Calling Different Constructors on each index of Array of Objects

Question

I am working on a project which handles the Data of a Library Section having multiple classes. Interface of interest of a class here is this.

class Author
{
  private:
    //data members
  public:
    Author (string _name) ;
    Author (string _name, int _books) ;
} ;

Data is present with at least the name of the author, and no. of books may come along. I cannot have a default constructor here naturally. But if create a static array like this.

Author auth_arr[100] ;

Compiler gives me error due to absence of default constructor. But this statement is invalid as well.

Author auth_arr[100] ("Jacob") ;

Is there any method by which i can create this array and call the constructor of my choice, instead of making another method?

Community · Accepted Answer · 2017-05-23 11:58:08Z

You can do this to a very limited extent. The trick is to call those constructors explicitly inside curly braces inside separated by commas, and instantiate the static array this way.

Author auth_arr [3] = {Author(“Kevin”, 7) ,Author(“Martha”) ,Author(“Shuan”, 15) } ;

But this method is literally a nightmare. You can do this for a size 5 or 10 array but not for a 50 size array or any more than that. So the better option is to make another method and call it passing the details as its arguments. Remember this is only for static arrays as you asked. For dynamic arrays, which are more frequently used than static, even this method is not applicable.

C++ does not allow you another method to call constructors of your choice. So default constructor should be defined in the class to get called at instantiation. Then, an appropriate method could be called afterwards with the data.

Another better thing would be to use std :: vector. Its detail you can find in another similar question here. Constructors and array of object in C++

I can't say this thing for sure. Maybe somebody else could explain it :P
Yeah, C++11 is almost 100% backwards compatable. Everything that compiled before still compiles. C++11 variadic templates make it possible to make a thing that constructs N Authors in an array using a much simpler syntax. (Also, in C++03 you could use placement new)

PiotrNycz · Accepted Answer · 2012-09-06 19:46:09Z

Are you really sure default c-tor has no sense here?

I would suggest this solution:

class Author
{
  private:
    //data members
  public:
    static string default_name;
    Author (string _name = default_name);
    Author (string _name, int _books);
} ; 
string Autor::default_name = "NOBODY";

If you really insist to have 100 Jacobs, then do it in this way:

Author::default_name = "Jacob";
Author auth_arr[100]; 
Author::default_name = "NONE";

[UPDATE1]

However this works for local arrays only. With a little trick you can use it for static/global arrays. I am not going to present it - because there is another solution to make static array of class without default constructor which I understand from your comment is your goal ;) With little help of macros you can even generate thousands of objects:

#define MAKE_10(X) X,X,X,X,X,X,X,X,X,X

Author auth_arr[101] = {
   MAKE_10(MAKE_10(Author("Jacob", 0))),
   Author("Jacob", 0)
};

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