9 Declarations [dcl.dcl]

9.2 Specifiers [dcl.spec]

9.2.9 Type specifiers [dcl.type]

9.2.9.1 General [dcl.type.general]

2
#
As a general rule, at most one defining-type-specifier is allowed in the complete decl-specifier-seq of a declaration or in a defining-type-specifier-seq, and at most one type-specifier is allowed in a type-specifier-seq.
The only exceptions to this rule are the following:
  • (2.1)
    const can be combined with any type specifier except itself.
  • (2.2)
    volatile can be combined with any type specifier except itself.
  • (2.3)
    signed or unsigned can be combined with char, long, short, or int.
  • (2.4)
    short or long can be combined with int.
  • (2.5)
    long can be combined with double.
  • (2.6)
    long can be combined with long.
3
#
Except in a declaration of a constructor, destructor, or conversion function, at least one defining-type-specifier that is not a cv-qualifier shall appear in a complete type-specifier-seq or a complete decl-specifier-seq.90
4
#
[Note 1:
enum-specifiers, class-specifiers, and typename-specifiers are discussed in [dcl.enum], [class], and [temp.res], respectively.
The remaining type-specifiers are discussed in the rest of [dcl.type].
— end note]
90)
There is no special provision for a decl-specifier-seq that lacks a type-specifier or that has a type-specifier that only specifies cv-qualifiers.
The “implicit int” rule of C is no longer supported.
 ⮥

9.2.9.2 The cv-qualifiers [dcl.type.cv]

1
#
There are two cv-qualifiers, const and volatile.
Each cv-qualifier shall appear at most once in a cv-qualifier-seq.
If a cv-qualifier appears in a decl-specifier-seq, the init-declarator-list or member-declarator-list of the declaration shall not be empty.
[Note 1:
[basic.type.qualifier] and [dcl.fct] describe how cv-qualifiers affect object and function types.
— end note]
Redundant cv-qualifications are ignored.
[Note 2:
For example, these could be introduced by typedefs.
— end note]
2
#
[Note 3:
Declaring a variable const can affect its linkage ([dcl.stc]) and its usability in constant expressions.
As described in [dcl.init], the definition of an object or subobject of const-qualified type must specify an initializer or be subject to default-initialization.
— end note]
3
#
A pointer or reference to a cv-qualified type need not actually point or refer to a cv-qualified object, but it is treated as if it does; a const-qualified access path cannot be used to modify an object even if the object referenced is a non-const object and can be modified through some other access path.
[Note 4:
Cv-qualifiers are supported by the type system so that they cannot be subverted without casting.
— end note]
4
#
Any attempt to modify ([expr.ass], [expr.post.incr], [expr.pre.incr]) a const object ([basic.type.qualifier]) during its lifetime ([basic.life]) results in undefined behavior.
[Example 1: 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
For another example, 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
— end example]
5
#
The semantics of an access through a volatile glvalue are implementation-defined.
If an attempt is made to access an object defined with a volatile-qualified type through the use of a non-volatile glvalue, the behavior is undefined.
6
#
[Note 5:
volatile is a hint to the implementation to avoid aggressive optimization involving the object because the value of the object might be changed by means undetectable by an implementation.
Furthermore, for some implementations, volatile might indicate that special hardware instructions are required to access the object.
See [intro.execution] for detailed semantics.
In general, the semantics of volatile are intended to be the same in C++ as they are in C.
— end note]

