9 Declarations [dcl.dcl]

[[using CC: opt(1), debug]]         // same as [[CC​::​opt(1), CC​::​debug]]
  void f() {}
[[using CC: opt(1)]] [[CC::debug]]  // same as [[CC​::​opt(1)]] [[CC​::​debug]]
  void g() {}
[[using CC: CC::opt(1)]]            // error: cannot combine using and scoped attribute token
  void h() {}

int p[10];
void f() {
  int x = 42, y[5];
  int(p[[x] { return x; }()]);  // error: invalid attribute on a nested declarator-id and
                                // not a function-style cast of an element of p.
  y[[] { return 2; }()] = 2;    // error even though attributes are not allowed in this context.
  int i [[vendor::attr([[]])]]; // well-formed implementation-defined attribute.
}

struct alignas(8) S {};
struct alignas(1) U {
  S s;
};  // error: U specifies an alignment that is less strict than if the alignas(1) were omitted.

// Translation unit #1:
struct S { int x; } s, *p = &s;

// Translation unit #2:
struct alignas(16) S;           // ill-formed, no diagnostic required: definition of S lacks alignment
extern S* p;

alignas(double) void f();                           // error: alignment applied to function
alignas(double) unsigned char c[sizeof(double)];    // array of characters, suitably aligned for a double
extern unsigned char c[sizeof(double)];             // no alignas necessary
alignas(float)
  extern unsigned char c[sizeof(double)];           // error: different alignment in declaration

/* Translation unit A. */

struct foo { int* a; int* b; };
std::atomic<struct foo *> foo_head[10];
int foo_array[10][10];

[[carries_dependency]] struct foo* f(int i) {
  return foo_head[i].load(memory_order::consume);
}

int g(int* x, int* y [[carries_dependency]]) {
  return kill_dependency(foo_array[*x][*y]);
}

/* Translation unit B. */

[[carries_dependency]] struct foo* f(int i);
int g(int* x, int* y [[carries_dependency]]);

int c = 3;

void h(int i) {
  struct foo* p;

  p = f(i);
  do_something_with(g(&c, p->a));
  do_something_with(g(p->a, &c));
}

void f(int n) {
  void g(), h(), i();
  switch (n) {
  case 1:
  case 2:
    g();
    [[fallthrough]];
  case 3:                       // warning on fallthrough discouraged
    do {
      [[fallthrough]];          // error: next statement is not part of the same substatement execution
    } while (false);
  case 6:
    do {
      [[fallthrough]];          // error: next statement is not part of the same substatement execution
    } while (n--);
  case 7:
    while (false) {
      [[fallthrough]];          // error: next statement is not part of the same substatement execution
    }
  case 5:
    h();
  case 4:                       // implementation may warn on fallthrough
    i();
    [[fallthrough]];            // error
  }
}

void g(int);
int f(int n) {
  if (n > 5) [[unlikely]] {     // n > 5 is considered to be arbitrarily unlikely
    g(0);
    return n * 2 + 1;
  }

  switch (n) {
  case 1:
    g(1);
    [[fallthrough]];

  [[likely]] case 2:            // n == 2 is considered to be arbitrarily more
    g(2);                       // likely than any other value of n
    break;
  }
  return 3;
}

[[maybe_unused]] void f([[maybe_unused]] bool thing1,
                        [[maybe_unused]] bool thing2) {
  [[maybe_unused]] bool b = thing1 && thing2;
  assert(b);
}

struct [[nodiscard]] my_scopeguard { /* ... */ };
struct my_unique {
  my_unique() = default;                                // does not acquire resource
  [[nodiscard]] my_unique(int fd) { /* ... */ }         // acquires resource
  ~my_unique() noexcept { /* ... */ }                   // releases resource, if any
  /* ... */
};
struct [[nodiscard]] error_info { /* ... */ };
error_info enable_missile_safety_mode();
void launch_missiles();
void test_missiles() {
  my_scopeguard();              // warning encouraged
  (void)my_scopeguard(),        // warning not encouraged, cast to void
    launch_missiles();          // comma operator, statement continues
  my_unique(42);                // warning encouraged
  my_unique();                  // warning not encouraged
  enable_missile_safety_mode(); // warning encouraged
  launch_missiles();
}
error_info &foo();
void f() { foo(); }             // warning not encouraged: not a nodiscard call, because neither
                                // the (reference) return type nor the function is declared nodiscard

[[ noreturn ]] void f() {
  throw "error";                // OK
}

[[ noreturn ]] void q(int i) {  // behavior is undefined if called with an argument <= 0
  if (i > 0)
    throw "positive";
}

template<typename Key, typename Value,
         typename Hash, typename Pred, typename Allocator>
class hash_map {
  [[no_unique_address]] Hash hasher;
  [[no_unique_address]] Pred pred;
  [[no_unique_address]] Allocator alloc;
  Bucket *buckets;
  // ...
public:
  // ...
};