c++ - 什么时候应用 ADL？

Question

有3个例子:

我.

typedef int foo;

namespace B
{
    struct S
    {
        operator int(){ return 24; }
    };
        int foo(B::S s){ return 0; }
}

int main()
{
    int t=foo(B::S()); //24, ADL does not apply
}

二.

namespace B
{
    struct S
    {
        operator int(){ return 24; }
    };
        int foo(B::S s){ return 0; }
}

int main()
{
    int t=foo(B::S()); //0, ADL applies
}

三.

namespace B
{
    struct S
    {
        operator int(){ return 24; }
    };
        int foo(B::S s){ return 0; }
}
int foo(B::S s){ return 12; }

int main()
{
    int t=foo(B::S()); //error: call of overloaded ‘foo(B::S)’ is ambiguous
                       //ADL applies
}

我不清楚 ADL 查找的实际条件是什么？我需要引用标准来描述它。

Answer 1

This Standard paragraph 澄清，甚至有一个非常像你的第一个例子的例子。

3.4.1/3:

The lookup for an unqualified name used as the postfix-expression of a function call is described in 3.4.2 [basic.lookup.argdep]. [Note: For purposes of determining (during parsing) whether an expression is a postfix-expression for a function call, the usual name lookup rules apply. The rules in 3.4.2 have no effect on the syntactic interpretation of an expression. For example,

typedef int f;
namespace N {
  struct A {
    friend void f(A &);
    operator int();
    void g(A a) {
      int i = f(a);    // f is the typedef, not the friend
                       // function: equivalent to int(a)
    }
  };
}

Because the expression is not a function call, the argument-dependent name lookup (3.4.2) does not apply and the friend function f is not found. -end note]