c++ - 使用命名空间标准；和载体

Question

我正在为我的 c++ 类开发一个项目，虽然我的代码对我(初学者)来说看起来不错，但每当我运行它时，我几乎每一行都会收到错误 C2065 未声明的标识符。我做了研究，感觉它与使用命名空间标准有关；但我不确定。任何帮助将不胜感激。

该项目是创建一个存储书籍列表的 vector ，然后为用户提供向列表中添加书籍、使用迭代器显示列表、使用迭代器删除书籍或将数据存储在外部文件。这是我目前所拥有的:

#include <iostream>
#include <vector>
#include <string>
#include "bookColl.h"
#include <algorithm>
#include <fstream>

using namespace std;

int main()
{
    vector <string> bookCollection;
    string book1, book2, book3;

    cout << "Enter the first book in your book collection: " << endl;
    getline(cin, book1);

    cout << "\nEnter the second book in your book collection: " << endl;
    getline(cin, book2);

    cout << "\nEnter the third book in your book collection: " << endl;
    getline(cin, book3);

    bookCollection.push_back(book1);
    bookCollection.push_back(book2);
    bookCollection.push_back(book3);

    vector <string> ::iterator myITER;
    vector <string> ::const_iterator cITER;

    double decide;

    do
    {
        int choice = 0;
        cout << "What would you like to do?/n1.Add a book to your collection." <<
            "/n2.Display your book collection." << "/n3.Remove a book from your collection" << "/n4. Stop adding books and store the data in an external file" << "/nChoose a number 1 through 4: " << endl;

        if (choice == 1)
        {
            string addNew;
            addBook(&addNew);
            bookCollection.push_back(addNew);
        }

        else if (choice == 2)
        {
            cout << "Here are your books: " << endl;
            for (cITER = bookCollection.begin(); cITER != bookCollection.end(); cITER++)
            {
                cout << *cITER << endl;
            }
        }

        else if (choice == 3)
        {
            int removeBook;
            myITER = bookCollection.erase(bookCollection.begin() + removeBook);
            cout << "enter the numbered book position for the book you would like to remove: " <<
                "/n(The books in your book collection are labels for 0 up)" << endl;
            cin >> removeBook;
            cout << *myITER << endl;
        }

        else if (choice == 4)
        {
            sort(bookCollection.begin(), bookCollection.end());

            ofstream bookFile;
            bookFile.open("BookCollection.txt", ios::out | ios::app);
            bookFile << " This is a line of text. \n";
            bookFile << " This is a second line of text. \n";
            bookFile.close();
        }

        cout << "\nWould you like to run this program again? If so select 1 for Yes, and 0 for No" << endl;
        cin >> decide;

    } while (decide > 0);
}

bookColl.h

string addBook(string* pValOne);

bookColl.cpp

#include <iostream>
#include <vector>
#include <string>
#include "bookColl.h"

using namespace std;

string addBook(string* pValOne)
{
    string newBook;
    *pValOne = newBook;
    cout << "What is the name of the book you would like to add?" << endl;
    getline(cin, *pValOne);
    return newBook;
}

Answer 1

bookColl.h 必须如下所示:

#ifndef BOOK_COLL_H
#define BOOK_COLL_H

#include <string>

std::string addBook(std::string* pValOne);

#endif

你需要一个 include guard ，类的全称是std::string , 不是 string ;这就是你用 #include <string> 得到的.您的代码无法编译，因为 using namespace std; 在 #include "bookColl.h" 之后.

说到 using namespace ，我的建议是忘记它，至少现在是这样。只需使用 std::无处不在。