使用istream参数读取的istream函数 [英] Istream function to read with istream parameter

问题描述

我正在尝试理解此代码

istream &read(istream &is, Sales_data &item) 
{
    double price = 0;    
    is >> item.bookNo >> item.units_sold >> price;   
    item.revenue = price * item.units_sold;    
    return is;
} 

ostream &print(ostream &os, const Sales_data &item)
{
    os << item.isbn() << " " << item.units_sold << " "    
        << item.revenue << " " << item.avg_price();    
    return os; 
}

我不明白这些功能是什么，我也不明白为什么我们使用 istream 进行阅读而使用 ostream 进行打印，而不是使用 cin 和 cout 。

I don't understand what are these function , also I can't understand why we use istream for reading and ostream for printing instead of using cin and cout.

推荐答案

还有很多您需要了解一些细微的作用，以了解 read 和 print 函数中正在发生的事情。从概述的角度来看， istream 和 ostream 是通用输入和输出类，它们为C ++中的流I / O提供了基础。正如其他答案和注释正确指出的那样， std :: cin 和 std :: cout 源自 istream 和 ostream 提供标准流 stdin 和 stdout 。

There are more than a few subtleties at play that you need to understand to understand what is taking place in the read and print functions. From the overview standpoint istream and ostream are general input and output classes that provide a basis for stream I/O in C++. As the other answer and comments correctly indicate, std::cin and std::cout are derived from istream and ostream to provide input/output from the standard stream stdin and stdout.

流本身具有确定流是否良好的状态，可以从中读取或写入或是否发生了流错误，从而阻止了进一步的I / O（某些故障是可恢复的，有些无法恢复），请参见 std :: basic_ios :: rdstate 和 std :: basic_ios :: setstate 讨论构成流状态的位（ goodbit，badbit，failbit，eofbit ）。

The stream themselves have a state that determines if the stream is good and can be read from or written to, or whether a stream error has occurred preventing further I/O (some failures are recoverable, some not) See std::basic_ios::rdstate and std::basic_ios::setstate for a discussion of the bits (goodbit, badbit, failbit, eofbit) that make up the stream state.

现在查看您的函数原型：

Now looking at your function prototyes:

istream &read(istream &is, Sales_data &item)

和

ostream &print(ostream &os, const Sales_data &item)

请注意第一个参数如何引用当前流？并注意返回也是如何引用相同的流的？这很重要，因为它传递了对流的引用，因此在函数中对 Steam状态进行的任何更改都可以在返回时使用。因此，如果在 read 函数中遇到 eof ，则状态更改将在返回时可用。与 print 函数相同（尽管会改变流的潜在错误更少且有所不同）

Notice how the first parameter is a refernce to the current stream? And notice how the return is also a reference to that same stream? That's important, because it passes a reference to the stream so any changes to the Steam State occurring within the function will be available on return. So if eof is encountered within the read function, that change in state will be available on return. The same for your print function (though the potential errors that would change the stream are fewer and different)

read ，您正在读取流中的3条信息， item.bookNo，item.units_sold 和价格并更新 item.revenue ：

Within read, you are reading 3-pieces of information from the stream, item.bookNo, item.units_sold and price and updating item.revenue:

is >> item.bookNo >> item.units_sold >> price; 
item.revenue = price * item.units_sold;

您可以从 std :: cin 中读取通过将其作为的参数是，或者可以传递打开的文件流。流操作将对任何有效流都有效。

You can read from std::cin by passing that as the argument for is or you can pass an open file-stream. The stream operations will work for any valid stream.

print函数执行相反的操作，它输出 item.isbn（）返回.units_sold，item.revenue 和 item.avg_price（）的返回作为输出。如果您通过 std :: cout 作为 os

The print function does the opposite, it outputs the return of item.isbn(), item.units_sold, item.revenue and the return from item.avg_price() as output. If you pass std::cout as os

这两个函数中的最后一个命令返回流，包括流状态中的任何更改，因此调用方可以检查读取还是打印成功。这是呼叫者能够确定是否发生I / O的关键。

The last command in both functions returns the stream including any change in the stream state so the caller can check whether the read or print was successful. That is key for the caller being able to determine whether the I/O took place.

过度简化的示例

使用该函数的过于简化的示例可能有助于使概念陷入其中。在这里，我们声明一个简化的结构 Sales_data 和 int bookno，units_sold; 和双重价格，收入; 例如：

An overly simplified example exercising the function may help the concepts sink in. Here we declare a simplified struct Sales_data with int bookno, units_sold; and double price, revenue; For example:

#include <iostream>

struct Sales_data
{
    int bookno, units_sold;
    double price, revenue;
};

我们然后将 read 简化为仅读取 bookno，units_sold 和 price 并为此计算收入单个项目，例如

We then simplify read to read only the bookno, units_sold and price and compute the revenue for that single item, e.g.

std::istream &read(std::istream &is, Sales_data &item) 
{
    is >> item.bookno >> item.units_sold >> item.price;   

    item.revenue = item.price * item.units_sold;    

    return is;
} 

并简化 print 只输出 units_sold 和收入：

std::ostream &print(std::ostream &os, const Sales_data &item)
{
    os << "units sold: " << item.units_sold << " revenue: $"  << item.revenue << '\n';

    return os; 
}

现在，您可以通过非常简单的方式看到提示用户输入 bookno，units_sold 和价格 读取和<$的方式c $ c> print 函数的工作方式以及该函数之一中发生的错误如何更改流状态。简短的 main（）可以是：

Now you can see in a very simple way prompting the user to enter a bookno, units_sold and price how both the read and print functions work, as well as how an error occurring within one of the function changes the stream state. A short main() could be:

int main (void) {

    Sales_data sd;

    std::cout << "enter bookno units_sold price: ";

    if (read (std::cin, sd))        /* if read succeeds */
        print (std::cout, sd);      /* print data */
    else
        std::cerr << "error: invalid input\n";
}

示例用法/输出

$ ./bin/readprintstream
enter bookno units_sold price: 12 100 12.95
units sold: 100 revenue: $1295

或如果发生错误：

$ ./bin/readprintstream
enter bookno units_sold price: 23 banannas 12.28
error: invalid input

希望讨论和示例有助于使概念更清晰。如果您还有其他问题，请告诉我。

Hopefully the discussion and example helps clear the concept up a bit. If you have further questions, let me know.

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