7 Expressions [expr]

template<typename ...T> int f(int n = 0, T ...t);
int x = f<int>();               // error: no argument for second function parameter

void f() {
  std::string s = "but I have heard it works even if you don't believe in it";
  s.replace(0, 4, "").replace(s.find("even"), 4, "only").replace(s.find(" don't"), 6, "");
  assert(s == "I have heard it works only if you believe in it");       // OK
}

struct S {
  S(int);
};
int operator<<(S, int);
int i, j;
int x = S(i=1) << (i=2);
int y = operator<<(S(j=1), j=2);

struct B { };
struct D : B { };
void foo(D* dp) {
  B*  bp = dynamic_cast<B*>(dp);    // equivalent to B* bp = dp;
}

class A { virtual void f(); };
class B { virtual void g(); };
class D : public virtual A, private B { };
void g() {
  D   d;
  B*  bp = (B*)&d;                  // cast needed to break protection
  A*  ap = &d;                      // public derivation, no cast needed
  D&  dr = dynamic_cast<D&>(*bp);   // fails
  ap = dynamic_cast<A*>(bp);        // fails
  bp = dynamic_cast<B*>(ap);        // fails
  ap = dynamic_cast<A*>(&d);        // succeeds
  bp = dynamic_cast<B*>(&d);        // ill-formed (not a runtime check)
}

class E : public D, public B { };
class F : public E, public D { };
void h() {
  F   f;
  A*  ap  = &f;                     // succeeds: finds unique A
  D*  dp  = dynamic_cast<D*>(ap);   // fails: yields null; f has two D subobjects
  E*  ep  = (E*)ap;                 // error: cast from virtual base
  E*  ep1 = dynamic_cast<E*>(ap);   // succeeds
}

class D { /* ... */ };
D d1;
const D d2;

typeid(d1) == typeid(d2);       // yields true
typeid(D)  == typeid(const D);  // yields true
typeid(D)  == typeid(d2);       // yields true
typeid(D)  == typeid(const D&); // yields true

struct B { };
struct D : public B { };
D d;
B &br = d;

static_cast<D&>(br);            // produces lvalue denoting the original d object

struct B { };
struct D : private B { };
void f() {
  static_cast<D*>((B*)0);               // error: B is a private base of D
  static_cast<int B::*>((int D::*)0);   // error: B is a private base of D
}

T* p1 = new T;
const T* p2 = static_cast<const T*>(static_cast<void*>(p1));
bool b = p1 == p2;  // b will have the value true.

typedef int *A[3];                  // array of 3 pointer to int
typedef const int *const CA[3];     // array of 3 const pointer to const int

CA &&r = A{};                       // OK, reference binds to temporary array object
                                    // after qualification conversion to type CA
A &&r1 = const_cast<A>(CA{});       // error: temporary array decayed to pointer
A &&r2 = const_cast<A&&>(CA{});     // OK

struct A { int i; };
struct B : A { };
... &B::i ...       // has type int A​::​*
int a;
int* p1 = &a;
int* p2 = p1 + 1;   // defined behavior
bool b = p2 > p1;   // defined behavior, with value true

template <typename T>
struct my_future {
  /* ... */
  bool await_ready();
  void await_suspend(std::coroutine_handle<>);
  T await_resume();
};

template <class Rep, class Period>
auto operator co_await(std::chrono::duration<Rep, Period> d) {
  struct awaiter {
    std::chrono::system_clock::duration duration;
    /* ... */
    awaiter(std::chrono::system_clock::duration d) : duration(d) {}
    bool await_ready() const { return duration.count() <= 0; }
    void await_resume() {}
    void await_suspend(std::coroutine_handle<> h) { /* ... */ }
  };
  return awaiter{d};
}

using namespace std::chrono;

my_future<int> h();

my_future<void> g() {
  std::cout << "just about go to sleep...\n";
  co_await 10ms;
  std::cout << "resumed\n";
  co_await h();
}

auto f(int x = co_await h());   // error: await-expression outside of function suspension context
int a[] = { co_await h() };     // error: await-expression outside of function suspension context

template<class... Types>
struct count {
  static const std::size_t value = sizeof...(Types);
};

new auto(1);                    // allocated type is int
auto x = new auto('a');         // allocated type is char, x is of type char*

template<class T> struct A { A(T, T); };
auto y = new A{1, 2};           // allocated type is A<int>

new int * i;                    // syntax error: parsed as (new int*) i, not as (new int)*i

void can_merge(int x) {
  // These allocations are safe for merging:
  std::unique_ptr<char[]> a{new (std::nothrow) char[8]};
  std::unique_ptr<char[]> b{new (std::nothrow) char[8]};
  std::unique_ptr<char[]> c{new (std::nothrow) char[x]};

  g(a.get(), b.get(), c.get());
}

void cannot_merge(int x) {
  std::unique_ptr<char[]> a{new char[8]};
  try {
    // Merging this allocation would change its catch handler.
    std::unique_ptr<char[]> b{new char[x]};
  } catch (const std::bad_alloc& e) {
    std::cerr << "Allocation failed: " << e.what() << std::endl;
    throw;
  }
}

• new T results in one of the following calls:

  operator new(sizeof(T))
  operator new(sizeof(T), std::align_val_t(alignof(T)))
  

• new(2,f) T results in one of the following calls:

  operator new(sizeof(T), 2, f)
  operator new(sizeof(T), std::align_val_t(alignof(T)), 2, f)
  

• new T[5] results in one of the following calls:

  operator new[](sizeof(T) * 5 + x)
  operator new[](sizeof(T) * 5 + x, std::align_val_t(alignof(T)))
  

• new(2,f) T[5] results in one of the following calls:

  operator new[](sizeof(T) * 5 + x, 2, f)
  operator new[](sizeof(T) * 5 + x, std::align_val_t(alignof(T)), 2, f)
  

struct S {
  // Placement allocation function:
  static void* operator new(std::size_t, std::size_t);

  // Usual (non-placement) deallocation function:
  static void operator delete(void*, std::size_t);
};

S* p = new (0) S;   // error: non-placement deallocation function matches
                    // placement allocation function