为什么不将此 C++ 代码打印到 cout?

Why isn't this C++ code printing to cout?

提问人:EE18 提问时间:11/7/2023 最后编辑:EE18 更新时间:11/7/2023 访问量:83

问:

我目前正在关注Lippman的C++入门第5版。我从本书的配套网站复制了以下代码:

#include <iostream>
#include "Sales_item.h"

int main() 
{
    Sales_item total; // variable to hold data for the next transaction

    // read the first transaction and ensure that there are data to process
    if (std::cin >> total) {
        Sales_item trans; // variable to hold the running sum
        // read and process the remaining transactions
        while (std::cin >> trans) {
            // if we're still processing the same book
            if (total.isbn() == trans.isbn()) 
                total += trans; // update the running total 
            else {              
                // print results for the previous book 
                std::cout << total << std::endl;  
                total = trans;  // total now refers to the next book
            }
        }
        std::cout << total << std::endl; // print the last transaction
    } else {
        // no input! warn the user
        std::cerr << "No data?!" << std::endl;
        return -1;  // indicate failure
    }
    std::cout << "Hi" << std::endl;;
    return 0;
}

标头位于工作目录中,一切正常,但如果需要,我已将其包含在此处:Sales_item.h


#ifndef SALESITEM_H
// we're here only if SALESITEM_H has not yet been defined 
#define SALESITEM_H

// Definition of Sales_item class and related functions goes here
#include <iostream>
#include <string>

class Sales_item {
// these declarations are explained section 7.2.1, p. 270 
// and in chapter 14, pages 557, 558, 561
friend std::istream& operator>>(std::istream&, Sales_item&);
friend std::ostream& operator<<(std::ostream&, const Sales_item&);
friend bool operator<(const Sales_item&, const Sales_item&);
friend bool 
operator==(const Sales_item&, const Sales_item&);
public:
    // constructors are explained in section 7.1.4, pages 262 - 265
    // default constructor needed to initialize members of built-in type
    Sales_item() = default;
    Sales_item(const std::string &book): bookNo(book) { }
    Sales_item(std::istream &is) { is >> *this; }
public:
    // operations on Sales_item objects
    // member binary operator: left-hand operand bound to implicit this pointer
    Sales_item& operator+=(const Sales_item&);
    
    // operations on Sales_item objects
    std::string isbn() const { return bookNo; }
    double avg_price() const;
// private members as before
private:
    std::string bookNo;      // implicitly initialized to the empty string
    unsigned units_sold = 0; // explicitly initialized
    double revenue = 0.0;
};

// used in chapter 10
inline
bool compareIsbn(const Sales_item &lhs, const Sales_item &rhs) 
{ return lhs.isbn() == rhs.isbn(); }

// nonmember binary operator: must declare a parameter for each operand
Sales_item operator+(const Sales_item&, const Sales_item&);

inline bool 
operator==(const Sales_item &lhs, const Sales_item &rhs)
{
    // must be made a friend of Sales_item
    return lhs.units_sold == rhs.units_sold &&
           lhs.revenue == rhs.revenue &&
           lhs.isbn() == rhs.isbn();
}

inline bool 
operator!=(const Sales_item &lhs, const Sales_item &rhs)
{
    return !(lhs == rhs); // != defined in terms of operator==
}

// assumes that both objects refer to the same ISBN
Sales_item& Sales_item::operator+=(const Sales_item& rhs) 
{
    units_sold += rhs.units_sold; 
    revenue += rhs.revenue; 
    return *this;
}

// assumes that both objects refer to the same ISBN
Sales_item 
operator+(const Sales_item& lhs, const Sales_item& rhs) 
{
    Sales_item ret(lhs);  // copy (|lhs|) into a local object that we'll return
    ret += rhs;           // add in the contents of (|rhs|) 
    return ret;           // return (|ret|) by value
}

std::istream& 
operator>>(std::istream& in, Sales_item& s)
{
    double price;
    in >> s.bookNo >> s.units_sold >> price;
    // check that the inputs succeeded
    if (in)
        s.revenue = s.units_sold * price;
    else 
        s = Sales_item();  // input failed: reset object to default state
    return in;
}

std::ostream& 
operator<<(std::ostream& out, const Sales_item& s)
{
    out << s.isbn() << " " << s.units_sold << " "
        << s.revenue << " " << s.avg_price();
    return out;
}

double Sales_item::avg_price() const
{
    if (units_sold) 
        return revenue/units_sold; 
    else 
        return 0;
}
#endif

我不确定这是否是 CLion 问题,或者我是否误解了事情应该如何运作。源文件表面上正在执行,但是当我给它输入(见附图)然后给它结束文件输入()或无效的输入时,我应该将我们从中解脱出来,我仍然没有看到任何输出。如果我只输入结束文件输入(),那么我确实会得到“没有数据?!”输出,所以我不确定为什么不起作用,但确实如此。^Dwhile^Dstd::coutstd::cerr

有关观察到的输出,请参阅附件。

enter image description here

编辑:

我现在已经设法获得了以下三个输出,其中一个在某种程度上是正确的(“l”后跟“l”最终打印了正确的东西,但使用“Command-D”则不然)。也许我误解了我打算如何终止输出?我的书说我可以使用文件结束字符(我认为这是 Mac 上的 Command-D),此外,每当我给它一个错误的输入时,while 循环都应该中断(当然,第一个“l”应该是一个错误的输入给定类型),但我仍然得到了这种不正确的行为。Sales_item

enter image description here

enter image description here

C++ Cion Cout

评论

1赞 Thomas Matthews 11/7/2023
调试器。调试会话的结果是什么?当您单步执行代码时,您的代码是否停留在输出语句处?
0赞 user4581301 11/7/2023
建议:备份你所写的内容,并将其缩减为最小的可重现示例(MRE)。如果在完成后仍有问题,通常制作 MRE 可以减少错误周围的噪音,足以发现并修复它,请将代码替换为 MRE。
0赞 Jarod42 11/7/2023
如果问题只是与 clion 有关,它不使用 (但为 ) 打印,您可以简化您的示例......std::coutstd::cerrstd::cout << "hello world" << std::endl;
1赞 drescherjm 11/7/2023
我只是想弄清楚如何在 CLion 中逐步执行代码。尽快学会使用调试器非常重要。与不知道如何使用它的学生相比,这将使您具有很大的优势,并为您节省大量时间。如果你以编程为职业,这将是一项必不可少的技能。
1赞 Jarod42 11/7/2023
您必须尝试最小化代码以缩小问题范围,要么从空代码开始,要么添加越来越多的代码,要么从现有代码中删除 /simplifying。该过程甚至可以让您自己了解问题;我的下一步是这样的:std::string s; while (std::cin >> s) { std::cout << "in loop:" << s << std::endl; } std::cout << "finished";

答: 暂无答案