9 Declarations [dcl.dcl]

const int ci = 3;                       // cv-qualified (initialized as required)
ci = 4;                                 // ill-formed: attempt to modify const

int i = 2;                              // not cv-qualified
const int* cip;                         // pointer to const int
cip = &i;                               // OK: cv-qualified access path to unqualified
*cip = 4;                               // ill-formed: attempt to modify through ptr to const

int* ip;
ip = const_cast<int*>(cip);             // cast needed to convert const int* to int*
*ip = 4;                                // defined: *ip points to i, a non-const object

const int* ciq = new const int (3);     // initialized as required
int* iq = const_cast<int*>(ciq);        // cast required
*iq = 4;                                // undefined: modifies a const object

struct X {
  mutable int i;
  int j;
};
struct Y {
  X x;
  Y();
};

const Y y;
y.x.i++;                                // well-formed: mutable member can be modified
y.x.j++;                                // ill-formed: const-qualified member modified
Y* p = const_cast<Y*>(&y);              // cast away const-ness of y
p->x.i = 99;                            // well-formed: mutable member can be modified
p->x.j = 99;                            // undefined: modifies a const subobject

const int&& foo();
int i;
struct A { double x; };
const A* a = new A();
decltype(foo()) x1 = 17;        // type is const int&&
decltype(i) x2;                 // type is int
decltype(a->x) x3;              // type is double
decltype((a->x)) x4 = x3;       // type is const double&

template<class T> struct A { ~A() = delete; };
template<class T> auto h()
  -> A<T>;
template<class T> auto i(T)     // identity
  -> T;
template<class T> auto f(T)     // #1
  -> decltype(i(h<T>()));       // forces completion of A<T> and implicitly uses A<T>​::​~A()
                                // for the temporary introduced by the use of h().
                                // (A temporary is not introduced as a result of the use of i().)
template<class T> auto f(T)     // #2
  -> void;
auto g() -> void {
  f(42);                        // OK: calls #2. (#1 is not a viable candidate: type deduction
                                // fails ([temp.deduct]) because A<int>​::​~A() is implicitly used in its
                                // decltype-specifier)
}
template<class T> auto q(T)
  -> decltype((h<T>()));        // does not force completion of A<T>; A<T>​::​~A() is not implicitly
                                // used within the context of this decltype-specifier
void r() {
  q(42);                        // error: deduction against q succeeds, so overload resolution selects
                                // the specialization “q(T) -> decltype((h<T>()))” with T$=$int;
                                // the return type is A<int>, so a temporary is introduced and its
                                // destructor is used, so the program is ill-formed
}

enum class E { a, b };
enum E x = E::a;                // OK

auto x = 5;                     // OK: x has type int
const auto *v = &x, u = 6;      // OK: v has type const int*, u has type const int
static auto y = 0.0;            // OK: y has type double
auto int r;                     // error: auto is not a storage-class-specifier
auto f() -> int;                // OK: f returns int
auto g() { return 0.0; }        // OK: g returns double
auto h();                       // OK: h's return type will be deduced when it is defined

auto x = 5, *y = &x;            // OK: auto is int
auto a = 5, b = { 1, 2 };       // error: different types for auto

auto  f() { }                   // OK, return type is void
auto* g() { }                   // error, cannot deduce auto* from void()

auto n = n;                     // error, n's initializer refers to n
auto f();
void g() { &f; }                // error, f's return type is unknown
auto sum(int i) {
  if (i == 1)
    return i;                   // sum's return type is int
  else
    return sum(i-1)+i;          // OK, sum's return type has been deduced
}

template <class T> auto f(T t) { return t; }    // return type deduced at instantiation time
typedef decltype(f(1)) fint_t;                  // instantiates f<int> to deduce return type
template<class T> auto f(T* t) { return *t; }
void g() { int (*p)(int*) = &f; }               // instantiates both fs to determine return types,
                                                // chooses second

auto f();
auto f() { return 42; }                         // return type is int
auto f();                                       // OK
int f();                                        // error, cannot be overloaded with auto f()
decltype(auto) f();                             // error, auto and decltype(auto) don't match

template <typename T> auto g(T t) { return t; } // #1
template auto g(int);                           // OK, return type is int
template char g(char);                          // error, no matching template
template<> auto g(double);                      // OK, forward declaration with unknown return type

template <class T> T g(T t) { return t; }       // OK, not functionally equivalent to #1
template char g(char);                          // OK, now there is a matching template
template auto g(float);                         // still matches #1

void h() { return g(42); }                      // error, ambiguous

template <typename T> struct A {
  friend T frf(T);
};
auto frf(int i) { return i; }                   // not a friend of A<int>

template <typename T> auto f(T t) { return t; }
extern template auto f(int);    // does not instantiate f<int>
int (*p)(int) = f;              // instantiates f<int> to determine its return type, but an explicit
                                // instantiation definition is still required somewhere in the program

template<class T> struct container {
    container(T t) {}
    template<class Iter> container(Iter beg, Iter end);
};
template<class Iter>
container(Iter b, Iter e) -> container<typename std::iterator_traits<Iter>::value_type>;
std::vector<double> v = { /* ... */ };

container c(7);                         // OK, deduces int for T
auto d = container(v.begin(), v.end()); // OK, deduces double for T
container e{5, 6};                      // error, int is not an iterator

const int ci = 10, *pc = &ci, *const cpc = pc, **ppc;
int i, *p, *const cp = &i;

i = ci;
*cp = ci;
pc++;
pc = cpc;
pc = p;
ppc = &pc;

ci = 1;             // error
ci++;               // error
*pc = 2;            // error
cp = &ci;           // error
cpc++;              // error
p = pc;             // error
ppc = &p;           // error

