【属性 (编程)】
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');
==具体语言实现举例==
=== 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
template  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  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  alpha;
property  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
==== C++, Microsoft & C++Builder的方言语法 ====
例子来自MSDN  documentation page date=20181003115649 ：
// 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 date=20201130055420 。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 ...