In programming language theory

Programming language theory

Programming language theory is a branch of computer science that deals with the design, implementation, analysis, characterization, and classification of programming languages and their individual features. It falls within the discipline of computer science, both depending on and affecting...

, a reference type is a data type

Data type

In computer programming, a data type is a classification identifying one of various types of data, such as floating-point, integer, or Boolean, that determines the possible values for that type; the operations that can be done on values of that type; the meaning of the data; and the way values of...

 that can only be accessed by references

Reference (computer science)

In computer science, a reference is a value that enables a program to indirectly access a particular data item, such as a variable or a record, in the computer's memory or in some other storage device. The reference is said to refer to the data item, and accessing those data is called...

. Unlike objects of value type

Value type

In computer science, the term value type is commonly used to refer to one of two kinds of data types: Types of values or Types of objects with deep copy semantics.- Types of Values :...

s, objects of reference types cannot be directly embedded into composite objects

Composite type

In computer science, a composite data type is any data type which can be constructed in a program using its programming language's primitive data types and other composite types...

 and are always dynamically allocated. They are usually destroyed automatically

Garbage collection (computer science)

In computer science, garbage collection is a form of automatic memory management. The garbage collector, or just collector, attempts to reclaim garbage, or memory occupied by objects that are no longer in use by the program...

 after they become unreachable

Unreachable memory

In computer science, unreachable memory is a block of memory allocated dynamically where the program that allocated the memory no longer has any reachable pointer that refers to it. Similarly, an unreachable object is a dynamically allocated object that has no reachable reference to it...

.

For immutable object

Immutable object

In object-oriented and functional programming, an immutable object is an object whose state cannot be modified after it is created. This is in contrast to a mutable object, which can be modified after it is created...

s, the distinction between reference type to an immutable object type and value type is sometimes unclear, because a reference type variable to an immutable object behaves with the same semantics as a value type variable—for example, in both cases the "value" of the data the variable represents can only be changed by direct assignment to the variable (whereas for mutable objects, the data could be changed by modifying the object through another reference to the object).

Language and platform support

• C++
  C++
  C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...
  
   has no built-in support for reference types. However this behavior can be emulated by restricting access to a type's constructor
  Constructor (computer science)
  In object-oriented programming, a constructor in a class is a special type of subroutine called at the creation of an object. It prepares the new object for use, often accepting parameters which the constructor uses to set any member variables required when the object is first created...
  
  .
• The .NET Framework
  .NET Framework
  The .NET Framework is a software framework that runs primarily on Microsoft Windows. It includes a large library and supports several programming languages which allows language interoperability...
  
   makes a clear distinction between value and reference types, and allows creation of user-defined types for both kinds.
  • In C#, the struct keyword defines a value type, while the class keyword defines a reference type.
  • In C++/CLI
    C++/CLI
    C++/CLI is Microsoft's language specification intended to supersede Managed Extensions for C++. It is a complete revision that aims to simplify the older Managed C++ syntax . C++/CLI is standardized by Ecma as ECMA-372...
    
    , the keyword pairs value class and value struct define managed
    Managed code
    Managed code is a term coined by Microsoft to identify computer program code that requires and will only execute under the "management" of a Common Language Runtime virtual machine ....
    
     value types, while the ref class and ref struct pairs define managed reference types.
• On the Java platform, all composite and user-defined types are reference types. Only primitive type
  Primitive type
  In computer science, primitive data type is either of the following:* a basic type is a data type provided by a programming language as a basic building block...
  
  s are value types.
• Windows COM
  Component Object Model
  Component Object Model is a binary-interface standard for software componentry introduced by Microsoft in 1993. It is used to enable interprocess communication and dynamic object creation in a large range of programming languages...
  
   objects are reference-counted
  Reference counting
  In computer science, reference counting is a technique of storing the number of references, pointers, or handles to a resource such as an object, block of memory, disk space or other resource...
  
   reference types.
• In many scripting languages, including Python
  Python (programming language)
  Python is a general-purpose, high-level programming language whose design philosophy emphasizes code readability. Python claims to "[combine] remarkable power with very clear syntax", and its standard library is large and comprehensive...
  
  , Ruby
  Ruby (programming language)
  Ruby is a dynamic, reflective, general-purpose object-oriented programming language that combines syntax inspired by Perl with Smalltalk-like features. Ruby originated in Japan during the mid-1990s and was first developed and designed by Yukihiro "Matz" Matsumoto...
  
  , all types are reference types, including primitive types.