[[noreturn]] void f [[noreturn]] (); // OK— end example
enum { }; // error typedef class { }; // error— end example
static_assert(sizeof(int) == sizeof(void*), "wrong pointer size");— end example
typedef char* Pc; static Pc; // error: name missing
void f(const Pc); // void f(char* const) (not const char*) void g(const int Pc); // void g(const int)
static char* f(); // f() has internal linkage char* f() // f() still has internal linkage { /* ... */ } char* g(); // g() has external linkage static char* g() // error: inconsistent linkage { /* ... */ } void h(); inline void h(); // external linkage inline void l(); void l(); // external linkage inline void m(); extern void m(); // external linkage static void n(); inline void n(); // internal linkage static int a; // a has internal linkage int a; // error: two definitions static int b; // b has internal linkage extern int b; // b still has internal linkage int c; // c has external linkage static int c; // error: inconsistent linkage extern int d; // d has external linkage static int d; // error: inconsistent linkage— end example
struct S; extern S a; extern S f(); extern void g(S); void h() { g(a); // error: S is incomplete f(); // error: S is incomplete }— end example
class X { mutable const int* p; // OK mutable int* const q; // error };— end example
using handler_t = void (*)(int); extern handler_t ignore; extern void (*ignore)(int); // redeclare ignore using cell = pair<void*, cell*>; // error— end example
typedef struct s { /* ... */ } s; typedef int I; typedef int I; typedef I I;— end example
struct S { typedef struct A { } A; // OK typedef struct B B; // OK typedef A A; // error };— end example
struct S; typedef struct S S; int main() { struct S* p; // OK } struct S { }; // OK— end example
class complex { /* ... */ }; typedef int complex; // error: redefinition— end example
typedef int complex; class complex { /* ... */ }; // error: redefinition— end example
struct S { S(); ~S(); }; typedef struct S T; S a = T(); // OK struct T * p; // error— end example
typedef struct { } *ps, S; // S is the class name for linkage purposes typedef decltype([]{}) C; // the closure type has no name for linkage purposes— end example
typedef struct { int f() {} } X; // error: struct with typedef name for linkage has member functions— end example
constexpr void square(int &x); // OK: declaration constexpr int bufsz = 1024; // OK: definition constexpr struct pixel { // error: pixel is a type int x; int y; constexpr pixel(int); // OK: declaration }; constexpr pixel::pixel(int a) : x(a), y(x) // OK: definition { square(x); } constexpr pixel small(2); // error: square not defined, so small(2) // not constant ([expr.const]) so constexpr not satisfied constexpr void square(int &x) { // OK: definition x *= x; } constexpr pixel large(4); // OK: square defined int next(constexpr int x) { // error: not for parameters return x + 1; } extern constexpr int memsz; // error: not a definition— end example
constexpr int square(int x) { return x * x; } // OK constexpr long long_max() { return 2147483647; } // OK constexpr int abs(int x) { if (x < 0) x = -x; return x; // OK } constexpr int first(int n) { static int value = n; // error: variable has static storage duration return value; } constexpr int uninit() { struct { int a; } s; return s.a; // error: uninitialized read of s.a } constexpr int prev(int x) { return --x; } // OK constexpr int g(int x, int n) { // OK int r = 1; while (--n > 0) r *= x; return r; }— end example
struct Length { constexpr explicit Length(int i = 0) : val(i) { } private: int val; };— end example
constexpr int f(bool b) { return b ? throw 0 : 0; } // OK constexpr int f() { return f(true); } // ill-formed, no diagnostic required struct B { constexpr B(int x) : i(0) { } // x is unused int i; }; int global; struct D : B { constexpr D() : B(global) { } // ill-formed, no diagnostic required // lvalue-to-rvalue conversion on non-constant global };— end example
constexpr int bar(int x, int y) // OK { return x + y + x*y; } // ... int bar(int x, int y) // error: redefinition of bar { return x * 2 + 3 * y; }— end example
struct pixel { int x, y; }; constexpr pixel ur = { 1294, 1024 }; // OK constexpr pixel origin; // error: initializer missing— end example
const int ci = 3; // cv-qualified (initialized as required) ci = 4; // error: 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; // error: 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 behavior: 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++; // error: 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 behavior: modifies a const subobject
Specifier(s) | Type |
type-name | the type named |
simple-template-id | the type as defined in [temp.names] |
decltype-specifier | the type as defined in [dcl.type.decltype] |
placeholder-type-specifier | the type as defined in [dcl.spec.auto] |
template-name | the type as defined in [dcl.type.class.deduct] |
char | “char” |
unsigned char | “unsigned char” |
signed char | “signed char” |
char8_t | “char8_t” |
char16_t | “char16_t” |
char32_t | “char32_t” |
bool | “bool” |
unsigned | “unsigned int” |
unsigned int | “unsigned int” |
signed | “int” |
signed int | “int” |
int | “int” |
unsigned short int | “unsigned short int” |
unsigned short | “unsigned short int” |
unsigned long int | “unsigned long int” |
unsigned long | “unsigned long int” |
unsigned long long int | “unsigned long long int” |
unsigned long long | “unsigned long long int” |
signed long int | “long int” |
signed long | “long int” |
signed long long int | “long long int” |
signed long long | “long long int” |
long long int | “long long int” |
long long | “long long int” |
long int | “long int” |
long | “long int” |
signed short int | “short int” |
signed short | “short int” |
short int | “short int” |
short | “short int” |
wchar_t | “wchar_t” |
float | “float” |
double | “double” |
long double | “long double” |
void | “void” |
friend class T;is ill-formed.
