Type

Objects, references, functions including function template specializations, and expressions have a property called type, which both restricts the operations that are permitted for those entities and provides semantic meaning to the otherwise generic sequences of bits.

Type classification

The C++ type system consists of the following types:

For every type other than reference and function, the type system supports three additional cv-qualified versions of that type (const, volatile, and const volatile).

Types are grouped in various categories based on their properties:

Type naming

A name can be declared to refer to a type by means of:

Types that do not have names often need to be referred to in C++ programs; the syntax for that is known as type-id. The syntax of the type-id that names type T is exactly the syntax of a declaration of a variable or function of type T, with the identifier omitted, except that decl-specifier-seq of the declaration grammar is constrained to type-specifier-seq. New types may be declared.

int* p;               // declration of a pointer to int
static_cast<int*>(p); // type-id is "int*"
 
int a[3];   // declaration of an array of 3 int
new int[3]; // type-id is "int[3]"
 
int (*(*x[2])())[3];      // declaration of an array of 2 pointers to functions
                          // returning pointer to array of 3 int
new (int (*(*[2])())[3]); // type-id is "int (*(*[2])())[3]"
 
void f(int); // declaration of a function that takes int and returns void
std::function<void(int)> x = f; // type template parameter is a type-id "void(int)"
 
std::vector<int> v;       // declaration of a vector of int
sizeof(std::vector<int>); // type-id is "std::vector<int>"
 
struct { int x; } b;       // creates a new type and declares an object b of that type
sizeof(struct{ int x; });  // error: cannot define new types in a sizeof expression
using t = struct { int x; }; // creates a new type and declares t as an alias of that type
 
sizeof(static int); // error: storage class specifiers not part of type-specifier-seq
std::function<inline void(int)> f; // error: function specifiers aren't either

The declarator part of the declaration grammar with the name removed is referred to as abstract-declarator.

Type-id may be used in the following situations:

Type-id can be used with some modifications in the following situations:

  • in the parameter list of a function (when the parameter name is omitted), type-id uses decl-specifier-seq instead of type-specifier-seq (in particular, some storage class specifiers are allowed);
  • in the name of a user-defined conversion function, the abstract declarator cannot include function or array operators.

Elaborated type specifier

Elaborated type specifiers may be used to refer to a previously-declared class name (class, struct, or union) or to a previously-declared enum name even if the name was hidden by a non-type declaration. They may also be used to declare new class names.

See elaborated type specifier for details.

Static type

The type of an expression that results from the compile-time analysis of the program is known as the static type of the expression. The static type does not change while the program is executing.

Dynamic type

If some glvalue expression refers to a polymorphic object, the type of its most derived object is known as the dynamic type.

// given
struct B { virtual ~B() {} }; // polymorphic type
struct D: B {}; // polymorphic type
D d; // most-derived object
B* ptr = &d;
// the static type of (*ptr) is B
// the dynamic type of (*ptr) is D

For prvalue expressions, the dynamic type is always the same as the static type.

Incomplete type

The following types are incomplete types:

  • the type void (possibly cv-qualified);
  • class type that has been declared (e.g. by forward declaration) but not defined;
  • array of unknown bound;
  • array of elements of incomplete type;
  • enumeration type from the point of declaration until its underlying type is determined.

Any of the following contexts requires class T to be complete:

(In general, when the size and layout of T must be known.).

If any of these situations occur in a translation unit, the definition of the type must appear in the same translation unit. Otherwise, it is not required.

See also

C documentation for Type
doc_CPP
2016-10-11 10:08:49
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