, global, local, struct, and classes are in fact namespaces. Now this leads to the conclusion that the baz(const &) is implicitly defined in the global scope.
This compiles:
namespace foo{
struct bar{
friend void baz(const bar&){};
void call_friend();
};
}
int main(){
foo::bar k;
baz(k);
return 0;
}
Therefore, there are two issues:
The friend declaration does not introduce a name in an enclosing scope, unless IV. Thus the original program cannot find baz() because it has not been properly declared.
If IV , i.e. ADL, then the function is found in foo, but cannot be accessed as foo::baz(k), due to ADL. You will have to explicitely define baz(const bar&) in foo to access it by qualified name.
Thanks, hope it helps, but certainly, I liked the challenge :) .
Interesting!
It seems that the compiler does not know what scope it belongs to (and to be honest there are no clues) and thus puts in in no scope. Some standard digging coming up I suppose.
Note: If you explicitly add a declaration to a particular scope then it starts to work as expected.
namespace foo
{
void baz(); // declare it here and now it works in foo namespace etc.
struct bar
{
friend void baz(){}
void call_friend();
};
}
Digging the standard I find:
11.3 Friends [class.friend]
Paragraph 6
A function can be defined in a friend declaration of a class if and only if the class is a non-local class (9.8), the function name is unqualified, and the function has namespace scope.
[ Example:
class M { friend void f() { } // definition of global f, a friend of M,
// not the definition of a member function
};
— end example ]
Paragraph 7
Such a function is implicitly inline. A friend function 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 (3.4.1).
Note:
A free standing function that does not take a parameter is not much use as a friend. As it will have no object on which to take advantage of its friendship (I suppose file scope static storage duration objects).
A friend function defined within an enclosing class, will only be found by argument dependent lookup (ADL). Calling it will succeed when one or more of the arguments is either of the enclosing class type; or of a type declared within the class. Here is an example, displaying "Hello World!", which (for variety) doesn't use an object of the enclosing class type to provide such an argument:
#include <iostream>
struct foo
{
struct local_class{};
friend void greet(local_class o, int) { std::cout << "Hello World!\n"; }
};
int main(int argc, char *argv[])
{
foo::local_class o;
greet(o,1);
return 0;
}
In this Example,
namespace foo{
struct bar{
friend void baz(){}
void call_friend();
};
}
int main(){
foo::baz(); // can't access through enclosing scope of the class
foo::bar::baz(); // can't access through class scope
}
namespace foo{
void bar::call_friend(){
baz(); // can't access through member function
}
}
foo::baz() is inaccessible because the name baz is not visible in scope of namespace foo. If I remember correctly (§ 3.4/2) applies here.
foo::bar::baz() is inaccessible because friends aren't members of class and the scope of inline friend function is namespace or class in which their definition exists therefore, you can't access them outside of that scope.
If you put the declaration of baz() in foo the name of function baz will be visible in foo and the definition will be looked up in the nested scope of bar.
namespace foo{
void baz(); // declaration at namespace scope
struct bar{
friend void baz(){}
};
void call_friend() {
baz(); // ok
}
}
int main()
{
foo::baz(); // ok, bar will be looked up in the nested scope of foo::bar.
}
I think you are confusing friend and private. By declaring function a friend you are granting it access to your private members, and not grating other functions access to it. Anyway, any member function of a struct is accessible by any object because struct members are public by default.
However, in your case baz isn't accessible because by doing friend void baz(){} you didn't really declare the function baz, you just said that it is a friend function. You can just remove the friend keyword, and it will solve all the issues.