enum class E { a, b }; enum E x = E::a; // OK struct S { } s; class S* p = &s; // OK— end example
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&— end example
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 Tint; // the return type is A<int>, so a temporary is introduced and its // destructor is used, so the program is ill-formed }— end example
( expression-list )the expression-list shall be a single assignment-expression.
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— end example
auto x = 5, *y = &x; // OK: auto is int auto a = 5, b = { 1, 2 }; // error: different types for auto— end example
auto f() { } // OK, return type is void auto* g() { } // error: cannot deduce auto* from void()— end example
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 }— end example
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— end example
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> extern int v; auto v = 17; // OK, redeclares v struct S { static int i; }; auto S::i = 23; // OK— end example
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— end example
auto x1 = { 1, 2 }; // decltype(x1) is std::initializer_list<int> auto x2 = { 1, 2.0 }; // error: cannot deduce element type auto x3{ 1, 2 }; // error: not a single element auto x4 = { 3 }; // decltype(x4) is std::initializer_list<int> auto x5{ 3 }; // decltype(x5) is int— end example
const auto &i = expr;
template <class U> void f(const U& u);
int i; int&& f(); auto x2a(i); // decltype(x2a) is int decltype(auto) x2d(i); // decltype(x2d) is int auto x3a = i; // decltype(x3a) is int decltype(auto) x3d = i; // decltype(x3d) is int auto x4a = (i); // decltype(x4a) is int decltype(auto) x4d = (i); // decltype(x4d) is int& auto x5a = f(); // decltype(x5a) is int decltype(auto) x5d = f(); // decltype(x5d) is int&& auto x6a = { 1, 2 }; // decltype(x6a) is std::initializer_list<int> decltype(auto) x6d = { 1, 2 }; // error: { 1, 2 } is not an expression auto *x7a = &i; // decltype(x7a) is int* decltype(auto)*x7d = &i; // error: declared type is not plain decltype(auto)— end example
template <class ...T> struct A { A(T...) {} }; A x[29]{}; // error: no declarator operators allowed const A& y{}; // error: no declarator operators allowed— end example
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— end example
T D1, D2, ... Dn;is usually equivalent to
T D1; T D2; ... T Dn;where T is a decl-specifier-seq and each Di is an init-declarator.
struct S { /* ... */ }; S S, T; // declare two instances of struct Swhich is not equivalent to
struct S { /* ... */ }; S S; S T; // error
auto i = 1, j = 2.0; // error: deduced types for i and j do not matchas opposed to
auto i = 1; // OK: i deduced to have type int auto j = 2.0; // OK: j deduced to have type double
void f1(int a) requires true; // error: non-templated function template<typename T> auto f2(T a) -> bool requires true; // OK template<typename T> auto f3(T a) requires true -> bool; // error: requires-clause precedes trailing-return-type void (*pf)() requires true; // error: constraint on a variable void g(int (*)() requires true); // error: constraint on a parameter-declaration auto* p = new void(*)(char) requires true; // error: not a function declaration— end example
int // int i int * // int *pi int *[3] // int *p[3] int (*)[3] // int (*p3i)[3] int *() // int *f() int (*)(double) // int (*pf)(double)name respectively the types “int”, “pointer to int”, “array of 3 pointers to int”, “pointer to array of 3 int”, “function of (no parameters) returning pointer to int”, and “pointer to a function of (double) returning int”.
struct S { S(int); }; void foo(double a) { S w(int(a)); // function declaration S x(int()); // function declaration S y((int(a))); // object declaration S y((int)a); // object declaration S z = int(a); // object declaration }— end example
template <class T> struct X {}; template <int N> struct Y {}; X<int()> a; // type-id X<int(1)> b; // expression (ill-formed) Y<int()> c; // type-id (ill-formed) Y<int(1)> d; // expression void foo(signed char a) { sizeof(int()); // type-id (ill-formed) sizeof(int(a)); // expression sizeof(int(unsigned(a))); // type-id (ill-formed) (int())+1; // type-id (ill-formed) (int(a))+1; // expression (int(unsigned(a)))+1; // type-id (ill-formed) }— end example
class C { }; void f(int(C)) { } // void f(int(*fp)(C c)) { } // not: void f(int C) { } int g(C); void foo() { f(1); // error: cannot convert 1 to function pointer f(g); // OK }
class C { }; void h(int *(C[10])); // void h(int *(*_fp)(C _parm[10])); // not: void h(int *C[10]);
T Dwhere T is of the form attribute-specifier-seq decl-specifier-seq and D is a declarator.
int unsigned i;the type specifiers int unsigned determine the type “unsigned int” ([dcl.type.simple]).
