Unqualified name lookup

int n = 1;     // declaration of n
int x = n + 1; // OK: lookup finds ::n
 
int z = y - 1; // Error: lookup fails
int y = 2;     // declaration of y

int n = 1; // declaration
 
namespace N
{
    int m = 2;
 
    namespace Y
    {
        int x = n; // OK, lookup finds ::n
        int y = m; // OK, lookup finds ::N::m
        int z = k; // Error: lookup fails
    }
 
    int k = 3;
}

namespace X
{
    extern int x; // declaration, not definition
    int n = 1;    // found 1st
}
 
int n = 2;        // found 2nd
int X::x = n;     // finds X::n, sets X::x to 1

namespace A
{
    namespace N
    {
        void f();
        int i = 3; // found 3rd (if 2nd is not present)
    }
 
    int i = 4;     // found 4th (if 3rd is not present)
}
 
int i = 5;         // found 5th (if 4th is not present)
 
void A::N::f()
{
    int i = 2;     // found 2nd (if 1st is not present)
 
    while (true)
    {
       int i = 1;  // found 1st: lookup is done
       std::cout << i;
    }
}
 
// int i;          // not found
 
namespace A
{
    namespace N
    {
        // int i;  // not found
    }
}

namespace M
{
    // const int i = 1; // never found
 
    class B
    {
        // static const int i = 3;     // found 3rd (but will not pass access check)
    };
}
 
// const int i = 5;                    // found 5th
 
namespace N
{
    // const int i = 4;                // found 4th
 
    class Y : public M::B
    {
        // static const int i = 2;     // found 2nd
 
        class X
        {
            // static const int i = 1; // found 1st
            int a[i]; // use of i
            // static const int i = 1; // never found
        };
 
        // static const int i = 2;     // never found
    };
 
    // const int i = 4;                // never found
}
 
// const int i = 5;                    // never found

class B
{
    // int i;         // found 3rd
};
 
namespace M
{
    // int i;         // found 5th
 
    namespace N
    {
        // int i;     // found 4th
 
        class X : public B
        {
            // int i; // found 2nd
            void f();
            // int i; // found 2nd as well
        };
 
        // int i;     // found 4th
    }
}
 
// int i;             // found 6th
 
void M::N::X::f()
{
    // int i;         // found 1st
    i = 16;
    // int i;         // never found
}
 
namespace M
{
    namespace N
    {
        // int i;     // never found
    }
}

struct X { void f(); };
 
struct B1: virtual X { void f(); };
 
struct B2: virtual X {};
 
struct D : B1, B2
{
    void foo()
    {
        X::f(); // OK, calls X::f (qualified lookup)
        f(); // OK, calls B1::f (unqualified lookup)
    }
};
 
// C++98 rules: B1::f hides X::f, so even though X::f can be reached from D
// through B2, it is not found by name lookup from D.
 
// C++11 rules: lookup set for f in D finds nothing, proceeds to bases
//  lookup set for f in B1 finds B1::f, and is completed
// merge replaces the empty set, now lookup set for f in C has B1::f in B1
//  lookup set for f in B2 finds nothing, proceeds to bases
//    lookup for f in X finds X::f
//  merge replaces the empty set, now lookup set for f in B2 has X::f in X
// merge into C finds that every subobject (X) in the lookup set in B2 is a base
// of every subobject (B1) already merged, so the B2 set is discarded
// C is left with just B1::f found in B1
// (if struct D : B2, B1 was used, then the last merge would *replace* C's 
//  so far merged X::f in X because every subobject already added to C (that is X)
//  would be a base of at least one subobject in the new set (B1), the end
//  result would be the same: lookup set in C holds just B1::f found in B1)

struct V { int v; };
 
struct B
{
    int a;
    static int s;
    enum { e };
};
 
struct B1 : B, virtual V {};
struct B2 : B, virtual V {};
struct D : B1, B2 {};
 
void f(D& pd)
{
    ++pd.v;       // OK: only one v because only one virtual base subobject
    ++pd.s;       // OK: only one static B::s, even though found in both B1 and B2
    int i = pd.e; // OK: only one enumerator B::e, even though found in both B1 and B2
    ++pd.a;       // error, ambiguous: B::a in B1 and B::a in B2 
}

int i = 3;                     // found 3rd for f1, found 2nd for f2
 
struct X
{
    static const int i = 2;    // found 2nd for f1, never found for f2
 
    friend void f1(int x)
    {
        // int i;              // found 1st
        i = x;                 // finds and modifies X::i
    }
 
    friend int f2();
 
