Create new object from an unknown class

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Hi I say my problem in an example.
for example i have an pointer from an unknown class. aPointer;
I just know it's base class is class ABaseClass;
now I another pointer : ABaseClass* bPointer;
now I want create an object from aPointer's class to bPointer;
can I do that ?

Posted 2-Jan-13 0:21am

mohammadali1375

Updated 2-Jan-13 0:31am

v2

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Comments

Jibesh 2-Jan-13 6:30am

your questions is not clear and its confused. what do you mean creating an instance of the same class type? do you have any sample code to share?

mohammadali1375 2-Jan-13 6:32am

I mean Object; Create new Object

Richard MacCutchan 2-Jan-13 6:41am

Unless there is some information that you can get from the pointer then it is not possible. C++ classes do not support reflection in the same way that C# and Java do.

mohammadali1375 2-Jan-13 6:42am

oh. so I should solve my problem with another way.
Thanks

Maximilien 2-Jan-13 7:30am

I don't think you can.

3 solutions

Solution 3

if u want make component base system for game or plugin
this can help u
:)

C++

class Component
{
public:
	virtual Component* getNewInstance(){ return new Component;}
	virtual const char* getName(){ return "Component";}
	bool isEqual(Component* _other){ return (getName() == _other->getName()); }
}

class MyComponent1 : Component
{
	Component* getNewInstance();
	const char* getName();
}

class MyComponent2 : Component
{
	Component* getNewInstance();
	const char* getName();
}

Component* MyComponent1::getNewInstance(){ return new MyComponent1;}
Component* MyComponent1::getName(){ return "MyComponent1";}
Component* MyComponent2::getNewInstance(){ return new MyComponent2;}
Component* MyComponent2::getName(){ return "MyComponent2";}

Component* g_cmp[4];
g_cmp[0] = new MyComponent1;
g_cmp[1] = new MyComponent2;
g_cmp[2] = g_cmp[0]->getNewInstance();	//MyComponent1
g_cmp[3] = g_cmp[1]->getNewInstance();	//MyComponent2
std::cout << g_cmp[0]->getName() << '\n';	//MyComponent1
std::cout << g_cmp[1]->getName() << '\n';	//MyComponent2
std::cout << g_cmp[0]->isEqual(g_cmp[2]) << '\n';	//true

Posted 2-Jan-13 4:03am

__UFNHGGI_H__

Solution 1

Quote:
now I want create an Instance from aPointer's class to bPointer;

What do you mean exactly?

You may, of course, write:

C++

ABaseClass * bPointer = aPointer;

Is this what you are asking for?

Posted 2-Jan-13 0:28am

CPallini

Comments

mohammadali1375 2-Jan-13 6:33am

no . I want create new object from apointer. but I dont Know whats taht class name

Jibesh 2-Jan-13 6:54am

aPointer means your object is already created. can you write your expected code. 'create a new object from pointer' to read your mind.

CPallini 2-Jan-13 7:01am

Technically the existence of a pointer doesn't imply the existence of a pointed object.

Jibesh 2-Jan-13 7:04am

your are right. i was under the impression that aPointer is already created.

CPallini 2-Jan-13 7:33am

For instance you may write
ABaseClass * bPointer = new ABaseClass(*dynamic_cast<ABaseClass*>(aPointer));
But, again, I don't know if it is what do you want.

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nv3 · Accepted Answer · 2013-01-02T01:51:00

Solution 2

No, there is no way in C++ to create an object of the same type when just having a base class pointer to a similar object.

This is what is usually done by a so-called factory function or class. In the cpp file that implements your derived class's object you could for example implement a function

MyObject* MyObjectFactory()
{
    return new MyObject;
}

and pass along a pointer to this function to places that want to create such objects.

Google the concept of Factory and you will find plenty of good examples how it is done.

Another approach is to implement a virtual function in the base class for creating objects of same type. Then override that function in each of your derived classes to create the proper type of object. This is in fact also kind of a factory mechanism.

Posted 2-Jan-13 1:51am

nv3

Updated 2-Jan-13 1:54am

v2

Comments

mohammadali1375 2-Jan-13 8:34am

I think your right. and I do that; but now I get these error :
Error 8 error LNK1120: 1 unresolved externals C:\Users\mohammadali1375\documents\visual studio 2010\Projects\MyEngine\MEngine\Debug\MEngine.exe 1

Error 7 error LNK2001: unresolved external symbol "public: virtual class MEngineComponent * __thiscall MEngineComponent::factory(void)" (?factory@MEngineComponent@@UAEPAV1@XZ) C:\Users\mohammadali1375\documents\visual studio 2010\Projects\MyEngine\MEngine\MEngine\main.obj

mohammadali1375 2-Jan-13 8:39am

does it true ? :
class TestComponent : public MEngineComponent
{
public:
TestComponent()
{
}
virtual MEngineComponent* factory()
{
return new TestComponent();
}
};

----

class MEngineComponent
{
public:
MEngineComponent(){}
virtual MEngineComponent* factory();

};

nv3 2-Jan-13 9:06am

You probably forgot to define the factory function for MEngineComponent as well. Either define it there, or make it abstract, e.g.

class MEngineComponent
{
...
virtual MEngineComponent: factory() = 0;
...
};

mohammadali1375 2-Jan-13 9:32am

Oh Yea . THANKS ALOT
You've solved my big problem. ; THANKS