CODEWITHSUNDEEP
c++c++11
vinnylinux
vinnylinux

Reputation: 7024

C++11 class attribute as another object?

I'm working on some C++11 examples, but i'm a little rusty. I'm trying to add an object instance to a class attribute. I have something like this: 

Entity.h

class Entity {
    private:
        MyClass object;

    public:
        Entity();
        void doTest();
};


Entity.cpp

#include "Entity.h"

Entity::Entity() {
}

void Entity::doTest(stuff) {
    object = new MyClass(stuff);
}

Is this correct? How can i do this in C++?

Upvotes: 0

Views: 215

Answers (4)

Chemistpp
Chemistpp

Reputation: 2056

You want to use in the decleration:

MyClass* object

Also, if you are going to use new MyClass make sure you use delete object to avoid leaks.

i.e.

Entity::Entity() { object = NULL; } //constructor

Entity::doTest(stuff) {
    delete object;
    object = new MyClass(stuff);
}

//Following rule of three, since we need to manage the resources properly
//you should define Copy Constructor, Copy Assignment Operator, and destructor.

Entity::Entity(const Entity& that) { //copy constructor
    object = that.object; 
    //assumes you've correctly implemented an assignment operator overload for MyClass
}

//invoke copy and swap idiom* if you wish, I'm too lazy
Entity& Entity::operator=(const Entity& source) {
    MyClass* temp = new MyClass(source.object)
    //assumes you've correctly implemented an copy constructor (or default one works) for MyClass.
    delete object;
    object = temp;
    return *this;
}

Entity::~Entity() {   //destuctor
   delete object;
}

You should avoid dynamic allocation if it is at all possible. You should also use smart pointers (like std::shared_ptr) but if you do wish to use raw pointers, then abide by the rule of three.

*copy and swap idiom

Upvotes: 0

Jean-Baptiste Yunès
Jean-Baptiste Yunès

Reputation: 36401

In C++ your object field is really an object. That means that there is an allocated memory inside every Entity object you may create. The problem is how you can initialize that object field ?

  • if MyClass has no ctor or a ctor callable with no parameter, everything is ok.

  • if not, you should define the initialization of the field at the same time you define the ctor of Entitythis way

    Entity::Entity() : object(parameters) { code for Entities initialization }

    this is a way to ensure the correctness of your initialization so that object is initialized before you have control on the initialization of the Entity.

Your object is statically initialized inside each Entity, this is a good way, in C++, to code what is called a composition in object oriented programming.

Upvotes: 0

Mike Seymour
Mike Seymour

Reputation: 254501

It's all correct apart from the new. Only use that when you need dynamic allocation; in this case, you just want to create an object:

object = MyClass(stuff);

Or perhaps you want to initialise it in the constructor instead:

Entity(stuff) : object(stuff) {}

Upvotes: 2

CS Pei
CS Pei

Reputation: 11047

It is wrong. object is an object not a pointer. but your code 

object = new MyClass(stuff);

treat object as a pointer.

You can either declare object as a pointer in the class Entity or change your function doTest;

If you want a pointer it is better to use smart pointers in C++, such as unique_ptr.

Upvotes: 0

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