Creating unknown number of multiple C++ objects automatically

Question

I would like to know C++ way of creating multiple indefinite number of objects (0..n) automatically. The use case is as below:

Let say I have a script that a user has to enter related data (width, length, height) that later on Box class will contain those data.

Box
{ 
    ...
    ...
    double width;
    double length;
    double height;
    ...
    ...
}

The script looks like:

<id="width" value="1"/>
<id="length" value="3"/>
<id="height" value="3"/>

<id="width2" value="3"/>
<id="length2" value="3"/>
<id="height2" value="4"/>

<id="width3" value="2"/>
<id="length3" value="3"/>
<id="height3" value="3"/>

In other words, how can I instantiate those object which number of the objects is not known in advance, rather, it depends on how many box information (width and so on) is entered by the user.

Also, is there any design pattern for that?

why isn't this just a loop calling new Box(width, length, height) many times? — djna
– djna, Commented Aug 21, 2014 at 18:35

quantdev · Accepted Answer · 2014-08-21 19:09:06Z

6

Use std::vector

http://www.cplusplus.com/reference/vector/vector/

It will allow to add an unknown number of objects to it, and works like an array

edited Aug 21, 2014 at 19:09

quantdev

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answered Aug 21, 2014 at 18:35

QuinnFTW

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Comments

frogatto · Accepted Answer · 2014-08-21 18:52:35Z

3

Suppose you want n objects type Box

Box **array = new Box*[n];
for(int i = 0; i < n; i += 1) {
    array[i] = new Box();
}

UPDATE #1

if you want a variable length list, you can try:

edited Aug 21, 2014 at 18:52

answered Aug 21, 2014 at 18:38

frogatto

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13 Comments

QuinnFTW Over a year ago

The issue here is that this is assuming there only n boxes

alfa_80 Over a year ago

n is a variable not a fixed number of objects. Therefore, n here should be fine.

frogatto Over a year ago

@alfa_80 My answer offers a dynamic memory allocation, so n can be variable at run-time and it works fine

Ferruccio Over a year ago

But you do need to know what n is in advance.

pqnet Over a year ago

the code fragment will not compile, unless Box can be casted to Box* in which case it will compile, run, and leak. Do yourself a favor and learn C++ before misleading people less experienced than you

|

Wojtek Surowka · Accepted Answer · 2014-08-21 18:49:48Z

2

You cannot during runtime add new members to your class based on XML. The best you can do is to you std::map in your class, which will map the ids to values. Something like

class Box
{
  std::map<std::string, int> values;
...

and then e.g. in Box constructor

values["width"] = 1;
values["width2] = 3;

Obviously instead of hard coding it you may read the values from XML and add them.

answered Aug 21, 2014 at 18:49

Wojtek Surowka

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Comments

Raydel Miranda · Accepted Answer · 2014-08-21 20:07:30Z

The obvious is that in first place you have to parse the xml file. All other answers assumes you already know how to do that. Here I will complement QuinnFTW's answer and also give you a link to Parsing xml with Boost. Why? Well, if you knew how to obtain a list of objects from the xml, you were not asking this question.

And, complementing QuinnFTW's answer a code sample using std::vector.

#include <vector>
#include <iostream>

using namespace std;

struct Box { 
    double with, length, height; 
    Box():with(0), length(0), height(0) {}
};

typedef std::vector<Box> box_list_t;


int main()
{

    Box a, b, c;
    // Create the vector.
    box_list_t box_list;

    box_list.push_back(a);  // Add a to the vector;
    box_list.push_back(b);  // Add b to the vector;
    box_list.push_back(c);  // Add c to the vector;

    box_list[0].with = 0;   // Change the with of a to 0;
    cout << box_list[1].with << endl; // Prints b.with. Have not been initialized.


    return 0;
}

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