9.2.9.3 Simple type specifiers [dcl.type.simple]

2
#
A placeholder-type-specifier is a placeholder for a type to be deduced ([dcl.spec.auto]).
A type-specifier of the form typename nested-name-specifier template-name is a placeholder for a deduced class type ([dcl.type.class.deduct]).
The nested-name-specifier, if any, shall be non-dependent and the template-name shall name a deducible template.
A deducible template is either a class template or is an alias template whose defining-type-id is of the form
typename nested-name-specifier template simple-template-id
where the nested-name-specifier (if any) is non-dependent and the template-name of the simple-template-id names a deducible template.
[Note 1:
An injected-class-name is never interpreted as a template-name in contexts where class template argument deduction would be performed ([temp.local]).
— end note]
The other simple-type-specifiers specify either a previously-declared type, a type determined from an expression, or one of the fundamental types ([basic.fundamental]).
Table 14 summarizes the valid combinations of simple-type-specifiers and the types they specify.
Table 14: simple-type-specifiers and the types they specify [tab:dcl.type.simple]
Specifier(s)
Type
the type named
the type as defined in [temp.names]
the type as defined in [dcl.type.decltype]
the type as defined in [dcl.spec.auto]
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”
3
#
When multiple simple-type-specifiers are allowed, they can be freely intermixed with other decl-specifiers in any order.
[Note 2:
It is implementation-defined whether objects of char type are represented as signed or unsigned quantities.
The signed specifier forces char objects to be signed; it is redundant in other contexts.
— end note]

9.2.9.4 Elaborated type specifiers [dcl.type.elab]

1
#
An attribute-specifier-seq shall not appear in an elaborated-type-specifier unless the latter is the sole constituent of a declaration.
If an elaborated-type-specifier is the sole constituent of a declaration, the declaration is ill-formed unless it is an explicit specialization ([temp.expl.spec]), an explicit instantiation ([temp.explicit]) or it has one of the following forms:
class-key attribute-specifier-seq identifier ;
friend class-key :: identifier ;
friend class-key :: simple-template-id ;
friend class-key nested-name-specifier identifier ;
friend class-key nested-name-specifier template simple-template-id ;
In the first case, the attribute-specifier-seq, if any, appertains to the class being declared; the attributes in the attribute-specifier-seq are thereafter considered attributes of the class whenever it is named.
2
#
[Note 1:
[basic.lookup.elab] describes how name lookup proceeds for the identifier in an elaborated-type-specifier.
— end note]
If the identifier or simple-template-id resolves to a class-name or enum-name, the elaborated-type-specifier introduces it into the declaration the same way a simple-type-specifier introduces its type-name ([dcl.type.simple]).
[Note 2:
This implies that, within a class template with a template type-parameter T, the declaration friend class T; is ill-formed.
However, the similar declaration friend T; is allowed ([class.friend]).
— end note]
3
#
The class-key or enum keyword present in the elaborated-type-specifier shall agree in kind with the declaration to which the name in the elaborated-type-specifier refers.
This rule also applies to the form of elaborated-type-specifier that declares a class-name or friend class since it can be construed as referring to the definition of the class.
Thus, in any elaborated-type-specifier, the enum keyword shall be used to refer to an enumeration ([dcl.enum]), the union class-key shall be used to refer to a union ([class.union]), and either the class or struct class-key shall be used to refer to a non-union class ([class.pre]).
[Example 1: enum class E { a, b }; enum E x = E::a; // OK struct S { } s; class S* p = &s; // OK — end example]

9.2.9.5 Decltype specifiers [dcl.type.decltype]

1
#
For an expression E, the type denoted by decltype(E) is defined as follows:
The operand of the decltype specifier is an unevaluated operand.
[Example 1: 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]
[Note 1:
The rules for determining types involving decltype(auto) are specified in [dcl.spec.auto].
— end note]
2
#
If the operand of a decltype-specifier is a prvalue and is not a (possibly parenthesized) immediate invocation ([expr.const]), the temporary materialization conversion is not applied ([conv.rval]) and no result object is provided for the prvalue.
The type of the prvalue may be incomplete or an abstract class type.
[Note 2:
As a result, storage is not allocated for the prvalue and it is not destroyed.
Thus, a class type is not instantiated as a result of being the type of a function call in this context.
In this context, the common purpose of writing the expression is merely to refer to its type.
In that sense, a decltype-specifier is analogous to a use of a typedef-name, so the usual reasons for requiring a complete type do not apply.
In particular, it is not necessary to allocate storage for a temporary object or to enforce the semantic constraints associated with invoking the type's destructor.
— end note]
[Note 3:
Unlike the preceding rule, parentheses have no special meaning in this context.
— end note]
[Example 2: 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]

