int x;

class B {
public:
  int i;
};

class D : public B {};

void f(D *pd) {
  pd->i = 3;    // finds B::i
  D::x = 2;      // ERROR: does not find ::x in the enclosing scope
}

extern int count;             // #1

int lookup_example(int count) // #2
{
  if (count < 0) {
    int count = 1;           // #3
    lookup_example(count);   // unqualified count refers to #3
  }
  return count + ::count;      // the first (unqualified) count refers to #2 ;
}                              // the second (qualified) count refers to #1

template<typename T>
T max (T a, T b) {
    return b < a ? a : b;
}

namespace BigMath {
  class BigNumber {
    ...
};

  bool operator < (BigNumber const&, BigNumber const&);
  ...
}

using BigMath::BigNumber;

void g (BigNumber const& a, BigNumber const& b) {
  ...
  BigNumber x = ::max(a,b);
  ...
}

#include <iostream>

namespace X {
  template <typename T> void f(T);
}

namespace N {
  using namespace X;
  enum E { e1 };
  void f(E) { std::cout << "N::f(N::E) called\n"; }
}    // namespace N

void f(int) { std::cout << "::f(int) called\n"; }

int main() {
  ::f(N::e1);    // qualified function name: no ADL
  f(N::e1);     // ordinary lookup finds ::f() and ADL finds N::f(), the latter is preferred
}      

#include <iostream>
int main() {
    std::cout << "Test\n"; // There is no operator<< in global namespace, but ADL
                           // examines std namespace because the left argument is in
                           // std and finds std::operator<<(std::ostream&, const char*)
    operator<<(std::cout, "Test\n"); // same, using function call notation
 
    // however,
    std::cout << endl; // Error: 'endl' is not declared in this namespace.
                       // This is not a function call to endl(), so ADL does not apply
 
    endl(std::cout); // OK: this is a function call: ADL examines std namespace
                     // because the argument of endl is in std, and finds std::endl
 
    (endl)(std::cout); // Error: 'endl' is not declared in this namespace.
                       // The sub-expression (endl) is not a function call expression
}

namespace A {
      struct X;
      struct Y;
      void f(int);
      void g(X);
}
 
namespace B {
    void f(int i) {
        f(i);      // calls B::f (endless recursion)
    }
    void g(A::X x) {
        g(x);   // Error: ambiguous between B::g (ordinary lookup)
                //        and A::g (argument-dependent lookup)
    }
    void h(A::Y y) {
        h(y);   // calls B::h (endless recursion): ADL examines the A namespace
                // but finds no A::h, so only B::h from ordinary lookup is used
    }
}

#include <iostream>
 
namespace A {
    template<typename T>
    class smart_ptr {
    public:
        smart_ptr() noexcept : ptr_(nullptr) {
 
        }
 
        smart_ptr(const T &ptr) noexcept : ptr_(new T(ptr)) {
 
        }
 
        smart_ptr(smart_ptr &rhs) noexcept {
            ptr_ = rhs.release();       // 释放所有权,此时rhs的ptr_指针为nullptr
        }
 
        smart_ptr &operator=(smart_ptr rhs) noexcept {
            swap(rhs);
            return *this;
        }
 
        void swap(smart_ptr &rhs) noexcept { // noexcept == throw() 保证不抛出异常
            using std::swap;
            swap(ptr_, rhs.ptr_);
        }
 
        T *release() noexcept {
            T *ptr = ptr_;
            ptr_ = nullptr;
            return ptr;
        }
 
        T *get() const noexcept {
            return ptr_;
        }
 
    private:
        T *ptr_;
    };
 
// 提供一个非成员swap函数for ADL(Argument Dependent Lookup)
    template<typename T>
    void swap(A::smart_ptr<T> &lhs, A::smart_ptr<T> &rhs) noexcept {
        lhs.swap(rhs);
    }
}
 
// 开启这个注释，会引发ADL冲突
//namespace std {
//    // 提供一个非成员swap函数for ADL(Argument Dependent Lookup)
//    template<typename T>
//    void swap(A::smart_ptr<T> &lhs, A::smart_ptr<T> &rhs) noexcept {
//        lhs.swap(rhs);
//    }
//
//}
 
int main() {
 
    using std::swap;
    A::smart_ptr<std::string> s1("hello"), s2("world");
    // 交换前
    std::cout << *s1.get() << " " << *s2.get() << std::endl;
    swap(s1, s2);      // 这里swap 能够通过Koenig搜索或者说ADL根据s1与s2的命名空间来查找swap函数
    // 交换后
    std::cout << *s1.get() << " " << *s2.get() << std::endl;
}