The definition of Object above should be:
class Object {
public:
int a;
};
BTW, this program as is, may work fine, but the error is exactly the sort of thing that will come and bite you when you don't expect it.
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Question
ReferenceandPointers
If in a class we store a pointer to an object and in its get
function return a reference to the object , in its set function take a reference to an object as input, is it going to fail under any condition?
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Answers
You wrote: "its set function take a reference to an object as input".
Who created this object? Who's responsible for deleting it? Is your own class suppose to adopt it? If so, you should make sure that:
1. It is heap allocated.
2. It will not be deleted by the part of the program that created it.
The possible error of someone taking the address of the object returned by get() and deleting it seems less likely to happen.
Some versions of set function:
X::SetObject(Object& obj);
X::SetObject(Object* obj);
I don't like using references to pass such objects. It is to easy to make one of stack, use the set function and get into trouble. Explicit use of pointers reminds me that I have to keep an eye on object ownership and lifetime.
I agree with comments above about taking ownership of objects in Set functions, pointers give you a better clue of what might happen to the object.
I also think the use of convention is very useful in these cases, for exaple we prefix functions that take ownership of an object (that is supposed to be heap allocated) with Adopt, so we would have:
X::AdoptObject(Object* obj)
just to give users of the class a further clue about what is expected.
The questioner doesn't say why he wants to do this, but maybe it is an efficiency thing. Scott Meyers talks about the ultimate futility of trying to avoid returning results by value in More Effective C++ (he also points out how Return Value Optimisation makes return by value more inexpensive than you might think).
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by: jasonclarkePosted on 1999-05-26 at 03:23:26ID: 1195215
Depends, you have to be very careful with lifetime and scope of objects when you do this kind of thing, for example:
class Object {
public:
};
class X {
public:
X() { thing = new Object; }
~X() { delete thing; }
Object* thing;
Object& getThing() { return *thing; }
};
main()
{
X* myx = new X;
Object& myThing = myx->getThing();
delete myx;
myThing.a = 2; // This could be very bad...
return 0;
}
In this example, the lifetime of the reference set up in the call to the get function is longer than the object that contains the pointer, and hence you have undefined behaviour when the reference is used after the object is destroyed.
It would be difficult to prevent this kind of thing with code, but it may be OK if you can enforce a suitable convention or if you know the circumstances in which the object is to be used.