C++/CLI学习入门(九):类的属性（转…

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属性是C++/CLI的类
成员，它类似于成员变量，但实际上不是。其主要区别在于，字段名引用了某个存储单元，而属性名则是调用某个函数。属性拥有访问属性的set()和
get()函数。当我们使用属性名时，实际上在调用该函数的get()或set()函数。如果一个属性仅提供了get()函数，则它是只读属性；如果一个
属性仅提供set()函数，则它是只写属性。

类可以有2种不同的属性：标量属性
和索引属性。标量属性是指通过属性名来访问的单值；索引属性是利用属性名加方框号来访问的一组值。如
String类，其Length属性为标量属性，用object->Length来访问其长度，且Length是个只读属性。String还包含了
索引属性，可以用object[idx]来访问字符串中第idx+1个字符。

属性可以与类的实例（类对象）相关，此时属性被称为实例属性，如String类的Length属性；如果用static修饰符指定属性，则属性为类属性，所有该类得实例都具有相同的属性值。

一、标量属性

标量属性是单值，用property关键字可定义标量属性，还需要定义其get()和set()函数，如下例所示

value class Height

{

private:

// Records the height in feet and inches

int feet;

int inches;

literal int inchesPerFoot = 12;

literal double inchesToMeter = 2.54/100;

public:

// Create a height from inches value

Height(int ins)

{

feet = ins/inchesPerFoot;

inches = ins%inchesPerFoot;

}

// Create a height from feet and inches

Height(int ft, int ins) : feet(ft), inches(ins) {};

// The height in meters as a property

property double meters

{

double get()

{

return inchesToMeters * (feet*inchesPerFoot + inches);

}

}

// Create a string representation of the object

virtual String^ ToString() overrides

{

return feet + L" feet " + inches + L" inches";

}

};

上面的例子定义了一个merters的属性，下面是属性的用法

Height ht = Height(6, 8);

Console::WriteLine(L"The height is {0} meters",
ht->meters);

属性不一定要定义成内联函数，也可以在.cpp中外部定义它，如在上例的定义中仅保留get()函数声明

property double meters

{

double get();

}

函数定义在.cpp中时，需要加类名和函数名的限定（但不需要返回值？），方法如下：

Height::meters::get()

{

return inchesToMeters*(feet*inchesPerFoot+inches);

}

如果定义一个属性时，不提供get()和set()函数定义，这种属性被称为平凡标量属性。对于此类属性，编译器将提供一个默认的get()和set()实现，如下例所示：

value class Point

{

public:

property int x;

proterty int y;

virtual String^ ToString() overrides

{

return L"("+x+L","+y+")"; // Result is (x,y)

}

};

下面是一个完整的例子，说明了标量属性的声明及其使用方法

- - - - - - - - - - - - - - - - <<==
华丽的分割线 ::开始==>>
[Ex7_16.cpp] - - - - - - - - - - - - - - - - - - - - - - - - - - -
- - - - - - - - - - - - - - - -// Ex7_16.cpp : main project
file.

#include "stdafx.h"

using namespace System;

// Class defining a person's height

value class Height

{

private:

// Record the height in feet and inches

int feet;

int inches;

literal int inchesPerFoot = 12;

literal double inchesToMeters = 2.54/100;

public:

// Create a height from inches value

Height(int ins)

{

feet = ins/inchesPerFoot;

inches = ins%inchesPerFoot;

}

// Create a height from feet and inches

Height(int ft, int ins) : feet(ft), inches(ins) {};

// The height in meters

property double meters

{

double get()

{

return inchesToMeters*(feet*inchesPerFoot+inches);

}

}

// Create a string representation of the object

virtual String^ ToString() override

{

return feet + L" feet " + inches + L" inches";

}

};

// Class defining a person's weight

value class Weight

{

private:

int lbs;

int oz;

literal int ouncesPerPound = 16;

literal double lbsToKg = 1.0/2.2;

public:

Weight(int pounds, int ounces)

{

lbs = pounds;

oz = ounces;

}

property int pounds

{

int get() { return lbs; }

void set(int value) { lbs = value; }

}

property int ounces

{

int get() { return oz; }

void set(int value) { oz = value; }

}

property double kilograms

{

double get() { return lbsToKg*(lbs+oz/ouncesPerPound); }

}

virtual String^ ToString() override

{

return lbs + L" pounds " + oz + L" ounces";

}

};

ref class Person

{

private:

Height ht;

Weight wt;

public:

property String^ Name;

Person(String^ name, Height h, Weight w) : ht(h), wt(w)

{

Name = name;

}

Height getHeight() { return ht; }

Weight getWeight() { return wt; }

};

int main(array<System::String ^>
^args)

{

Weight hisWeight = Weight(185, 7);

Height hisHeight = Height(6, 3);

Person^ him = gcnew Person(L"Fred", hisHeight, hisWeight);

Weight herWeight = Weight(105, 3);

Height herHeight = Height(5, 2);

Person^ her = gcnew Person(L"Freda", herHeight, herWeight);

Console::WriteLine(L"She is {0}", her->Name);

Console::WriteLine(L"Her weight is {0:F2} kilograms.",
her->getWeight().kilograms);

Console::WriteLine(L"Her height is {0} which is {1:F2} meters.",
her->getHeight(),

her->getHeight().meters);

Console::WriteLine(L"He is {0}", him->Name);

Console::WriteLine(L"His weight is {0}",
him->getWeight());

Console::WriteLine(L"His height is {0} which is {1:F2} meters.",
him->getHeight(),

him->getHeight().meters);

return 0;

}- - - - - - - - - - - - - - - - <<==
华丽的分割线 ::结束==>>
[Ex7_16.cpp] - - - - - - - - - - - - - - - - - - - - - - - - - - -
- - - - - - - - - - - - - - - -输出为She is Freda

Her weight is 47.73 kilograms.