T D1
const int ci = 10, *pc = &ci, *const cpc = pc, **ppc; int i, *p, *const cp = &i;declare ci, a constant integer; pc, a pointer to a constant integer; cpc, a constant pointer to a constant integer; ppc, a pointer to a pointer to a constant integer; i, an integer; p, a pointer to integer; and cp, a constant pointer to integer.
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; // error: lvalue reference to non-const initialized with rvalue— end example
void f(double& a) { a += 3.14; } // ... double d = 0; f(d);declares a to be a reference parameter of f so the call f(d) will add 3.14 to d.
int v[20]; // ... int& g(int i) { return v[i]; } // ... g(3) = 7;declares the function g() to return a reference to an integer so g(3)=7 will assign 7 to the fourth element of the array v.
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); }declares p to be a reference to a pointer to link so h(q) will leave q with the value zero.
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&— end example
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;declares pmi, pmf, pmd and pmc to be a pointer to a member of X of type int, a pointer to a member of X of type void(int), a pointer to a member of X of type double and a pointer to a member of Y of type char respectively.
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”— end example
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 }— end example
int x3d[3][5][7];declares an array of three elements, each of which is an array of five elements, each of which is an array of seven integers.
typedef int FIC(int) const; FIC f; // error: does not declare a member function struct S { FIC f; // OK }; FIC S::*pm = &S::f; // OK— end example
typedef void F(); struct S { const F f; // OK: equivalent to: void f(); };— end example
int fseek(FILE*, long, int);declares a function taking three arguments of the specified types, and returning int ([dcl.type]).
typedef void F(); F fv; // OK: equivalent to void fv(); F fv { } // error void fv() { } // OK: definition of fv— end example
int i, *pi, f(), *fpi(int), (*pif)(const char*, const char*), (*fpif(int))(int);declares an integer i, a pointer pi to an integer, a function f taking no arguments and returning an integer, a function fpi taking an integer argument and returning a pointer to an integer, a pointer pif to a function which takes two pointers to constant characters and returns an integer, a function fpif taking an integer argument and returning a pointer to a function that takes an integer argument and returns an integer.
typedef int IFUNC(int); IFUNC* fpif(int);or
auto fpif(int)->int(*)(int);
template <class T, class U> auto add(T t, U u) -> decltype(t + u);rather than
template <class T, class U> decltype((*(T*)0) + (*(U*)0)) add(T t, U u);
template<typename T> concept C1 = /* ... */; template<typename T> concept C2 = /* ... */; template<typename... Ts> concept C3 = /* ... */; void g1(const C1 auto*, C2 auto&); void g2(C1 auto&...); void g3(C3 auto...); void g4(C3 auto);
template<C1 T, C2 U> void g1(const T*, U&); template<C1... Ts> void g2(Ts&...); template<C3... Ts> void g3(Ts...); template<C3 T> void g4(T);
template<> void g1<int>(const int*, const double&); // OK, specialization of g1<int, const double>— end example
template<typename> concept C = /* ... */; template <typename T, C U> void g(T x, U y, C auto z);
template<typename T, C U, C W> void g(T x, U y, W z); template<typename T, typename U, typename W> requires C<U> && C<W> void g(T x, U y, W z);— end example
template<typename... T> void f(T (* ...t)(int, int)); int add(int, int); float subtract(int, int); void g() { f(add, subtract); }— end example
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)— end example
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— end example
void f() { int i; extern void g(int x = i); // error extern void h(int x = sizeof(i)); // OK // ... }— end example
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— end example
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— end example
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() }— end example
int f(int); int a = 2; int b = f(a); int c(b);— end example
X a();is not the declaration of an object of class X, but the declaration of a function taking no argument and returning an X.
int a; struct X { static int a; static int b; }; int X::a = 1; int X::b = a; // X::b = X::a— end example
struct A { int a; int&& r; }; int f(); int n = 10; A a1{1, f()}; // OK, lifetime is extended A a2(1, f()); // well-formed, but dangling reference A a3{1.0, 1}; // error: narrowing conversion A a4(1.0, 1); // well-formed, but dangling reference A a5(1.0, std::move(n)); // OK— end example
struct C { union { int a; const char* p; }; int x; } c = { .a = 1, .x = 3 };initializes c.a with 1 and c.x with 3. — end example
struct A { int x; struct B { int i; int j; } b; } a = { 1, { 2, 3 } };initializes a.x with 1, a.b.i with 2, a.b.j with 3.
