属性 (编程)
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(重定向自点表示法)
属性(Property),在一些面向对象编程语言中,是类的特殊成员,功能上居于字段(或数据成员)与方法之间。可读可写属性的语法类似于字段,但属性的读写操作通常被编译为getter与setter方法调用。属性使用类似于字段的读写语法,比普通的方法调用的语法形式更易于读写操作;但属性读写编译为内部的方法调用,则可以支持数据确认、主动修改或实现只读字段等功能。
语言支持
支持属性的编程语言有:ActionScript 3、C#、D、Delphi/Free Pascal、eC、F#、Kotlin、JavaScript、Objective-C 2.0、Python、Scala、Swift、Lua、Visual Basic。
一些面向对象语言,如Java与C++,不支持属性,而要求编程者写一对accessor与mutator方法。 C++可以通过运算符重载来模拟属性。
不同的语法
一些语言用点表示,另一些语言用方括号表示,来访问属性。
点表示法
如JavaScript:
document.createElement('pre');
方括号表示法
JavaScript也可以用方括号来访问属性:
document['createElement']('pre');
具体语言实现舉例
C#
class Pen
{
private int color; // private field
// public property
public int Color
{
get
{
return this.color;
}
set
{
if (value > 0) {
this.color = value;
}
}
}
}
// accessing:
Pen pen = new Pen();
int color_tmp = 0;
// ...
pen.Color = 17;
color_tmp = pen.Color;
// ...
pen.Color = ~pen.Color; // bitwise complement ...
// another silly example:
pen.Color += 1; // a lot clearer than "pen.set_Color(pen.get_Color() + 1)"!
高版本的C#支持编译器自动实现属性。
class Shape
{
public Int32 Height { get; set; }
public Int32 Width { get; private set; }
}
C++
C++有多种方法模拟属性实现。
#include <iostream>
template <typename T> class property {
T value;
public:
T & operator = (const T &i) {
return value = i;
}
// This template class member function template serves the purpose to make
// typing more strict. Assignment to this is only possible with exact identical types.
// The reason why it will cause an error is temporary variable created while implicit type conversion in reference initialization.
template <typename T2> T2 & operator = (const T2 &i) {
T2 &guard = value;
throw guard; // Never reached.
}
// Implicit conversion back to T.
operator T const & () const {
return value;
}
};
struct Foo {
// Properties using unnamed classes.
class {
int value;
public:
int & operator = (const int &i) { return value = i; }
operator int () const { return value; }
} alpha;
class {
float value;
public:
float & operator = (const float &f) { return value = f; }
operator float () const { return value; }
} bravo;
};
struct Bar {
// Using the property<>-template.
property <bool> alpha;
property <unsigned int> bravo;
};
int main () {
Foo foo;
foo.alpha = 5;
foo.bravo = 5.132f;
Bar bar;
bar.alpha = true;
bar.bravo = true; // This line will yield a compile time error
// due to the guard template member function.
::std::cout << foo.alpha << ", "
<< foo.bravo << ", "
<< bar.alpha << ", "
<< bar.bravo
<< ::std::endl;
return 0;
}
C++, Microsoft & C++Builder的方言语法
例子来自MSDN documentation page (页面存档备份,存于互联网档案馆):
// declspec_property.cpp
struct S
{
int i;
void putprop(int j)
{
i = j;
}
int getprop()
{
return i;
}
__declspec(property(get = getprop, put = putprop)) int the_prop;
};
int main()
{
S s;
s.the_prop = 5;
return s.the_prop;
}
JavaScript
function Pen() {
this._color = 0;
}
// Add the property to the Pen type itself, can also
// be set on the instance individually
Object.defineProperties(Pen.prototype, {
color: {
get: function () {
return this._color;
},
set: function (value) {
this._color = value;
}
}
});
var pen = new Pen();
pen.color = ~pen.color; // bitwise complement
pen.color += 1; // Add one
Python
Python 2.2开始的new-style classes (即派生自object
的类)支持属性。见the relevant section of the tutorial Unifying types and classes in Python 2.2 (页面存档备份,存于互联网档案馆)。Python 2.6支持了定义属性的装饰器语法。
class Pen:
def __init__(self) -> None:
self._color = 0 # "private" variable
@property
def color(self):
return self._color
@color.setter
def color(self, color):
self._color = color
pen = Pen()
# Accessing:
pen.color = ~pen.color # Bitwise complement ...