Overview of data objects

A data object is a region of storage that contains a value or group of values. Each value can be accessed using its identifier or a more complex expression that refers to the object. In addition, each object has a unique data type. The data type of an object determines the storage allocation for that object and the interpretation of the values during subsequent access. It is also used in any type checking operations. Both the identifier and data type of an object are established in the object declaration.

C++ only begins An instance of a class type is commonly called a class object. The individual class members are also called objects.C++ only ends

Data types are often grouped into type categories that overlap, such as:
Fundamental types versus derived types
Fundamental data types are also known as "basic", "fundamental" or "built-in" to the language. These include integers, floating-point numbers, and characters. Derived types, also known as "compound" types in C++, are created from the set of basic types, and include arrays, pointers, structures, unions, enumerations. All C++ classes are considered compound types.
Built-in types versus user-defined types
Built-in data types include all of the fundamental types, plus types that refer to the addresses of basic types, such as arrays and pointers. User-defined types are created by the user from the set of basic types, in typedef, structure, union, and enumeration definitions. C++ classes are considered user-defined types.
Scalar types versus aggregate types
Scalar types represent a single data value, while aggregate types represent multiple values, of the same type or of different types. Scalars include the arithmetic types and pointers. Aggregate types include arrays, structures. C++ classes are considered aggregate types.
The following matrix lists the supported data types and their classification into fundamental, derived, scalar, and aggregate types.
Table 1. C/C++ data types
Data object Basic Compound
Scalar Aggregate
integer types + + +
floating-point types1 + + +
character types + +
Booleans + + +
void type +2 + +
pointers + + +
arrays + + +
structures + + +
unions + +
enumerations + + see note3  
C++ only classes + + +
  1. C only. Although complex floating-point types are represented internally as an array of two elements, they behave in the same way as real floating-pointing types in terms of alignment and arithmetic operations, and can therefore be considered scalar types.
  2. The void type is really an incomplete type, as discussed in Incomplete types. Nevertheless, the C++ standard defines it as a fundamental type.
  3. C only begins The C standard does not classify enumerations as either scalar or aggregate.C only ends C++ only The C++ standard classifies enumerations as scalars.C++ only ends

Incomplete types

The following are incomplete types:
  • The void type
  • Arrays of unknown size
  • Arrays of elements that are of incomplete type
  • Structure, union, or enumerations that have no definition
  • C++ only Pointers to class types that are declared but not defined
  • C++ only Classes that are declared but not defined

C only However, if an array size is specified by [*], indicating a variable length array, the size is considered as having been specified, and the array type is then considered a complete type. For more information, see Variable length arrays.

The following examples illustrate incomplete types:
      void *incomplete_ptr;
      struct dimension linear; /* no previous definition of dimension */

Compatible and composite types

Begin C only
In C, compatible types are defined as:
  • two types that can be used together without modification (as in an assignment expression)
  • two types that can be substituted one for the other without modification

A composite type is constructed from two compatible types. Determining the resultant composite type for two compatible types is similar to following the usual binary conversions of integral types when they are combined with some arithmetic operators.

Obviously, two types that are identical are compatible; their composite type is the same type. Less obvious are the rules governing type compatibility of non-identical types, user-defined types, type-qualified types, and so on. Type specifiers discusses compatibility for basic and user-defined types in C.

End C

C++ only A separate notion of type compatibility as distinct from being of the same type does not exist in C++. Generally speaking, type checking in C++ is stricter than in C: identical types are required in situations where C would only require compatible types.C++ only ends