struct base1 { int b1, b2 = 42; }; struct base2 { base2() { b3 = 42; } int b3; }; struct derived : base1, base2 { int d; }; derived d1{{1, 2}, {}, 4}; derived d2{{}, {}, 4};initializes d1.b1 with 1, d1.b2 with 2, d1.b3 with 42, d1.d with 4, and d2.b1 with 0, d2.b2 with 42, d2.b3 with 42, d2.d with 4. — end example
struct S { int a; const char* b; int c; int d = b[a]; }; S ss = { 1, "asdf" };initializes ss.a with 1, ss.b with "asdf", ss.c with the value of an expression of the form int{} (that is, 0), and ss.d with the value of ss.b[ss.a] (that is, 's'), and in
struct X { int i, j, k = 42; }; X a[] = { 1, 2, 3, 4, 5, 6 }; X b[2] = { { 1, 2, 3 }, { 4, 5, 6 } };a and b have the same value
struct A { string a; int b = 42; int c = -1; };
int x[] = { 1, 3, 5 };declares and initializes x as a one-dimensional array that has three elements since no size was specified and there are three initializers.
struct S { int y[] = { 0 }; // error: non-static data member of incomplete type };— end example
struct A; extern A a; struct A { const A& a1 { A{a,a} }; // OK const A& a2 { A{} }; // error }; A a{a,a}; // OK struct B { int n = B{}.n; // error };— end example
struct S { } s; struct A { S s1; int i1; S s2; int i2; S s3; int i3; } a = { { }, // Required initialization 0, s, // Required initialization 0 }; // Initialization not required for A::s3 because A::i3 is also not initialized— end example
float y[4][3] = { { 1, 3, 5 }, { 2, 4, 6 }, { 3, 5, 7 }, };is a completely-braced initialization: 1, 3, and 5 initialize the first row of the array y[0], namely y[0][0], y[0][1], and y[0][2].
float y[4][3] = { 1, 3, 5, 2, 4, 6, 3, 5, 7 };
union u { int a; const char* b; }; u a = { 1 }; u b = a; u c = 1; // error u d = { 0, "asdf" }; // error u e = { "asdf" }; // error u f = { .b = "asdf" }; u g = { .a = 1, .b = "asdf" }; // error— end example
char msg[] = "Syntax error on line %s\n";shows a character array whose members are initialized with a string-literal.
int g(int) noexcept; void f() { int i; int& r = i; // r refers to i r = 1; // the value of i becomes 1 int* p = &r; // p points to i int& rr = r; // rr refers to what r refers to, that is, to i int (&rg)(int) = g; // rg refers to the function g rg(i); // calls function g int a[3]; int (&ra)[3] = a; // ra refers to the array a ra[1] = i; // modifies a[1] }— end example
int& r1; // error: initializer missing extern int& r2; // OK— end example
double d = 2.0; double& rd = d; // rd refers to d const double& rcd = d; // rcd refers to d struct A { }; struct B : A { operator int&(); } b; A& ra = b; // ra refers to A subobject in b const A& rca = b; // rca refers to A subobject in b int& ir = B(); // ir refers to the result of B::operator int&— end example
double& rd2 = 2.0; // error: not an lvalue and reference not const int i = 2; double& rd3 = i; // error: type mismatch and reference not const— end example
struct A { }; struct B : A { } b; extern B f(); const A& rca2 = f(); // bound to the A subobject of the B rvalue. A&& rra = f(); // same as above struct X { operator B(); operator int&(); } x; const A& r = x; // bound to the A subobject of the result of the conversion int i2 = 42; int&& rri = static_cast<int&&>(i2); // bound directly to i2 B&& rrb = x; // bound directly to the result of operator B— end example
struct Banana { }; struct Enigma { operator const Banana(); }; struct Alaska { operator Banana&(); }; void enigmatic() { typedef const Banana ConstBanana; Banana &&banana1 = ConstBanana(); // error Banana &&banana2 = Enigma(); // error Banana &&banana3 = Alaska(); // error } const double& rcd2 = 2; // rcd2 refers to temporary with value 2.0 double&& rrd = 2; // rrd refers to temporary with value 2.0 const volatile int cvi = 1; const int& r2 = cvi; // error: cv-qualifier dropped struct A { operator volatile int&(); } a; const int& r3 = a; // error: cv-qualifier dropped // from result of conversion function double d2 = 1.0; double&& rrd2 = d2; // error: initializer is lvalue of related type struct X { operator int&(); }; int&& rri2 = X(); // error: result of conversion function is lvalue of related type int i3 = 2; double&& rrd3 = i3; // rrd3 refers to temporary with value 2.0— end example
