Archived

This topic is now archived and is closed to further replies.

empty references

This topic is 5359 days old which is more than the 365 day threshold we allow for new replies. Please post a new topic.

Recommended Posts

I tried to put an empty reference in C++ just like in Java or C#.
class Foo
{
MyClass&  otherClass;

public:
void SetMyClass( MyClass& other )
{
otherClass = other;
}
}; 
But then the compiler gives me an error saying that I have to initialize otherClass in the ctor. But I don't want to initialize it in ctor. Even if I wanted to, how do I initialize it? otherClass is supposed to be referencing an object outside, it's not supposed to be instantiated. I tried to create a ctor that accepts MyClass& argument, it still gives me the error. The compiler expects it to be initialized in the initializer list. Any help please? [edited by - alnite on November 14, 2003 12:10:56 PM]

Share on other sites
If for some reason you cannot initialise the reference at the point of the object's construction, or what to change what it is bound to, use a pointer (MyClass*) instead. You cannot change what a reference is bound to, yet you can assign a new address to a pointer.

[ Google || Start Here || ACCU || STL || Boost || MSDN || GotW || CUJ || MSVC++ Library Fixes || BarrysWorld || E-Mail Me ]

[edited by - Lektrix on November 14, 2003 12:14:37 PM]

Share on other sites
Oh goody! A question I might be able to answer! References in C++ are not a replacement for pointers. A reference always has to refer to something that actually exists, from the time of initialization, to it’s end. That means you have to initialize a reference to some object.

If ever a reference can point to nothing, it’s a sure sign you need a pointer instead. Pointers can point to null, or random garbage if you want. I suppose you could initialize the reference to a dummy value. But I wouldn’t.

Bah! Lektrix! Trust someone to beat me to it with a concise and yet complete answer. :/
[/edit]

Actually, a concise, complete, but not entirely correct answer:
quote:

You cannot change what a reference is bound to...

Here is an example of Nonsense: (See posts below for comments)
[/edit]
int int1 = 0;int int2 = 1;int &refint = int1;refint = int1;refint = int2;refint = int1;refint = int2;// etc.

Are you thinking of const references?
const int &const_refint = int1;const_refint = int2; // The compiler won't like this. Assigning to a const l-value indeed!

[/edit]

Ro_Akira

P.S. Edits eat "source" tags for breakfast.

[edited by - Ro_Akira on November 14, 2003 12:13:17 PM]

[edited by - Ro_Akira on November 14, 2003 12:20:02 PM]

[edited by - Ro_Akira on November 14, 2003 12:34:01 PM][/source]

[edited by - Ro_Akira on November 14, 2003 12:35:24 PM]

[edited by - Ro_Akira on November 14, 2003 12:36:18 PM]

[edited by - Ro_Akira on November 14, 2003 1:03:24 PM]

Share on other sites
Hi alnite.

Section 8.3.2 paragraph 4 of the C++ standard states :

quote:

The decla-ration of a reference shall contain an initializer (8.5.3) except when the declaration contains an explicit
extern specifier (7.1.1), is a class member (9.2) declaration within a class declaration, or is the declara-tion
of a parameter or a return type (8.3.5); see 3.1. A reference shall be initialized to refer to a valid object
or function. [Note: in particular, a null reference cannot exist in a well-defined program, because the only
way to create such a reference would be to bind it to the “object” obtained by dereferencing a null pointer,
which causes undefined behavior. As described in 9.6, a reference cannot be bound directly to a bit-field. ]

So in short, you need to initialize it. The only way you can initialize it when it is a class member is through the initializer list, so your stuck with it .

Share on other sites
quote:
Original post by Lektrix
If for some reason you cannot initialise the reference, use a pointer (MyClass*) instead.
But I hate the indirection operator (->) it''s just not pretty to look at.

Share on other sites
Or maybe I could write a pointer_reference class, a class that can be used both as a pointer and a reference...something like smart pointers, except that it doesn''t delete the pointer.

Oh wait, I can''t overload . (dot) dang it!

Share on other sites
Well if you dont like -> you could always just not use it

pclass->member or
*pclass.member

But i think -> looks a lot better

Share on other sites
something i have never understood about references, i have always seen everyone say a reference cannot point to null, it always has to point to an object

so what happens in this case if there is no memory to allocate?

MyClass & obj = new(std::nothrow) MyClass;

?

Share on other sites
quote:

Well if you dont like -> you could always just not use it

pclass->member or
*pclass.member

But i think -> looks a lot better

Actually, it's worse than that. My compiler at least, MSVC++ 6.0 SP5, likes (*pclass).member. And it's quite right too. Consider *pclass.myint = 5; This is really *(pclass.myint) = 5; That's assigning 5 to the deferenced pointer pclass.myint. "But pclass.myint isn't a pointer!" - compiler.

quote:

something i have never understood about references, i have always seen everyone say a reference cannot point to null, it always has to point to an object

so what happens in this case if there is no memory to allocate?

MyClass & obj = new(std::nothrow) MyClass;

I'm putting a bet on "Crash".

Consider also:

int &myclass::returnint (){   return *pint; // pint is a pointer to an int. If it's pointing to null, or some other God forsaken place...}

Ro_Akira

[edited by - Ro_Akira on November 14, 2003 12:30:16 PM]

Share on other sites
Original post by Ro_Akira
quote:

You cannot change what a reference is bound to...

Nonsense.
int int1 = 0;int int2 = 1;int &refint = int1;refint = int1;refint = int2;refint = int1;refint = int2;// etc.

ummm, surely that is just assigning the value, not changing what the reference refers to..

1. 1
2. 2
Rutin
22
3. 3
JoeJ
20
4. 4
5. 5

• 27
• 40
• 23
• 13
• 13
• Forum Statistics

• Total Topics
631735
• Total Posts
3001941
×