' const & "use...

Hey guys,.

An another newbie question c#, I checked a few samples that uses ' const type & nom_de_variable.

So I thought... hmm what is it... I discovered that const is a constant, but... after I thought he thinks as a constant, it's a little weird to understand things, why do I have to declare variables constant functions?

My before and after examples had no difference to me:

Before:

MyPaths::MyPaths(QObject *parent, QVariant *currentRoute) : QObject(parent)
{
...
favorite = *currentRoute.toMap()["favorite"].toBool();
}

After:

MyPaths::MyPaths(QObject *parent, const QVariant ¤tRoute) : QObject(parent)
{
...
favorite = currentRoute.toMap()["favorite"].toBool();
}

I think that the front was theoretically bad... but the effect was the same... can someone explain to me if there is a good or bad on these two?

QVariant * currentRouter declares a pointer to QVariant.

Pointer to QVariant is an address of memory pointing to the object of type QVariant.

If you not pass the instance of QVariant itself, but its memory address.

* operator dereferences memory address. Then result of (* currentRoute) is the object itself. You may also experience this style: currentRoute-> name. It is essentially the same.

The variable pointer itself is not constant - you can assign the address of an another QVariant to it.

It can be made constant, but there is no reason to do so. It's just a local variable address.

On the second example you pass the reference object. Reference looks like another name for the object. It behaves exactly like the original object. It cannot be NULL, it still exists. You work with a reference as you work with the original object.

The references are initialized on the creation and cannot be reallocated later. If the references are always constant.

In this case reference points to constant object, so you cannot modify the object through this reference.

const QVariant ¤tRoute

Other examples:

QVariant * currentRoute;

V pointer to the object that is not constant.

Object can be changed: currentRoute-> setXxx();

Pointer that can be changed: currentRoute = & someOtherObject.

const QVariant * currentRoute;

V pointer to the constant object.

Object cannot be changed: currentRoute-> setXxx();

Pointer that can be changed: currentRoute = & someOtherObject.

const QVariant * currentRoute const;

Constant constant object pointer.

Cannot be changed.

QVariant & currentRoute = "SomeObject";

Object reference not constant.

Object can be changed: currentRoute.setXxx ();

Reference cannot be changed after initialization.

const QVariant & currentRoute = "SomeObject";

Reference to the constant object.

Object cannot be changed: currentRouter.setXxx ();

Reference cannot be changed after initialization.

That you prefer?

Well, if you need a way to pass NULL in function then use the pointer.

In most cases, the references are sufficient.

Compiler usually generates the same code for References and pointers.

Pointers have been used actively in C, references have been introduced later in C++.

Tags: BlackBerry Developers

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