int a = {1}; std::complex<double> z{1,2}; new std::vector<std::string>{"once", "upon", "a", "time"}; // 4 string elements f( {"Nicholas","Annemarie"} ); // pass list of two elements return { "Norah" }; // return list of one element int* e {}; // initialization to zero / null pointer x = double{1}; // explicitly construct a double std::map<std::string,int> anim = { {"bear",4}, {"cassowary",2}, {"tiger",7} };— end example
struct A { int x; int y; int z; }; A a{.y = 2, .x = 1}; // error: designator order does not match declaration order A b{.x = 1, .z = 2}; // OK, b.y initialized to 0— end example
double ad[] = { 1, 2.0 }; // OK int ai[] = { 1, 2.0 }; // error: narrowing struct S2 { int m1; double m2, m3; }; S2 s21 = { 1, 2, 3.0 }; // OK S2 s22 { 1.0, 2, 3 }; // error: narrowing S2 s23 { }; // OK: default to 0,0,0— end example
struct S { S(std::initializer_list<double>); // #1 S(std::initializer_list<int>); // #2 S(); // #3 // ... }; S s1 = { 1.0, 2.0, 3.0 }; // invoke #1 S s2 = { 1, 2, 3 }; // invoke #2 S s3 = { }; // invoke #3— end example
struct Map { Map(std::initializer_list<std::pair<std::string,int>>); }; Map ship = {{"Sophie",14}, {"Surprise",28}};— end example
struct S { // no initializer-list constructors S(int, double, double); // #1 S(); // #2 // ... }; S s1 = { 1, 2, 3.0 }; // OK: invoke #1 S s2 { 1.0, 2, 3 }; // error: narrowing S s3 { }; // OK: invoke #2— end example
enum byte : unsigned char { }; byte b { 42 }; // OK byte c = { 42 }; // error byte d = byte{ 42 }; // OK; same value as b byte e { -1 }; // error struct A { byte b; }; A a1 = { { 42 } }; // error A a2 = { byte{ 42 } }; // OK void f(byte); f({ 42 }); // error enum class Handle : uint32_t { Invalid = 0 }; Handle h { 42 }; // OK— end example
int x1 {2}; // OK int x2 {2.0}; // error: narrowing— end example
struct S { S(std::initializer_list<double>); // #1 S(const std::string&); // #2 // ... }; const S& r1 = { 1, 2, 3.0 }; // OK: invoke #1 const S& r2 { "Spinach" }; // OK: invoke #2 S& r3 = { 1, 2, 3 }; // error: initializer is not an lvalue const int& i1 = { 1 }; // OK const int& i2 = { 1.1 }; // error: narrowing const int (&iar)[2] = { 1, 2 }; // OK: iar is bound to temporary array struct A { } a; struct B { explicit B(const A&); }; const B& b2{a}; // error: cannot copy-list-initialize B temporary from A— end example
int** pp {}; // initialized to null pointer— end example
struct A { int i; int j; }; A a1 { 1, 2 }; // aggregate initialization A a2 { 1.2 }; // error: narrowing struct B { B(std::initializer_list<int>); }; B b1 { 1, 2 }; // creates initializer_list<int> and calls constructor B b2 { 1, 2.0 }; // error: narrowing struct C { C(int i, double j); }; C c1 = { 1, 2.2 }; // calls constructor with arguments (1, 2.2) C c2 = { 1.1, 2 }; // error: narrowing int j { 1 }; // initialize to 1 int k { }; // initialize to 0— end example
struct X { X(std::initializer_list<double> v); }; X x{ 1,2,3 };
const double __a[3] = {double{1}, double{2}, double{3}}; X x(std::initializer_list<double>(__a, __a+3));assuming that the implementation can construct an initializer_list object with a pair of pointers.
typedef std::complex<double> cmplx; std::vector<cmplx> v1 = { 1, 2, 3 }; void f() { std::vector<cmplx> v2{ 1, 2, 3 }; std::initializer_list<int> i3 = { 1, 2, 3 }; } struct A { std::initializer_list<int> i4; A() : i4{ 1, 2, 3 } {} // ill-formed, would create a dangling reference };
int x = 999; // x is not a constant expression const int y = 999; const int z = 99; char c1 = x; // OK, though it might narrow (in this case, it does narrow) char c2{x}; // error: might narrow char c3{y}; // error: narrows (assuming char is 8 bits) char c4{z}; // OK: no narrowing needed unsigned char uc1 = {5}; // OK: no narrowing needed unsigned char uc2 = {-1}; // error: narrows unsigned int ui1 = {-1}; // error: narrows signed int si1 = { (unsigned int)-1 }; // error: narrows int ii = {2.0}; // error: narrows float f1 { x }; // error: might narrow float f2 { 7 }; // OK: 7 can be exactly represented as a float bool b = {"meow"}; // error: narrows int f(int); int a[] = { 2, f(2), f(2.0) }; // OK: the double-to-int conversion is not at the top level— end example
int max(int a, int b, int c) { int m = (a > b) ? a : b; return (m > c) ? m : c; }
static const char __func__[] = "function-name";had been provided, where function-name is an implementation-defined string.