9.2.9.6 Placeholder type specifiers [dcl.spec.auto]

9.2.9.6.1 General [dcl.spec.auto.general]

1
#
A placeholder-type-specifier designates a placeholder type that will be replaced later by deduction from an initializer.
2
#
A placeholder-type-specifier of the form type-constraint auto can be used as a decl-specifier of the decl-specifier-seq of a parameter-declaration of a function declaration or lambda-expression and, if it is not the auto type-specifier introducing a trailing-return-type (see below), is a generic parameter type placeholder of the function declaration or lambda-expression.
[Note 1:
Having a generic parameter type placeholder signifies that the function is an abbreviated function template ([dcl.fct]) or the lambda is a generic lambda ([expr.prim.lambda]).
— end note]
3
#
The placeholder type can appear with a function declarator in the decl-specifier-seq, type-specifier-seq, conversion-function-id, or trailing-return-type, in any context where such a declarator is valid.
If the function declarator includes a trailing-return-type ([dcl.fct]), that trailing-return-type specifies the declared return type of the function.
Otherwise, the function declarator shall declare a function.
If the declared return type of the function contains a placeholder type, the return type of the function is deduced from non-discarded return statements, if any, in the body of the function ([stmt.if]).
4
#
The type of a variable declared using a placeholder type is deduced from its initializer.
This use is allowed in an initializing declaration ([dcl.init]) of a variable.
The placeholder type shall appear as one of the decl-specifiers in the decl-specifier-seq and the decl-specifier-seq shall be followed by one or more declarators, each of which shall be followed by a non-empty initializer.
If the initializer is a parenthesized expression-list, the expression-list shall be a single assignment-expression.
[Example 1: 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]
The auto type-specifier can also be used to introduce a structured binding declaration ([dcl.struct.bind]).
6
#
A program that uses a placeholder type in a context not explicitly allowed in [dcl.spec.auto] is ill-formed.
7
#
If the init-declarator-list contains more than one init-declarator, they shall all form declarations of variables.
The type of each declared variable is determined by placeholder type deduction, and if the type that replaces the placeholder type is not the same in each deduction, the program is ill-formed.
[Example 2: auto x = 5, *y = &x; // OK: auto is int auto a = 5, b = { 1, 2 }; // error: different types for auto — end example]
8
#
If a function with a declared return type that contains a placeholder type has multiple non-discarded return statements, the return type is deduced for each such return statement.
If the type deduced is not the same in each deduction, the program is ill-formed.
9
#
If a function with a declared return type that uses a placeholder type has no non-discarded return statements, the return type is deduced as though from a return statement with no operand at the closing brace of the function body.
[Example 3: auto f() { } // OK, return type is void auto* g() { } // error: cannot deduce auto* from void() — end example]
10
#
An exported function with a declared return type that uses a placeholder type shall be defined in the translation unit containing its exported declaration, outside the private-module-fragment (if any).
[Note 2:
The deduced return type cannot have a name with internal linkage ([basic.link]).
— end note]
11
#
If the name of an entity with an undeduced placeholder type appears in an expression, the program is ill-formed.
Once a non-discarded return statement has been seen in a function, however, the return type deduced from that statement can be used in the rest of the function, including in other return statements.
[Example 4: 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]
12
#
Return type deduction for a templated entity that is a function or function template with a placeholder in its declared type occurs when the definition is instantiated even if the function body contains a return statement with a non-type-dependent operand.
[Note 3:
Therefore, any use of a specialization of the function template will cause an implicit instantiation.
Any errors that arise from this instantiation are not in the immediate context of the function type and can result in the program being ill-formed ([temp.deduct]).
— end note]
[Example 5: 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]
13
#
Redeclarations or specializations of a function or function template with a declared return type that uses a placeholder type shall also use that placeholder, not a deduced type.
Similarly, redeclarations or specializations of a function or function template with a declared return type that does not use a placeholder type shall not use a placeholder.
[Example 6: 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]
14
#
A function declared with a return type that uses a placeholder type shall not be virtual.
15
#
A function declared with a return type that uses a placeholder type shall not be a coroutine ([dcl.fct.def.coroutine]).
16
#
An explicit instantiation declaration does not cause the instantiation of an entity declared using a placeholder type, but it also does not prevent that entity from being instantiated as needed to determine its type.
[Example 7: 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]

