为什么运算子>> (或<<)重载函数需要接收i \ ostream引用吗? [英] Why does operator>> (or <<) overloading function need to receive an i\ostream reference?

问题描述

在cplusplus.com上，我看到ostream类的成员函数operator<<看起来像这样:

From cplusplus.com, I saw that ostream class's member function operator<< looks like this:

ostream& operator<< (bool val);   ostream& operator<< (int val);   

....依此类推.

这确实是有道理的，因为当您使用像cout<<x这样的Cout对象时，您会激活ostream& operator<< (int val)函数，因此实际上是在Cout对象上使用了<<运算符.这与其他所有运算符非常相似，并将int变量发送给函数.当我要流式传输自己的对象时，有什么区别?究竟发生了什么?为什么语法突然是ostream& operator<< (**ostream &os**, object ob)? 为什么需要添加ostream var?我仍在使用cout<<ob，所以不是ostream& operator<< (object obj)吗?我所通过的只是我的对象. cout对象已经准备就绪.

It does make sense because when you use the Cout object like cout<<x you activate the ostream& operator<< (int val) function, so you actually use the << operator on Cout object. This is very much like every other operator and sends the int variable to the function. What is the difference and what exactly happens when I want to stream an object of my own? Why does the syntax is suddenly ostream& operator<< (**ostream &os**, object ob)? Why do I need to add the ostream var? I am still using cout<<ob so whay isnt it just ostream& operator<< (object obj)? All I pass is my object. The cout object is allready there.

推荐答案

operator<<通常定义为自由函数；即不是成员函数.由于它是重载的二进制运算符，这意味着它首先获得其左参数，然后获得其右参数.

operator<< is generally defined as a free function; that is, not a member function. Since it is an overloaded binary operator, that means it get's its left argument first and its right argument second.

operator<<传统上会返回对其左参数的引用，以启用惯用的输出链.

The operator<< traditionally returns a reference to its left argument to enable the idiomatic chain of output.

为了使读者容易理解，我倾向于使用lhs和rhs缩写来定义我的operator重载；对于某些类型的T，operator<<看起来与此类似.

To make it obvious to the reader, I tend to define my operator overloads using the lhs and rhs abbreviations; an operator<< would look similar to this, for some type T.

std::ostream& operator<<(std::ostream& lhs, T const& rhs)
{
    // TODO: Do something
    return lhs;
}

作为成员函数

与其他二进制文件一样，可以将其定义为成员函数.也就是说，让我们假设您定义了自己的iostream.除其他事项外，您的类声明可能看起来像这样.同样，T是一种特殊类型.

As a member function

As with other binary it could be defined as a member function. That is, let us suppose that you with defining your own iostream. Amongst other things, your class declaration may look like this. Again, T is a particular type.

class MyIOStream : public iostream
{

public:
    MyIOStream& operator<<(T const& rhs);

}

在这种情况下，operator<<是成员函数.与<<一样具有相同的语义.

In this case operator<< is a member function. It has the same semantics when used as <<.

1. C和C ++中的运算符-您所有运算符的摘要可以重载及其典型参数.

1. Operators in C and C++ - a great summary of all the operators you can overload and their typical arguments.

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