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++ 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 12. C/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
C++ only classes + + +
typedef types + +
  1. 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, Standard C++ defines it as a fundamental type.
  3. C onlyThe C standard does not classify enumerations as either scalar or aggregate. C++ Standard C++ classifies enumerations as scalars.

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Incomplete types

The following are incomplete types:

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 */

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Compatible types

C only

In C, compatible types are defined as:

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.

End of C only
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.

End of C++ only

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