    // static const int i = 2; // found 2nd for f1 anywhere in class scope
};
 
void f2(int x)
{
    // int i;                  // found 1st
    i = x;                     // finds and modifies ::i
}

template<class T>
struct S;
 
// the class whose member functions are friended
struct A
{ 
    typedef int AT;
 
    void f1(AT);
    void f2(float);
 
    template<class T>
    void f3();
 
    void f4(S<AT>);
};
 
// the class that is granting friendship for f1, f2 and f3
struct B
{
    typedef char AT;
    typedef float BT;
 
    friend void A::f1(AT);    // lookup for AT finds A::AT (AT found in A)
    friend void A::f2(BT);    // lookup for BT finds B::BT (BT not found in A)
    friend void A::f3<AT>();  // lookup for AT finds B::AT (no lookup in A, because
                              //     AT is in the declarator identifier A::f3<AT>)
};
 
// the class template that is granting friendship for f4
template<class AT>
struct C
{
    friend void A::f4(S<AT>); // lookup for AT finds A::AT
                              // (AT is not in the declarator identifier A::f4)
};

class X
{
    int a, b, i, j;
public:
    const int& r;
 
    X(int i): r(a),      // initializes X::r to refer to X::a
              b(i),      // initializes X::b to the value of the parameter i
              i(i),      // initializes X::i to the value of the parameter i
              j(this->i) // initializes X::j to the value of X::i
    {}
};
 
int a;
int f(int a, int b = a); // error: lookup for a finds the parameter a, not ::a
                         // and parameters are not allowed as default arguments

struct X
{
    static int x;
    static const int n = 1; // found 1st
};
 
int n = 2;                  // found 2nd
int X::x = n;               // finds X::n, sets X::x to 1, not 2

const int RED = 7;
 
enum class color
{
    RED,
    GREEN = RED + 2, // RED finds color::RED, not ::RED, so GREEN = 2
    BLUE = ::RED + 4 // qualified lookup finds ::RED, BLUE = 11
};

int n = 3;          // found 3rd
int f(int n = 2)    // found 2nd
 
try
{
    int n = -1;     // never found
}
catch(...)
{
    // int n = 1;   // found 1st
    assert(n == 2); // loookup for n finds function parameter f
    throw;
}

struct A {};
void operator+(A, A);  // user-defined non-member operator+
 
struct B
{
    void operator+(B); // user-defined member operator+
    void f();
};
 
A a;
 
void B::f() // definition of a member function of B
{
    operator+(a, a); // error: regular name lookup from a member function
                     // finds the declaration of operator+ in the scope of B
                     // and stops there, never reaching the global scope
 
    a + a; // OK: member lookup finds B::operator+, non-member lookup
           // finds ::operator+(A, A), overload resolution selects ::operator+(A, A)
}

void f(char); // first declaration of f
 
template<class T> 
void g(T t)
{
    f(1);    // non-dependent name: lookup finds ::f(char) and binds it now
    f(T(1)); // dependent name: lookup postponed
    f(t);    // dependent name: lookup postponed
//  dd++;    // non-dependent name: lookup finds no declaration
}
 
enum E { e };
void f(E);   // second declaration of f
void f(int); // third declaration of f
double dd;
 
void h()
{
    g(e);  // instantiates g<E>, at which point
           // the second and the third uses of the name 'f'
           // are looked up and find ::f(char) (by lookup) and ::f(E) (by ADL)
           // then overload resolution chooses ::f(E).
           // This calls f(char), then f(E) twice
 
    g(32); // instantiates g<int>, at which point
           // the second and the third uses of the name 'f'
           // are looked up and find ::f(char) only
           // then overload resolution chooses ::f(char)
           // This calls f(char) three times
}
 
typedef double A;
 
template<class T>
class B
{
    typedef int A;
};
 
template<class T>
struct X : B<T>
{
    A a; // lookup for A finds ::A (double), not B<T>::A
};