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.

An instance of a class type is commonly called a class object. The individual class members are also called objects. The set of all member objects comprises a class object.

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 Standard C++, are created from the set of basic types, and include arrays, pointers, structures, unions, and 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 and 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 9. C/C++ data types

Data object	Basic	Compound	Built- in	User- defined	Scalar	Aggregate
integer types	+		+		+
floating-point types	+		+		+
character types			+		+
Booleans	+		+		+
void type	+1		+		+
pointers		+	+		+
arrays		+	+			+
structures		+		+		+
unions		+		+
enumerations		+		+	see note2
classes		+		+		+
typedef types		+		+