C ++-将一个ostream中的数据发送到另一个ostream [英] C++ - Send data in one ostream to another ostream

问题描述

我不明白此ostream函数声明的含义:

I don't understand what this ostream function declaration means:

ostream& operator<< (ostream& (*pf)(ostream&));

(特别是(*pf)(ostream&)部分).我想做类似的事情:

(specifically, the (*pf)(ostream&) part). I want to do something like:

void print(ostream& os){
    cout << os;
}

但是我得到了错误:

 Invalid operands to binary expression ('ostream' . . . and 'ostream')

推荐答案

我不明白此ostream函数声明的含义:

I don't understand what this ostream function declaration means:

ostream& operator<< (ostream& (*pf)(ostream&));

您是否看到过std::endl，std::flush，std::hex，std::dec，std::setw ...之类的功能?都可以使用<<"将它们全部发送"到流中，然后使用流作为函数参数来调用它们，并对流进行魔术处理.它们实际上与上面的ostream& (*pf)(ostream&)参数匹配，并且该运算符是允许使用它们的那个.如果我们看一下Visual C ++实现...

Have you seen functions like std::endl, std::flush, std::hex, std::dec, std::setw...? They can all be "sent" to a stream using "<<", then they get called with the stream as a function argument and do their magic on the stream. They actually match the ostream& (*pf)(ostream&) argument above, and that operator's the one that lets them be used. If we look at the Visual C++ implementation...

 _Myt& operator<<(_Myt& (__cdecl *_Pfn)(_Myt&))
 {
      return ((*_Pfn)(*this));
 }

...您可以看到是否只是调用函数，并将其用作参数的流传递.期望这些函数返回对相同流参数的引用，以便可以进一步链接<<操作，或者可以将该流隐式转换为bool，以测试流状态.

...you can see if just calls the function, passing the stream it's used on as an argument. The functions are expected to return a reference to the same stream argument, so that further << operations may be chained, or the stream may be implicitly converted to bool as a test of stream state.

请参见 http://en.cppreference.com/w/cpp/io/manip 有关io机械手的更多信息.

See http://en.cppreference.com/w/cpp/io/manip for more information about io manipulators.

您需要帮助:

void print(ostream& os){
    cout << os;
}

这里的问题是，您正在将ostream参数发送到另一个流-cout-并且它不知道您希望它如何处理.

The issue here is that you're sending the ostream argument to another stream - cout - and it doesn't know what you want it to do with it.

要将os的当前内容发送到cout，请尝试:

To send the current content of os to cout, try:

void print(ostream& os){
    cout << os.rdbuf();
}

或者如果您要将一些实际数据打印到参数表示的流中:

Or if you want to print some actual data to the stream represented by the argument:

void print(ostream& os){
    os << "show this!\n";
}

print(std::cout);   // to write "show this!\n" to `std::cout`
print(std::cerr);   // to write "show this!\n" to `std::cerr`

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