C++/CLI学习入门(九):类的属性
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。
