CODEWITHSUNDEEP
c++argument-passingthis-pointer
user63898
user63898

Reputation: 30885

C++ how to pass 'this' to pointer reference

i have main class that i like to pass its pointer reference to on of the objects i creating but it gives me error :

Error 1 error C2664: 'GameController::GameController(GameLayer *&)' : cannot convert parameter 1 from 'GameLayer *const ' to 'GameLayer *&'

what i have in the GameLayer ( the main object ) 

m_pGameController = new GameController(this);

and in the GameController i have this constructor 

GameController(GameLayer*& GameLayer)
{
 setGameLayer(gameLayer); // to GameLayer memeber ) 
}

the resone is i need to be able to modify the data in GameLayer from GameController (GUI stuff)

Upvotes: 5

Views: 15100

Answers (4)

Uchia Itachi
Uchia Itachi

Reputation: 5315

First, the type of this could be GameLayer * const or const GameLayer * const depending upon whether the member function is const or not. But, it is a const pointer.

Having said that, a reference to a pointer type *&ref means indicates that you will be modifying the pointer, the object which it is pointing to.

"Since, this pointer is a constant pointer you cannot assign that to a non-const pointer(GameLayer * p)" in the context of references.

In case of references to pointer, you cannot assign to a reference which will modify the value in this. You cannot assign it to a reference to a non-const pointer(GameLayer *&p), but you can assign a reference to a const pointer i.e. GameLayer *const &p.

So, changing the constructor to GameController(GameLayer * const & gameLayer) should work.
But I don't see any use of it here currently.

And if you're calling from a const member function then this has the type const *const so you will have to change it to GameController(const GameLayer * const & gameLayer).

Upvotes: 6

Steve Jessop
Steve Jessop

Reputation: 279245

"No Idea For Name"'s answer is almost certainly what you need, but for additional information the reason your code doesn't work is that GameLayer*& doesn't just mean, "I can modify the GameLayer object referred to by the argument value I receive", it means "I am passed a reference to a pointer to a GameLayer object I can modify, and also I can use this reference to change the pointer value itself".

So for example:

void foo(GameLayer *&layerptr) {
    layerptr = new GameLayer();
}

void main() {
    GameLayer *ptr = 0;
    std::cout << (void*)ptr << "\n";
    foo(ptr);
    std::cout << (void*)ptr << "\n";
}

The value of the ptr variable in main has changed. That's what pass-by-non-const-reference is for, and so you do not need pass-by-non-const-reference here. Either pass a pointer to a GameLayer object (type GameLayer*) or a reference (type GameLayer&), not both.

The reason it is not valid to pass this to a function that accepts GameLayer*& is that this is not a variable and it cannot be assigned to -- in C++ jargon it is not an lvalue and so you can't take an lvalue reference to it. this tells you what object the current member function is being called on, and you cannot suddenly decide that your member function will from now on be operating on a different object.

Upvotes: 1

alexbuisson
alexbuisson

Reputation: 8469

' this' is a pointer constant so you can pass it as reference or const reference.

 m_pGameController = new GameController(*this);

 GameController(GameLayer& GameLayer) {}

Upvotes: 1

No Idea For Name
No Idea For Name

Reputation: 11577

i don't know why are you using *& but it'll work fine if you do:

GameController(GameLayer* GameLayer)
{
 setGameLayer(gameLayer); // to GameLayer memeber ) 
}

Upvotes: 5

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