Re: Array of objects from another class

From:
"Jim Langston" <tazmaster@rocketmail.com>
Newsgroups:
comp.lang.c++
Date:
Mon, 6 Apr 2009 20:01:19 -0700
Message-ID:
<2mzCl.18511$%e2.12669@newsfe24.iad>
"sr98p47" <sr98p47@gmail.com> wrote in message
news:42014bd7-8f31-46f0-ac81-aec735aa9c11@w9g2000yqa.googlegroups.com...

I have no experience with C++ other than going through the FAQ to find
my answer but I'm stuck. I did a search on Google Books through
Deitel's C++ book but the preview is limited. I have bought Deitel's
book on C++ on Amazon but it hasn't been delivered yet. There's no
Barnes & Nobles within 50 miles from where we live and the localy
library isn't barren of computer books.

I'm trying to create a simple table in order to start learning C++,
something I can try hashing numbers or characters or strings into. I
don't know much about tables so it's possible I have the concept
wrong.

In my header file, I put the following, leaving out any directives to
save space:

class data {
       public:
               data(int);
               ~data(int);
       private:
               int item;
};

In my definitions file, my constructor is simple:

data::data(int x) {
              item = x;
}

I then built another class to create a table, a table consisting of an
array of data items so I can hash numbers or strings into them, but I
don't think I'm constructing it correctly:

Here's my class definition file for the table I want to create:

#include "data.h"

class data_table {
       public:
               data_table(int);
               ~data_table();
       private:
               data *data_entries; // pointer to an array of data
items.
               int table_size;
};

#include "data.h"

data_table::data_table(int size) {
       table_size = size;
       data_entries = new data[size]; // compiler error


new data[size]; would call the default constructor for data, which you don't
have. The only constructor for data you have is data(int).
You have a few options. 1. Create a default constructor for data that sets
item to some value (maybe zero?).

Basically your data_table is duplicated what std::vector does. I understand
you are doing this as a learning experience so that's fine, but in
applications you probalby want to use std::vector. I.E.
std::vector<data> MyData;
MyData.push_back( /*...*/ );

You will notice that std::vector usually doesn't try to put in blank
entries, but you can add them to the end, getting rid of your problem also.
Not having to make default constructed entries.

       ...
       }

The warning I'm getting is: no matching function for call to
`data::data()'

I modified my data class definition to include an array, a pointer,
and a copy constructor, but I can't seem to get this to work.

I can stop the compiler errors if I change this

       data_entries = new data[size];

to this:

       data_entries = new data(size};

but then when I try to write code to access the data_entires array,
there's more compiler errors.

I know I should use a vector, but that's beyond me right now. Is
there a simple solution to this error? I'm sorry if this is an easy
question.

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