struct S { S() : s(__func__) { } // OK const char* s; }; void f(const char* s = __func__); // error: __func__ is undeclared— end example
struct S { constexpr S() = default; // error: implicit S() is not constexpr S(int a = 0) = default; // error: default argument void operator=(const S&) = default; // error: non-matching return type ~S() noexcept(false) = default; // OK, despite mismatched exception specification private: int i; S(S&); // OK: private copy constructor }; S::S(S&) = default; // OK: defines copy constructor struct T { T(); T(T &&) noexcept(false); }; struct U { T t; U(); U(U &&) noexcept = default; }; U u1; U u2 = static_cast<U&&>(u1); // OK, calls std::terminate if T::T(T&&) throws— end example
struct trivial { trivial() = default; trivial(const trivial&) = default; trivial(trivial&&) = default; trivial& operator=(const trivial&) = default; trivial& operator=(trivial&&) = default; ~trivial() = default; }; struct nontrivial1 { nontrivial1(); }; nontrivial1::nontrivial1() = default; // not first declaration— end example
struct onlydouble { onlydouble() = delete; // OK, but redundant template<class T> onlydouble(T) = delete; onlydouble(double); };
struct sometype { void* operator new(std::size_t) = delete; void* operator new[](std::size_t) = delete; }; sometype* p = new sometype; // error: deleted class operator new sometype* q = new sometype[3]; // error: deleted class operator new[]— end example
struct moveonly { moveonly() = default; moveonly(const moveonly&) = delete; moveonly(moveonly&&) = default; moveonly& operator=(const moveonly&) = delete; moveonly& operator=(moveonly&&) = default; ~moveonly() = default; }; moveonly* p; moveonly q(*p); // error: deleted copy constructor— end example
struct sometype { sometype(); }; sometype::sometype() = delete; // error: not first declaration— end example
task<int> f(); task<void> g1() { int i = co_await f(); std::cout << "f() => " << i << std::endl; } template <typename... Args> task<void> g2(Args&&...) { // OK, ellipsis is a pack expansion int i = co_await f(); std::cout << "f() => " << i << std::endl; } task<void> g3(int a, ...) { // error: variable parameter list not allowed int i = co_await f(); std::cout << "f() => " << i << std::endl; }— end example
#include <iostream> #include <coroutine> // ::operator new(size_t, nothrow_t) will be used if allocation is needed struct generator { struct promise_type; using handle = std::coroutine_handle<promise_type>; struct promise_type { int current_value; static auto get_return_object_on_allocation_failure() { return generator{nullptr}; } auto get_return_object() { return generator{handle::from_promise(*this)}; } auto initial_suspend() { return std::suspend_always{}; } auto final_suspend() { return std::suspend_always{}; } void unhandled_exception() { std::terminate(); } void return_void() {} auto yield_value(int value) { current_value = value; return std::suspend_always{}; } }; bool move_next() { return coro ? (coro.resume(), !coro.done()) : false; } int current_value() { return coro.promise().current_value; } generator(generator const&) = delete; generator(generator && rhs) : coro(rhs.coro) { rhs.coro = nullptr; } ~generator() { if (coro) coro.destroy(); } private: generator(handle h) : coro(h) {} handle coro; }; generator f() { co_yield 1; co_yield 2; } int main() { auto g = f(); while (g.move_next()) std::cout << g.current_value() << std::endl; }— end example
auto f() -> int(&)[2]; auto [ x, y ] = f(); // x and y refer to elements in a copy of the array return value auto& [ xr, yr ] = f(); // xr and yr refer to elements in the array referred to by f's return value— end example
struct S { int x1 : 2; volatile double y1; }; S f(); const auto [ x, y ] = f();
enum color { red, yellow, green=20, blue }; color col = red; color* cp = &col; if (*cp == blue) // ...makes color a type describing various colors, and then declares col as an object of that type, and cp as a pointer to an object of that type.
color c = 1; // error: type mismatch, no conversion from int to color int i = yellow; // OK: yellow converted to integral value 1, integral promotion
enum class Col { red, yellow, green }; int x = Col::red; // error: no Col to int conversion Col y = Col::red; if (y) { } // error: no Col to bool conversion
enum direction { left='l', right='r' }; void g() { direction d; // OK d = left; // OK d = direction::right; // OK } enum class altitude { high='h', low='l' }; void h() { altitude a; // OK a = high; // error: high not in scope a = altitude::low; // OK }— end example
struct X { enum direction { left='l', right='r' }; int f(int i) { return i==left ? 0 : i==right ? 1 : 2; } }; void g(X* p) { direction d; // error: direction not in scope int i; i = p->f(left); // error: left not in scope i = p->f(X::right); // OK i = p->f(p->left); // OK // ... }— end example
enum class fruit { orange, apple }; struct S { using enum fruit; // OK, introduces orange and apple into S }; void f() { S s; s.orange; // OK, names fruit::orange S::orange; // OK, names fruit::orange }— end example
enum class fruit { orange, apple }; enum class color { red, orange }; void f() { using enum fruit; // OK using enum color; // error: color::orange and fruit::orange conflict }— end example
export module M; namespace N1 {} // N1 is not exported export namespace N2 {} // N2 is exported namespace N3 { export int n; } // N3 is exported— end example
namespace Outer { int i; namespace Inner { void f() { i++; } // Outer::i int i; void g() { i++; } // Inner::i } }— end example
namespace Q { namespace V { void f(); // enclosing namespaces are the global namespace, Q, and Q::V class C { void m(); }; } void V::f() { // enclosing namespaces are the global namespace, Q, and Q::V extern void h(); // ... so this declares Q::V::h } void V::C::m() { // enclosing namespaces are the global namespace, Q, and Q::V } }— end example
namespace E { inline namespace I { B } }where the optional inline is present if and only if the identifier I is preceded by inline.
