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
为什么不将此 C++ 代码打印到 cout?
Why isn't this C++ code printing to cout?
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。^D
while
^D
std::cout
std::cerr
有关观察到的输出,请参阅附件。
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
答: 暂无答案
评论
std::cout
std::cerr
std::cout << "hello world" << std::endl;
std::string s; while (std::cin >> s) { std::cout << "in loop:" << s << std::endl; } std::cout << "finished";