c++ - 从文件读取和写入
问题描述
我需要为图书馆系统制作卡片目录条目,并且需要一个菜单并保存卡片并从文件中读取卡片。我只是在寻求帮助,而不是为任何人保存,如果你有任何提示,我只是想学习,而不仅仅是复制和粘贴。
我已经完成了他们需要添加的信息,但是在让他们保存并从文件中读取并从菜单中选择添加新条目、查看条目时遇到问题。
#include <iostream>
#include <fstream>
#include <cstdlib>
using namespace std;
struct libary
{
string title;
string author;
string ISBN_code;
int page_count;
int publish_year;
} s[10];
bool processMenu()
{
int choice, spot;
cout << "Main Menu" << endl;
cout << "Select your options" << endl;
cout << "1. Add a new entry to the system" << endl;
cout << "2. Review an entry" << endl;
cout << "3. Save entries to a file" << endl;
cout << "4. Read entries from a file" << endl;
cin >> choice;
switch (choice)
{
case 1:
cout << "which spot do you want to add a entry to the system " << endl;
cin >> spot;
break;
case 2:
cout << "which spot do you want to review to a file " << endl;
cin >> spot;
break;
case 3:
cout << "Save entries to a file" << endl;
ofstream myfile;
myfile.open("file.txt");
myfile << "this will show in the file";
break;
case 4:
cout << "read entries from a file :" << endl;
break;
case 7:
return 0;
}
}
int main()
{
while (!processMenu())
{
cout << "Sorry, that is not a valid choice." << endl;
cout << "Please try again." << endl << endl << endl;
}
cout << " Enter information of each card catalog" << endl;
for (int i = 0; i < 10; i++)
{
cout << endl;
cout << " Enter title " << endl;
cin >> s[i].title;
cout << endl;
cout << " Enter author " << endl;
cin >> s[i].author;
cout << endl;
cout << " Enter Page count " << endl;
cin >> s[i].page_count;
cout << endl;
cout << " Enter publish year " << endl;
cin >> s[i].publish_year;
cout << endl;
cout << " Enter ISBN code, it should be 13 digits" << endl;
cin >> s[i].ISBN_code;
while (s[i].ISBN_code.length() != 13)
{
cout << " Please enter a ISBN code thats 13 digits" << endl;
cin >> s[i].ISBN_code;
cout << endl;
cout << endl;
}
}
cout << " Displaying Information" << endl;
for (int i = 0; i < 10; i++)
{
cout << " title: " << s[i].title << endl;
cout << " author: " << s[i].author << endl;
cout << " page count: " << s[i].page_count << endl;
cout << " publish year: " << s[i].publish_year << endl;
cout << " ISBN code: " << s[i].ISBN_code << endl;
}
return 0;
}
它让我选择一个位置,但不会添加新条目,也不会审查或保存/阅读。
解决方案
您可以通过覆盖其输出 ( ) 和/或输入运算符 ( )使您自己的类型可写入流 ( cout
or ) 和/或从流 ( or )ofstream
中读取。cin
ifstream
operator<<
operator>>
#include <fstream> // ifstream, ofstream
#include <iostream> // cin, cout
#include <string> // getline, string
using namespace std;
struct Card
{
// Writing to the standard output ('cout').
friend ostream& operator<<(ostream& os, const Card& card);
// Reading from the standard input ('cin').
friend istream& operator>>(istream& is, Card& card);
// Writing to a file ('ofstream').
friend ofstream& operator<<(ofstream& os, const Card& card);
// Reading from a file ('ifstream).
friend ifstream& operator>>(ifstream& is, Card& card);
string title = "";
string author = "";
string isbn = "";
int page_count = -1;
int publish_year = -1;
};
ostream& operator<<(ostream& os, const Card& card)
{
os << "Title: " << card.title << '\n'
<< "Author: " << card.author << '\n'
<< "ISBN: " << card.isbn << '\n'
<< "Page count: " << card.page_count << '\n'
<< "Publish year: " << card.publish_year << '\n';
return os;
}
istream& operator>>(istream& is, Card& card)
{
// 'getline' needs for the reading of strings because the title of a book or
// its author's name may contain Space (' ') characters. 'cin' and
// 'ifstream' separate input by every whitespace. So, if we want to allow
// spaces in our fields we have to separate them by a special delimiter
// character. We use the Newline ('\n') character here to separate fields.
// `page_count` and `publish_year` are numbers, they don't contain
// whitespaces, so they simply can be read with 'operator>>'.
cout << "Title: ";
std::getline(is, card.title);
cout << "Author: ";
std::getline(is, card.author);
cout << "ISBN code: ";
std::getline(is, card.isbn);
cout << "Page count: ";
is >> card.page_count;
cout << "Publish year: ";
is >> card.publish_year;
return is;
}
// The next two functions is for files.
ofstream& operator<<(ofstream& os, const Card& card)
{
os << card.title << '\n'
<< card.author << '\n'
<< card.isbn << '\n'
<< card.page_count << '\n'
<< card.publish_year << '\n';
return os;
}
ifstream& operator>>(ifstream& is, Card& card)
{
std::getline(is, card.title);
std::getline(is, card.author);
std::getline(is, card.isbn);
is >> card.page_count;
is >> card.publish_year;
return is;
}
int main()
{
Card card;
// Read the informations of a card from the standard input.
cin >> card;
// Write the informations of a card to the standard output.
cout << card;
// Write the informations of a card to "file.txt".
ofstream ofs("file.txt");
ofs << card;
ofs.close();
// Read the informations of a card from "file.txt".
ifstream ifs("file.txt");
ifs >> card;
ifs.close();
return 0;
}
重要提示:为简单起见,我省略了打开文件和写入/读取流时的错误检查。在您的代码中,您应该执行这些操作。
推荐阅读
- django - Django 与 BeautifulSoup 的 lxml 解析器的兼容性
- javascript - 如何使用codemirror显示像mozilla开发网络一样的代码输出?
- css - CSS @keyframes 动画在悬停时不断闪烁
- encryption - 如何使用 Virgil 安全加密和解密消息?
- google-app-engine - 谷歌应用引擎项目多区域部署
- angular - 我们如何在 Angular 4 应用程序中为特定的 html 页面导入条件 css 文件?
- android - 更改系统语言时,Android 复数无法正常工作
- scala - 如何在 Play 自己的代码中使用 Status
- javascript - javascript中的mouseChildren之类的东西
- spring - Spring Security - 从本地调用安全方法