“Value type and reference type”的意思、由来-开放百科全书

In computer programming, data types can be divided into two categories: value types and reference types. A value of value type is the actual value. A value of reference type is a reference to another value.^[1]^[2]

Classification per language

Language

Value type

Reference type

C++^[3]

booleans, characters, integer numbers, floating-point numbers, classes (including strings, lists, maps, sets, stacks, queues), enumerations

references, pointers

Java^[4]

booleans, characters, integer numbers, floating-point numbers

arrays, classes (including immutable strings, lists, dictionaries, sets, stacks, queues, enumerations), interfaces, null pointer

C#^[5]

structures (including booleans, characters, integer numbers, floating-point numbers, fixed-point numbers, lists, dictionaries, sets, stacks, queues, optionals), enumerations

classes (including immutable strings, arrays, tuples, lists, dictionaries, sets, stacks, queues), interfaces, pointers

Swift^[6]^[7]

structures (including booleans, characters, integer numbers, floating-point numbers, fixed-point numbers, immutable strings, tuples, lists, dictionaries, sets, stacks, queues), enumerations (including optionals)

functions, classes, interfaces

Python^[8]

classes (including immutable booleans, immutable integer numbers, immutable floating-point numbers, immutable complex numbers, immutable strings, byte strings, immutable byte strings, immutable tuples, immutable ranges, immutable memory views, lists, dictionaries, sets, immutable sets, null pointer)

JavaScript^[9]

immutable booleans, immutable floating-point numbers, immutable symbols, immutable strings, undefined, prototypes (including lists, null pointer)

OCaml^[10]^[11]

immutable characters, immutable integer numbers, immutable floating-point numbers, immutable tuples, immutable enumerations (including immutable units, immutable booleans, immutable lists, immutable optionals), immutable exceptions, immutable formatting strings

arrays, immutable strings, byte strings, dictionaries (including pointers)

Boxing and unboxing

Programming languages that distinguish between value types and reference types typically offer a mechanism, called boxing, to wrap some or all of their value types in reference types. This permits the use of value types in contexts expecting reference types. The converse process (to unwrap the value type) is known as unboxing.

See also

References

1. ^{{cite book |last=Brown |first=Erik E. |date=2006 |title=Windows Forms in Action |url=https://books.google.fr/books?id=PrZQAAAAMAAJ&q=%22value+type%22+%22reference+type%22&dq=%22value+type%22+%22reference+type%22&hl=fr&sa=X&ved=0ahUKEwiamJGq0bncAhVPExoKHR21DQA4HhDoAQhZMAY |location=Shelter Island, New York |publisher=Manning |page=703 |isbn= 978-1-932-39465-8 |author-link= }}
2. ^{{cite book |last=Stephens |first=Rod |date=2014 |title=C# 5.0 Programmer's Reference |url=https://books.google.fr/books?id=DEM_AwAAQBAJ&pg=PA57&dq=%22value+type%22+%22reference+type%22&hl=fr&sa=X&ved=0ahUKEwiJyMCA1LncAhVhxYUKHQZECVI4ZBDoAQhCMAQ#v=onepage&q=%22value%20type%22%20%22reference%20type%22&f=false |location=Indianapolis, Indiana |publisher=John Wiley & Sons |page=57 |isbn=978-1-118-84728-2 |author-link= }}
3. ^http://en.cppreference.com/w/cpp/language/type
4. ^https://docs.oracle.com/javase/specs/jls/se10/html/jls-4.html#jls-4.3
5. ^https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/types
6. ^https://docs.swift.org/swift-book/LanguageGuide/ClassesAndStructures.html
7. ^https://docs.swift.org/swift-book/LanguageGuide/Closures.html
8. ^https://docs.python.org/3/library/stdtypes.html
9. ^https://www.ecma-international.org/ecma-262/#sec-ecmascript-language-types
10. ^https://caml.inria.fr/pub/docs/manual-ocaml/core.html
11. ^https://caml.inria.fr/pub/docs/oreilly-book/html/book-ora026.html