【C/C++开发】运算符重载二
C++中预定义的运算符的操作对象只能是基本数据类型。但实际上,对于许多用户自定义类型(例如类),也需要类似的运算操作。这时就必须在C++中重新定义这些运算符,赋予已有运算符新的功能,使它能够用于特定类型执行特定的操作。运算符重载的实质是函数重载,它提供了C++的可扩展性,也是C++最吸引人的特性之一。
运算符重载是通过创建运算符函数实现的,运算符函数定义了重载的运算符将要进行的操作。运算符函数的定义与其他函数的定义类似,惟一的区别是运算符函数的函数名是由关键字operator和其后要重载的运算符符号构成的。运算符函数定义的一般格式如下:
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运算符重载时要遵循以下规则:
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运算符函数重载一般有两种形式:重载为类的成员函数和重载为类的非成员函数。非成员函数通常是友元。(可以把一个运算符作为一个非成员、非友元函数重载。但是,这样的运算符函数访问类的私有和保护成员时,必须使用类的公有接口中提供的设置数据和读取数据的函数,调用这些函数时会降低性能。可以内联这些函数以提高性能。)
成员函数运算符
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当运算符重载为类的成员函数时,函数的参数个数比原来的操作数要少一个(后置单目运算符除外),这是因为成员函数用this指针隐式地访问了类的一个对象,它充当了运算符函数最左边的操作数。因此:
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友元函数运算符
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当运算符重载为类的友元函数时,由于没有隐含的this指针,因此操作数的个数没有变化,所有的操作数都必须通过函数的形参进行传递,函数的参数与操作数自左至右一一对应。
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两种重载形式的比较
在多数情况下,将运算符重载为类的成员函数和类的友元函数都是可以的。但成员函数运算符与友元函数运算符也具有各自的一些特点:
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