C++ accessing object method through array of pointers to objects

Question

I have trouble figuring out how to access bject method through array of pointers to objects.

I have an env object of Environment class:

Environment env;

My Environment has some object pointers as well as a dynamic array of pointers:

    static Robot *robot;
    static Obstacle *obstacle;
    static Object **objects;

So inside objects I can have robots and obstacles:

But now when I want to access a method of an object in the objects array, how can I do that? I tried

    Environment env;
    Robot *robot;

    robot = env.objects[0];
    robot->setSpeed(175);

But it didn't work. I got an error:

error: invalid conversion from ‘Object*’ to ‘Robot*’

What am I doing wrong?

PS: Robot inherits from Object.

Thank you in advance!

Does Robot inherit from Object?

Some programmer dude
– Some programmer dude

2012-11-03 20:04:33 +00:00
Commented Nov 3, 2012 at 20:04 — Some programmer dude
– Some programmer dude, Commented Nov 3, 2012 at 20:04

Some programmer dude · Accepted Answer · 2012-11-03 20:08:05Z

4

Since Robot inherits from Object you have to use either static_cast or dynamic_cast to cast the pointer:

robot = static_cast<Robot*>(Environment::objects[0]);

As a side note, I also recommend you to use std::vector instead of the array, and a smart pointer like std::shared_ptr instead of the raw pointers.

answered Nov 3, 2012 at 20:08

Some programmer dude

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Comments

Lightness Races in Orbit · Accepted Answer · 2012-11-03 20:29:13Z

2

You need to cast the Object* to a Robot*. Assuming Robot inherits from Object. I advise you to use dynamic_cast:

Robot* robot = dynamic_cast<Robot*>(env.objects[0]);
if (robot != NULL) {
    robot->setSpeed(14);
}

edited Nov 3, 2012 at 20:29

Lightness Races in Orbit

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answered Nov 3, 2012 at 20:06

gvd

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Comments

Jesus H · Accepted Answer · 2012-11-03 20:04:47Z

1

You should cast your object from Object* to Robot*. However, you have to make sure it is a Robot object otherwise your application will crash.

Here is an example:

#include <iostream>
class Object
{
};

class Robot : public Object
{
  public:
  int speed;
  void setSpeed(int newSpeed){ speed = newSpeed; }
};

int main()
{
  Object* obj = new Robot();
  ((Robot*)obj)->setSpeed(4);
  std::cout << "Speed is: " << ((Robot*)obj)->speed << std::endl;
}

answered Nov 3, 2012 at 20:04

Jesus H

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SomeWittyUsername · Accepted Answer · 2012-11-03 20:08:28Z

1

You can't implicitly assign pointer of base class to pointer of derived class. If you need to do this, use dynamic_cast.

answered Nov 3, 2012 at 20:08

SomeWittyUsername

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Angew is no longer proud of SO · Accepted Answer · 2012-11-03 20:09:09Z

1

objects is declared with type Object**, which means that objects[0] is of type Object*. You cannot assign an Object* to a Robot*. Assuming Robot is a class derived from Object and has at least one virtual member function, you can do

robot = dynamic_cast<Robot*>(object[0]);

This will perform the cast, or set robot to the null pointer value if object[0] happens to not be a Robot. If you know for certain that it is a Robot, you can use static_cast() instead.

answered Nov 3, 2012 at 20:09

Angew is no longer proud of SO

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Comments

Benedikt Bünz · Accepted Answer · 2012-11-03 20:05:49Z

0

your object does not seem to be a Robot* or a pointer to a subclass of it.

answered Nov 3, 2012 at 20:05

Benedikt Bünz

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