9.2.9.6.2 Placeholder type deduction [dcl.type.auto.deduct]

1
#
Placeholder type deduction is the process by which a type containing a placeholder type is replaced by a deduced type.
2
#
A type T containing a placeholder type, and a corresponding initializer E, are determined as follows:
  • (2.1)
    for a non-discarded return statement that occurs in a function declared with a return type that contains a placeholder type, T is the declared return type and E is the operand of the return statement.
    If the return statement has no operand, then E is void();
  • (2.2)
    for a variable declared with a type that contains a placeholder type, T is the declared type of the variable and E is the initializer.
    If the initialization is direct-list-initialization, the initializer shall be a braced-init-list containing only a single assignment-expression and E is the assignment-expression;
  • (2.3)
    for a non-type template parameter declared with a type that contains a placeholder type, T is the declared type of the non-type template parameter and E is the corresponding template argument.
In the case of a return statement with no operand or with an operand of type void, T shall be either type-constraint decltype(auto) or cv type-constraint auto.
3
#
If the deduction is for a return statement and E is a braced-init-list ([dcl.init.list]), the program is ill-formed.
4
#
If the placeholder-type-specifier is of the form type-constraint auto, the deduced type replacing T is determined using the rules for template argument deduction.
Obtain P from T by replacing the occurrences of type-constraint auto either with a new invented type template parameter U or, if the initialization is copy-list-initialization, with std​::​initializer_­list<U>.
Deduce a value for U using the rules of template argument deduction from a function call, where P is a function template parameter type and the corresponding argument is E.
If the deduction fails, the declaration is ill-formed.
Otherwise, is obtained by substituting the deduced U into P.
[Example 1: 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]
[Example 2: const auto &i = expr;
The type of i is the deduced type of the parameter u in the call f(expr) of the following invented function template: template <class U> void f(const U& u);
— end example]
5
#
If the placeholder-type-specifier is of the form type-constraint decltype(auto), T shall be the placeholder alone.
The type deduced for T is determined as described in [dcl.type.decltype], as though E had been the operand of the decltype.
[Example 3: 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]
6
#
For a placeholder-type-specifier with a type-constraint, the immediately-declared constraint ([temp.param]) of the type-constraint for the type deduced for the placeholder shall be satisfied.

9.2.9.7 Deduced class template specialization types [dcl.type.class.deduct]

1
#
If a placeholder for a deduced class type appears as a decl-specifier in the decl-specifier-seq of an initializing declaration ([dcl.init]) of a variable, the declared type of the variable shall be cv T, where T is the placeholder.
[Example 1: 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]
The placeholder is replaced by the return type of the function selected by overload resolution for class template deduction ([over.match.class.deduct]).
If the decl-specifier-seq is followed by an init-declarator-list or member-declarator-list containing more than one declarator, the type that replaces the placeholder shall be the same in each deduction.
2
#
A placeholder for a deduced class type can also be used in the type-specifier-seq in the new-type-id or type-id of a new-expression ([expr.new]), as the simple-type-specifier in an explicit type conversion (functional notation), or as the type-specifier in the parameter-declaration of a template-parameter ([temp.param]).
A placeholder for a deduced class type shall not appear in any other context.
3
#
[Example 2: 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]