Class 在 C++ 中不包含名为...的成员

Question

Assuming I am making a library of books in C++ this way:

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

class Book
{
public:
     Book(string name, string author)
};

Simple, just a constructor, now I create a vector of Book and push books back:

int main()
{
   vector<Book> books;
   books.push_back(Book("Gatsby", "Fitzgerald"));

But when I try to print out some member (name or author):

   cout << books[0].name << endl;

   return 0;
}

My boy compiler is angry:

error: ‘__gnu_cxx::__alloc_traits >::value_type {aka class Book}’ has no member named ‘name’
     cout << books[0].name << endl;

I'm a relative beginner, does this approach make sense at all? And if it does, what did I do wrong?

Thank you!

Answer 1

The class Book has no members to store name and author. And, the constructor that you defined is syntactically wrong.

With public data members, it would look like this:

class Book
{
public:
    // data members
    std::string name;
    std::string author;

    // parameterized constructor
    Book( std::string name, std::string author )
    {
        this->name = name;
        this->author = author;
    }
};

Please note that:

1. exposing data members like that is in violation of data-hiding principle of OOP. Ideally, the data members should be private and adequate accessor methods should be used.
2. the assignments of name and author in the body of the constructor is just for your understanding. If you've already studied the initializer list for constructor then use that.

---

Here's an example ( live ):

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

class Book final
{
public:
    // constructor with initializer list
    Book( std::string name_, std::string author_ ) : name{name_}, author{author_} {}

    // accessor methods
    std::string getName() const { return name; }
    std::string getAuthor() const { return author; }

private:
    std::string name;
    std::string author;
};

int main()
{
    std::vector<Book> books;

    books.push_back( Book{"The Alchemist", "Paulo Coehlo"} );
    books.push_back( Book{"Fight Club", "Chuck Palahniuk"} );
    books.push_back( Book{"No Country for Old Men", "Cormac McCarthy"} );

    books.emplace_back( "Brave New World", "Aldous Huxley" );
    books.emplace_back( "1984", "George Orwell" );
    books.emplace_back( "Animal Farm", "George Orwell" );

    for ( const auto& book : books )
    {
        std::cout << book.getName() << " by " << book.getAuthor() << '\n';
    }

    return 0;
}

Some relevant threads to read:

• Why is "using namespace std;" considered bad practice?
• push_back vs emplace_back
• range-for loop
• C++: "std::endl" vs "\n"

Answer 2

Assuming I am making a library of books in C++ this way:

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

class Book
{
public:
     Book(string name, string author)
};

Simple, just a constructor, now I create a vector of Book and push books back:

int main()
{
   vector<Book> books;
   books.push_back(Book("Gatsby", "Fitzgerald"));

But when I try to print out some member (name or author):

   cout << books[0].name << endl;

   return 0;
}

My boy compiler is angry:

error: ‘__gnu_cxx::__alloc_traits >::value_type {aka class Book}’ has no member named ‘name’
     cout << books[0].name << endl;

I'm a relative beginner, does this approach make sense at all? And if it does, what did I do wrong?

Thank you!