c++ - 我需要对 Stroustrup 关于 ADL 的新书上的这个例子做一些澄清

Question

我在下面重现了 Stroustrup 书(第 4 版)第 396 和 397 页中给出的参数相关查找 (ADL) 示例:

namespace N {
    struct S { int i; };
    void f(S);
    void g(S);
    void h(int);
};

struct Base {
    void f(N::S);
};

struct D : Base {
    void mf(N::S);

    void g(N::S x)
    {
        f(x);   // call Base::f()
        mf(x);  // call D::mf()
        h(1);   // error: no h(int) available
    }
};

上面的评论是正确的(我已经测试过了)，但这似乎与作者在下一段中所说的不一致:

In the standard, the rules for argument-dependent lookup are phrased in terms of associated namespaces (iso §3.4.2). Basically:

• If an argument is a class member , the associated namespaces are the class itself (including its base classes) and the class's enclosing namespaces.
• If an argument is a member of a namespace, the associated namespaces are the enclosing namespaces.
• If an argument is a built-in type, there are no associated namespaces.

在示例中，类型为N::S 的x 不是D 类的成员，也不是其基类基础。但它是 namespace N 的成员。根据上面的第二个项目符号，函数 N::f(S) 应该被调用，而不是 Base::f()。

上面的结果似乎也不符合标准中第 3.4.2p2 段中的第二个项目符号，它说:

If T is a class type (including unions), its associated classes are: the class itself; the class of which it is a member, if any; and its direct and indirect base classes. Its associated namespaces are the namespaces of which its associated classes are members. Furthermore, if T is a class template specialization, its associated namespaces and classes also include: the namespaces and classes associated with the types of the template arguments provided for template type parameters (excluding template template parameters); the namespaces of which any template template arguments are members; and the classes of which any member templates used as template template arguments are members.

Answer 1

3.4.2/3 Let X be the lookup set produced by unqualified lookup (3.4.1) and let Y be the lookup set produced by argument dependent lookup (defined as follows). If X contains

• a declaration of a class member, or
• a block-scope function declaration that is not a using-declaration, or
• a declaration that is neither a function or a function template

then Y is empty. Otherwise...

所以基本上，当普通查找找到成员函数或局部( block 作用域)函数声明(或不是函数的东西)时，ADL 不会启动。当普通查找找到一个独立的命名空间作用域函数时，或者当它什么也没找到时，它确实会启动。