namespace A::inline B::C { int i; }
namespace A { inline namespace B { namespace C { int i; } } }
namespace { int i; } // unique::i void f() { i++; } // unique::i++ namespace A { namespace { int i; // A::unique::i int j; // A::unique::j } void g() { i++; } // A::unique::i++ } using namespace A; void h() { i++; // error: unique::i or A::unique::i A::i++; // A::unique::i j++; // A::unique::j }— end example
namespace X { void f() { /* ... */ } // OK: introduces X::f() namespace M { void g(); // OK: introduces X::M::g() } using M::g; void g(); // error: conflicts with X::M::g() }— end example
namespace Q { namespace V { void f(); } void V::f() { /* ... */ } // OK void V::g() { /* ... */ } // error: g() is not yet a member of V namespace V { void g(); } } namespace R { void Q::V::g() { /* ... */ } // error: R doesn't enclose Q }— end example
// Assume f and g have not yet been declared. void h(int); template <class T> void f2(T); namespace A { class X { friend void f(X); // A::f(X) is a friend class Y { friend void g(); // A::g is a friend friend void h(int); // A::h is a friend // ::h not considered friend void f2<>(int); // ::f2<>(int) is a friend }; }; // A::f, A::g and A::h are not visible here X x; void g() { f(x); } // definition of A::g void f(X) { /* ... */ } // definition of A::f void h(int) { /* ... */ } // definition of A::h // A::f, A::g and A::h are visible here and known to be friends } using A::x; void h() { A::f(x); A::X::f(x); // error: f is not a member of A::X A::X::Y::g(); // error: g is not a member of A::X::Y }— end example
namespace Company_with_very_long_name { /* ... */ } namespace CWVLN = Company_with_very_long_name; namespace CWVLN = Company_with_very_long_name; // OK: duplicate namespace CWVLN = CWVLN;— end example
namespace A { int i; namespace B { namespace C { int i; } using namespace A::B::C; void f1() { i = 5; // OK, C::i visible in B and hides A::i } } namespace D { using namespace B; using namespace C; void f2() { i = 5; // ambiguous, B::C::i or A::i? } } void f3() { i = 5; // uses A::i } } void f4() { i = 5; // error: neither i is visible }— end example
namespace M { int i; } namespace N { int i; using namespace M; } void f() { using namespace N; i = 7; // error: both M::i and N::i are visible }
namespace A { int i; } namespace B { int i; int j; namespace C { namespace D { using namespace A; int j; int k; int a = i; // B::i hides A::i } using namespace D; int k = 89; // no problem yet int l = k; // ambiguous: C::k or D::k int m = i; // B::i hides A::i int n = j; // D::j hides B::j } }
namespace A { class X { }; extern "C" int g(); extern "C++" int h(); } namespace B { void X(int); extern "C" int g(); extern "C++" int h(int); } using namespace A; using namespace B; void f() { X(1); // error: name X found in two namespaces g(); // OK: name g refers to the same entity h(); // OK: overload resolution selects A::h }
namespace D { int d1; void f(char); } using namespace D; int d1; // OK: no conflict with D::d1 namespace E { int e; void f(int); } namespace D { // namespace extension int d2; using namespace E; void f(int); } void f() { d1++; // error: ambiguous ::d1 or D::d1? ::d1++; // OK D::d1++; // OK d2++; // OK: D::d2 e++; // OK: E::e f(1); // error: ambiguous: D::f(int) or E::f(int)? f('a'); // OK: D::f(char) }— end example
struct B { void f(char); void g(char); enum E { e }; union { int x; }; }; struct D : B { using B::f; void f(int) { f('c'); } // calls B::f(char) void g(int) { g('c'); } // recursively calls D::g(int) };— end example
enum class button { up, down }; struct S { using button::up; button b = up; // OK };— end example
template <typename... bases> struct X : bases... { using bases::g...; }; X<B, D> x; // OK: B::g and D::g introduced— end example
class C { int g(); }; class D2 : public B { using B::f; // OK: B is a base of D2 using B::e; // OK: e is an enumerator of base B using B::x; // OK: x is a union member of base B using C::g; // error: C isn't a base of D2 };— end example
struct A { template <class T> void f(T); template <class T> struct X { }; }; struct B : A { using A::f<double>; // error using A::X<int>; // error };— end example
struct X { int i; static int s; }; void f() { using X::i; // error: X::i is a class member and this is not a member declaration. using X::s; // error: X::s is a class member and this is not a member declaration. }— end example
void f(); namespace A { void g(); } namespace X { using ::f; // global f using A::g; // A's g } void h() { X::f(); // calls ::f X::g(); // calls A::g }— end example
namespace A { int i; } namespace A1 { using A::i, A::i; // OK: double declaration } struct B { int i; }; struct X : B { using B::i, B::i; // error: double member declaration };— end example
namespace A { void f(int); } using A::f; // f is a synonym for A::f; that is, for A::f(int). namespace A { void f(char); } void foo() { f('a'); // calls f(int), even though f(char) exists. } void bar() { using A::f; // f is a synonym for A::f; that is, for A::f(int) and A::f(char). f('a'); // calls f(char) }— end example
