This may sound stupid (I'm pretty sure you can't) but I'm wondering if I have:
class a {}
class b
{
public:
a GetA(){return MyObject;}
private:
a MyObject;
};
Instead of GetA() returning MyObject as an object is it possible to return it as a pointer(*)?
Thanks
Yes.
As a pointer:
a* GetA() {return &MyObject;}
MyB.GetA()->DoSomethingToMyObject();
or as a reference:
a& GetA(){return MyObject;}
MyB.GetA().DoSomethingToMyObject();
Returning a pointer or a reference to internal class data is of course valid, but one has to be careful not to use the pointer or reference if the object is destroyed.
a *my_a_pointer;
{
b my_b;
my_a_pointer = b.GetA();
}
my_a_pointer->DoSomethingToMyObject(); // <-- Trouble! my_b has been destroyed already
This is not too different from what happens when you obtain a char const * from a string using std::c_str(). But it's still worth mentioning.
If you want to return a reference to the class member, I suppose the reason is that you want to modify the member. But if the reason is that the object is big, and you want to improve performance by not having to return a copy, it may be a good idea to return a const pointer to the object, or const reference.
Returning a pointer (const or not) is valid C++, but it violates some principles of object oriented programming. If you want the object updated, the usual principle to have the update logic in a member function instead.
To expose internal design of a class to the outside is usually a bad idea. That means you have an extra dependency, and dependencies should be kept to a minimum. Suppose you later find out that the data need to be managed in another way, you can no longer update only the class 'b'. You will also have to find all users of GetA(), and update them. If it is a library, it may not be possible, and the API is suddenly incompatible.
Maybe I'm nitpicky, but isn't it a bit wrong (and slightly confusing) to say that you "return an object as a pointer"?
Too me, that doesn't really make sense.
What you do is "returning a pointer that points to the object".
The pointer itself is an object too, a very different object then the class object it (maybe) points to.
I think that remembering this makes the answer more obvious.
This is an excellent point. And, while OT, another important one: people will talk about pass-by-value/pass-by-reference. in C/C++, everything is pass-by-value; it's just that sometimes, you pass values that are pointers, so the objects they represent don't copy.
in C/C++, everything is pass-by-value...
[/quote]
Please identify the value is being passed in the following code:
void frobnicate(int &example) {
example = 42;
}
in C/C++, everything is pass-by-value...
Please identify the value is being passed in the following code:
void frobnicate(int &example) { example = 42; }
I suppose he would've been correct if he said, "In C, everything is passed by value...."
But, not so for C++, as you point out.
