成员变量

在面向对象程序设计中，成员变量（有时称为成员字段）是与特定对象相关联的变量，并且特定对象的所有方法（成员函数）均能访问該變量。^[1]

例子

C++

class Foo {  int bar; // Member variable  public:  void setBar(const int newBar) {   bar = newBar;  } }; int main () {  Foo rect; // Local variable  return 0; } 

Java

public class Program { public static void main(String[] args) { // This is a local variable. Its lifespan // is determined by lexical scope. Foo foo; } } public class Foo { /* This is a member variable - a new instance  of this variable will be created for each   new instance of Foo. The lifespan of this  variable is equal to the lifespan of "this"  instance of Foo  */ int bar; } 

Python

class Foo: def __init__(self): self._bar = 0 @property def bar(self): return self._bar @bar.setter def bar(self, new_bar): self._bar = new_bar f = Foo() f.bar = 100 print(f.bar) 

Common Lisp

(defclass foo () (bar)) (defvar f (make-instance 'foo)) (setf (slot-value f 'bar) 100) (print (slot-value f 'bar)) 

Ruby

/*  Ruby has three member variable types: class, class instance, and instance. */ class Dog # The class variable is defined within the class body with two at-signs # and describes data about all Dogs *and* their derived Dog breeds (if any) @@sniffs = true end mutt = Dog.new mutt.class.sniffs #=> true class Poodle < Dog # The "class instance variable" is defined within the class body with a single at-sign # and describes data about only the Poodle class. It makes no claim about its parent class # or any possible subclass derived from Poodle @sheds = false # When a new Poodle instance is created, by default it is untrained. The 'trained' variable # is local to the initialize method and is used to set the instance variable @trained # An instance variable is defined within an instance method and is a member of the Poodle instance def initialize(trained = false) @trained = trained end def has_manners? @trained end end p = Poodle.new p.class.sheds #=> false p.has_manners? #=> false 

PHP

<?php class Example { /**  * Example instance member variable.  *  * Member variables may be public, protected or private.  *  * @var int  */ public int $foo; /**  * Example static member variable.  *  * @var bool  */ protected static int $bar; /**  * Example constructor method.  *  * @param int $foo  */ public function __construct(int $foo) { // Sets foo. $this->foo = $foo; } } // Create a new Example object. // Set the "foo" member variable to 5. $example = new Example(5); // Overwrite the "foo" member variable to 10. $example->foo = 10; // Prints 10. echo $example->foo; 

Lua

--region example --- @class example_c --- @field foo number Example "member variable". local example_c = {} local example_mt = {__index = example_c} --- Creates an object from example. --- @return example_c function example_c.new(foo) -- The first table argument is our object's member variables. -- In a Lua object is a metatable and its member variables are table key-value pairs. return setmetatable({ foo = foo }, example_mt) end --endregion -- Create an example object. -- Set the "foo" member variable to 5. local example = example_c.new(5) -- Overwrite the "foo" member variable to 10. example.foo = 10 -- Prints 10. print(example.foo) 

参考文献

1. Richard G. Baldwin. . Richard G Baldwin Programming Tutorials. 1999-03-10 [2011-08-12]. （原始内容存档于2022-04-12）. A member variable is a member of a class (class variable) or a member of an object instantiated from that class (instance variable). It must be declared within a class, but not within the body of a method of the class.