namespace A { int x; } namespace B { int i; struct g { }; struct x { }; void f(int); void f(double); void g(char); // OK: hides struct g } void func() { int i; using B::i; // error: i declared twice void f(char); using B::f; // OK: each f is a function f(3.5); // calls B::f(double) using B::g; g('a'); // calls B::g(char) struct g g1; // g1 has class type B::g using B::x; using A::x; // OK: hides struct B::x x = 99; // assigns to A::x struct x x1; // x1 has class type B::x }— end example
namespace B { void f(int); void f(double); } namespace C { void f(int); void f(double); void f(char); } void h() { using B::f; // B::f(int) and B::f(double) using C::f; // C::f(int), C::f(double), and C::f(char) f('h'); // calls C::f(char) f(1); // error: ambiguous: B::f(int) or C::f(int)? void f(int); // error: f(int) conflicts with C::f(int) and B::f(int) }— end example
struct B { virtual void f(int); virtual void f(char); void g(int); void h(int); }; struct D : B { using B::f; void f(int); // OK: D::f(int) overrides B::f(int); using B::g; void g(char); // OK using B::h; void h(int); // OK: D::h(int) hides B::h(int) }; void k(D* p) { p->f(1); // calls D::f(int) p->f('a'); // calls B::f(char) p->g(1); // calls B::g(int) p->g('a'); // calls D::g(char) } struct B1 { B1(int); }; struct B2 { B2(int); }; struct D1 : B1, B2 { using B1::B1; using B2::B2; }; D1 d1(0); // error: ambiguous struct D2 : B1, B2 { using B1::B1; using B2::B2; D2(int); // OK: D2::D2(int) hides B1::B1(int) and B2::B2(int) }; D2 d2(0); // calls D2::D2(int)— end example
struct A { int x(); }; struct B : A { }; struct C : A { using A::x; int x(int); }; struct D : B, C { using C::x; int x(double); }; int f(D* d) { return d->x(); // error: overload resolution selects A::x, but A is an ambiguous base class }— end example
class A { private: void f(char); public: void f(int); protected: void g(); }; class B : public A { using A::f; // error: A::f(char) is inaccessible public: using A::g; // B::g is a public synonym for A::g };— end example
complex sqrt(complex); // C++ linkage by default extern "C" { double sqrt(double); // C linkage }— end example
extern "C" // the name f1 and its function type have C language linkage; void f1(void(*pf)(int)); // pf is a pointer to a C function extern "C" typedef void FUNC(); FUNC f2; // the name f2 has C++ language linkage and the // function's type has C language linkage extern "C" FUNC f3; // the name of function f3 and the function's type have C language linkage void (*pf2)(FUNC*); // the name of the variable pf2 has C++ linkage and the type // of pf2 is “pointer to C++ function that takes one parameter of type // pointer to C function” extern "C" { static void f4(); // the name of the function f4 has internal linkage (not C language linkage) // and the function's type has C language linkage. } extern "C" void f5() { extern void f4(); // OK: Name linkage (internal) and function type linkage (C language linkage) // obtained from previous declaration. } extern void f4(); // OK: Name linkage (internal) and function type linkage (C language linkage) // obtained from previous declaration. void f6() { extern void f4(); // OK: Name linkage (internal) and function type linkage (C language linkage) // obtained from previous declaration. }— end example
extern "C" typedef void FUNC_c(); class C { void mf1(FUNC_c*); // the name of the function mf1 and the member function's type have // C++ language linkage; the parameter has type “pointer to C function” FUNC_c mf2; // the name of the function mf2 and the member function's type have // C++ language linkage static FUNC_c* q; // the name of the data member q has C++ language linkage and // the data member's type is “pointer to C function” }; extern "C" { class X { void mf(); // the name of the function mf and the member function's type have // C++ language linkage void mf2(void(*)()); // the name of the function mf2 has C++ language linkage; // the parameter has type “pointer to C function” }; }— end example
int x; namespace A { extern "C" int f(); extern "C" int g() { return 1; } extern "C" int h(); extern "C" int x(); // error: same name as global-space object x } namespace B { extern "C" int f(); // A::f and B::f refer to the same function extern "C" int g() { return 1; } // error: the function g with C language linkage has two definitions } int A::f() { return 98; } // definition for the function f with C language linkage extern "C" int h() { return 97; } // definition for the function h with C language linkage // A::h and ::h refer to the same function— end example
extern "C" double f(); static double f(); // error extern "C" int i; // declaration extern "C" { int i; // definition } extern "C" static void g(); // error— end example
[[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() {}— end example
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. }— end example
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.— end example
// 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;— end example
alignas(T) alignas(A) T buffer[N];
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— end example
/* 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 } }— end example
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; }— end example
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— end example
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: // ... };