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.

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 are created from the set of basic types, and include arrays, pointers, structures, unions, enumerations, and vectors.
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.
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, and vectors.

The following matrix lists the supported data types and their classification into fundamental, derived, scalar, and aggregate types.

Table 1. C data types
Data object	Basic	Compound	Built- in	User- defined	Scalar	Aggregate
integer types	+		+		+
floating-point types¹	+		+		+
character types			+		+
Booleans	+		+		+
void type	+²		+		+
pointers		+	+		+
arrays		+	+			+
structures		+		+		+
unions		+		+
enumerations		+		+	see note³
vector types			+			+

Note:

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.
The void type is really an incomplete type, as discussed in Incomplete types.
The C standard does not classify enumerations as either scalar or aggregate.

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

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

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

When two compatible types are combined, the result is a composite type. 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.