c++ - 在 lambda 中调用函数时 ADL 失败

Question

我正在尝试从作为在特定成员函数中运行 std::for_each 的一部分的 lambda 调用函数。这里的例子: https://godbolt.org/g/KCBLjL

namespace A {
  struct Foo {
    int a;
    int b;
  };

  void dump(const A::Foo& v) {
      std::cout << v.a << v.b << std::endl;
  }
} // ns A

class B {
public:
   void dump() const {
       (void)std::for_each(std::begin(foo), std::end(foo),
                           [](const A::Foo &f){
                           dump(f); // <- fails here, I expected ADL to kick in
       });
   }
private:
   std::vector<A::Foo> foo = { A::Foo{}, A::Foo{} };
};

获得:

<source>: In lambda function:
21 : <source>:21:19: error: no matching function for call to 
'B::dump(const A::Foo&)'
         dump(f);
               ^
18 : <source>:18:10: note: candidate: void B::dump() const
         void dump() const {

为什么ADL在这里失败了？我假设编译器会找到 B::dump 但也因为 A::Foo 在同一个命名空间中有 dump(const Foo&) ，编译器会将其添加为一个选项。但它不起作用。使用 -std=c++17 编译

Answer 1

[basic.lookup.argdep]/3 ，强调我的:

Let X be the lookup set produced by unqualified lookup 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 nor a function template

then Y is empty.