Her height is 5 feet 2 inches which is 1.57 meters.

He is Fred

His weight is 185 pounds 7 ounces

His height is 6 feet 3 inches which is 1.91
meters.二、索引属性

索引属性是类的一组属性值，其访问方法同数组元素那样，在方括号内加索引值来访问。如果在方括号前面的是类对象的名称，则该索引属性被称为默认索引属性
（如String^
obj可以用obj[idx]来访问字符串中第idx+1个字符），如果用属性名[idx]来访问索引属性值，则称为有名索引属性。下面的代码在类
Name中定义了一个默认索引属性，

ref class Name

{

private:

array<String^>^ Names;

public:

Name(...array<String^>^ names) :
Names(names) {}

// Indexed property to return any name

property String^ default[int]

{

// Retrieve indexed property value

String^ get(int index)

{

if(index >= Names->Length)

throw gcnew Exception(L"Index out of range");

return Names[index];

}

}

};

在上面的例子中，如果将default换成别的名字，则该属性就成为一个有名索引属性。在定义索引属性时，方括号内用int指定了索引的数据类型为int
型，它也可以是别的数据类型。访问索引属性的get()函数的形参其数据类型必须与属性名后方括号内类型相同；set()函数必须有2个形参，第一个指定
索引，第二个指定属性元素的值。

下面是一个完整的例子，说明了索引属性的定义与使用方法。

- - - - - - - - - - - - - - - - <<==
华丽的分割线 ::开始==>>
[Ex7_17.cpp] - - - - - - - - - - - - - - - - - - - - - - - - - - -
- - - - - - - - - - - - - - - -// Ex7_17.cpp : main project
file.

#include "stdafx.h"

using namespace System;

ref class Name

{

private:

array<String ^>^ Names;

public:

Name(...array<String ^>^ names) :
Names(names) {}

// Scalar property specifying number of names

property int NameCount

{

int get() { return Names->Length; }

}

// Indexed property to return names

property String^ default[int]

{

String ^ get(int index)

{

if(index >= Names->Length)

throw gcnew Exception(L"Index out of range");

return Names[index];

}

}

};

int main(array<System::String ^>
^args)

{

Name^ myName = gcnew Name(L"Ebenezer", L"Isaiah", L"Ezra",
L"Inigo", L"Whelkwhistle");

// List the names

for(int i=0; i<myName->NameCount;
i++)

Console::WriteLine(L"Name {0} is {1}", i+1, myName[i]);

return 0;

}- - - - - - - - - - - - - - - - <<==
华丽的分割线 ::结束==>>
[Ex7_17.cpp] - - - - - - - - - - - - - - - - - - - - - - - - - - -
- - - - - - - - - - - - - - - -输出为Name 1 is Ebenezer

Name 2 is Isaiah

Name 3 is Ezra

Name 4 is Inigo

Name 5 is Whelkwhistle

索引属性的索引也可以不是整型，甚至可以不是数字，下面的例子定义了一个商店类，其属性Opening指定了商店的开门时间，访问该属性的索引有2个参数，如下面的例子所示

enum class Day { Monday, Tuesday, Wednesday, Thursday, Friday,
Saturday, Sunday };

// class defining a shop

ref class Shop

{

public:

property String^ Opening[Day, String^] // Opening times

{

String ^ get(Day day, String^ AmOrPm)

{

switch(day)

{

case Day::Saturday:

if(AmOrPm == L"am")

return L"9:00";

else

return L"14:30";

break;

case Day::Sunday:

return L"closed";

break;

default:

if(AmOrPm == L"am")

return L"9:30";

else

return L"14:00";

break;

}

}

}

};使用该类的方法如下Shop^ shop = gcnew Shop;

Console::WriteLine(shop->Opening(Day::Saturday,
L"pm");三、静态属性

静态属性为类的所有实例共有，类似于类的静态成员变量。通过在属性定义前添加修饰符static来定义，如下面的例子所示

value class Length

{

public:

static property String ^ Units

{

String ^ get() { return L"feet and inches"; }

}

};

无论是否创建类实例，静态属性都存在。如果已经定义了类实例，则可以用实例名.属性名来访问静态属性。对于上面的例子如果已经定义了一个类对象len，则可以如此访问其静态属性：

Console::WriteLine(L"Class units are {0}.", len.Units);

?注意：在定义了属性之后，对应的get_属性名和set_属性名自动成为系统保留名称，不能为其它目的而使用他们。如果定义了默认索引属性，则set_Item和get_Item也成为系统保留名称，不可被使用。