*ppc = &ci;         // OK, but would make p point to ci because of previous error
*p = 5;             // clobber ci

typedef int& A;
const A aref = 3;   // ill-formed; lvalue reference to non-const initialized with rvalue

void f(double& a) { a += 3.14; }
// ...
double d = 0;
f(d);

int v[20];
// ...
int& g(int i) { return v[i]; }
// ...
g(3) = 7;

struct link {
  link* next;
};

link* first;

void h(link*& p) {  // p is a reference to pointer
  p->next = first;
  first = p;
  p = 0;
}

void k() {
   link* q = new link;
   h(q);
}

int i;
typedef int& LRI;
typedef int&& RRI;

LRI& r1 = i;                    // r1 has the type int&
const LRI& r2 = i;              // r2 has the type int&
const LRI&& r3 = i;             // r3 has the type int&

RRI& r4 = i;                    // r4 has the type int&
RRI&& r5 = 5;                   // r5 has the type int&&

decltype(r2)& r6 = i;           // r6 has the type int&
decltype(r2)&& r7 = i;          // r7 has the type int&

struct X {
  void f(int);
  int a;
};
struct Y;

int X::* pmi = &X::a;
void (X::* pmf)(int) = &X::f;
double X::* pmd;
char Y::* pmc;

X obj;
// ...
obj.*pmi = 7;       // assign 7 to an integer member of obj
(obj.*pmf)(7);      // call a function member of obj with the argument 7

typedef int A[5], AA[2][3];
typedef const A CA;             // type is “array of 5 const int”
typedef const AA CAA;           // type is “array of 2 array of 3 const int”

float fa[17], *afp[17];

int x3d[3][5][7];

extern int x[10];
struct S {
  static int y[10];
};

int x[];                // OK: bound is 10
int S::y[];             // OK: bound is 10

void f() {
  extern int x[];
  int i = sizeof(x);    // error: incomplete object type
}

int printf(const char*, ...);

printf("hello world");
printf("a=%d b=%d", a, b);

typedef int FIC(int) const;
FIC f;              // ill-formed: does not declare a member function
struct S {
  FIC f;            // OK
};
FIC S::*pm = &S::f; // OK

typedef void F();
struct S {
  const F f;        // OK: equivalent to: void f();
};

int fseek(FILE*, long, int);

typedef void F();
F  fv;              // OK: equivalent to void fv();
F  fv { }           // ill-formed
void fv() { }       // OK: definition of fv

int i,
    *pi,
    f(),
    *fpi(int),
    (*pif)(const char*, const char*),
    (*fpif(int))(int);

template<typename... T> void f(T (* ...t)(int, int));

int add(int, int);
float subtract(int, int);

void g() {
  f(add, subtract);
}

void point(int = 3, int = 4);

point(1,2);  point(1);  point();

void g(int = 0, ...);           // OK, ellipsis is not a parameter so it can follow
                                // a parameter with a default argument
void f(int, int);
void f(int, int = 7);
void h() {
  f(3);                         // OK, calls f(3, 7)
  void f(int = 1, int);         // error: does not use default from surrounding scope
}
void m() {
  void f(int, int);             // has no defaults
  f(4);                         // error: wrong number of arguments
  void f(int, int = 5);         // OK
  f(4);                         // OK, calls f(4, 5);
  void f(int, int = 5);         // error: cannot redefine, even to same value
}
void n() {
  f(6);                         // OK, calls f(6, 7)
}
template<class ... T> struct C {
  void f(int n = 0, T...);
};
C<int> c;                       // OK, instantiates declaration void C​::​f(int n = 0, int)

int a = 1;
int f(int);
int g(int x = f(a));            // default argument: f(​::​a)

void h() {
  a = 2;
  {
  int a = 3;
  g();                          // g(f(​::​a))
  }
}

class C {
  void f(int i = 3);
  void g(int i, int j = 99);
};

void C::f(int i = 3) {}         // error: default argument already specified in class scope
void C::g(int i = 88, int j) {} // in this translation unit, C​::​g can be called with no argument

void f() {
  int i;
  extern void g(int x = i);         // error
  extern void h(int x = sizeof(i)); // OK
  // ...
}

int a;
int f(int a, int b = a);            // error: parameter a used as default argument
typedef int I;
int g(float I, int b = I(2));       // error: parameter I found
int h(int a, int b = sizeof(a));    // OK, unevaluated operand

int b;
class X {
  int a;
  int mem1(int i = a);              // error: non-static member a used as default argument
  int mem2(int i = b);              // OK;  use X​::​b
  static int b;
};

int f(int = 0);

void h() {
  int j = f(1);
  int k = f();                      // OK, means f(0)
}

int (*p1)(int) = &f;
int (*p2)() = &f;                   // error: type mismatch

struct A {
  virtual void f(int a = 7);
};
struct B : public A {
  void f(int a);
};
void m() {
  B* pb = new B;
  A* pa = pb;
  pa->f();          // OK, calls pa->B​::​f(7)
  pb->f();          // error: wrong number of arguments for B​::​f()
}