为什么矢量会发生这种情况

Question

我一直在编写代码以将书添加到库中。当我将书添加到库中并尝试显示它时，库中什么都没有存储（显示）

在这里，我将库可以容纳的最大书籍数量设置为
100。int maxbooks = 100; 向量图书馆（maxbooks）; 当我删除“（maxbooks）”时，代码工作正常。但是我不明白为什么？

#include <iostream>
#include <vector> 
#include <string>

using namespace std;

//struct model a book 
    struct book {
    string authorname;
    string bookname;
}; //struct ends here 

//function to display number of books in the library
void displayLibrary(vector <book> &cmini_library,int cnum_of_books);

//function to add books to the library;
void add (vector <book> &cminilibrary,book dummylibrary,int &num_of_books);

int main() {
    int option;
    int curr_num_of_books = 0; //current number of books in the library
    int maxbooks = 100; //max number of books our library can hold 
    vector<book> minilibrary(maxbooks); //minilibrary vector of max capacity 100
    book dummylibrary; // a variable used to hold entries for a short period 

    cout << "1.Display all books in the library" << endl;
    cout << "2.Add a book to the library" << endl;
    cout << "option entered: " << endl;
    cin >> option;

    switch (option) {
        case 2:
            add (minilibrary,dummylibrary,curr_num_of_books);
            displayLibrary(minilibrary,curr_num_of_books);
            break;
        }

    return 0;
}

void displayLibrary(vector <book> &cmini_library,int cnum_of_books)
{
    for(int i=0;i<cnum_of_books;i++) {
        cout << "Book name: \t" << cmini_library[i].bookname << endl;
        cout << "Author: \t" << cmini_library[i].authorname << endl;
    }
}

void add(vector<book> &cmini_library,book dummylibrary, int &num_of_books) 
{
    int booksadded=0; // number of books added to the library
    cout << "How many books do you wish to add: ";
    cin >> booksadded;
    cin.ignore(1,'\n');
    for(int i=0;i<booksadded;i++) {
        cout << "Book name: ";
        getline(cin,dummylibrary.bookname); 
        cout << endl;
        cout << "Author: ";
        getline(cin,dummylibrary.authorname);
        cout << endl;
        cmini_library.push_back(dummylibrary);
    }

    num_of_books = num_of_books + booksadded;
}

Answer 1

int maxbooks=100;
vector <book> minilibrary(maxbooks);

这些行创建了带有100本书的向量，每本书都是默认构造函数创建的。

现在，当你添加新书入载体，其大小为101，但displayLibrary需要1作为被印刷图书的数量，并打印minilibrary[0]- 拖欠您添加即书，不是书minilibrary[100]。

您应该将其创建minilibrary为空向量。然后，您可以将书推入其中。