C++ question

kamransoomro84 · 02-20-2007, 11:03 AM

Hi. I was going through the code and saw this statement:

Logger &logger;

Can anybody tell me what kind of declaration is that? I've never come across it before.

Thanks.

xhi · 02-20-2007, 11:32 AM

well that is declaring a reference, but in that context alone it is wrong as a reference always must be initialized;

how about showing a little code that surrounds it..

kamransoomro84 · 02-21-2007, 03:02 AM

Well, I don't understand what declaring a reference like that means. I mean I don't understand what happens internally. It's not a function argument, if that's what you're wondering. It's a private variable of a class. What possible context could it be in?

dmail · 02-21-2007, 04:21 AM

Well if its a member then it is valid and the constructor will initialise it. A reference is like constant pointer to a type(on steroids); yet it does not use the pointer syntax.
http://www.parashift.com/c++-faq-lite/references.html

graemef · 02-21-2007, 04:24 AM

When an object of the class (that this member variable belongs to) is created the constructor will be required to create an instance of the Logger class. The private member variable logger will then be assigned to the instance of this object.

Think of a reference of an object the same as a pointer to an object. The big difference is that whilst a pointer can point to different objects the reference is fixed to the same object for the life of the reference (in this case the life of the object)

kamransoomro84 · 02-21-2007, 04:50 AM

>> The private member variable logger will then be assigned to the instance of this object.

Shouldn't it be the other way round? The constructor creates an instance of the Logger class and stores its reference in this variable?

If the above is correct, does this mean that this kind of reference is used to make sure that a particular member variable is initialised whenever an instance of the containing class is created?

graemef · 02-21-2007, 05:01 AM

Yes you are correct, sorry I wasn't very clear with my wording there.

This technique is used to ensure that a logger instance exists for the duration of the object.

I guess that it's not the most common technique to achieve this type of guarantee, you would probably more often see the use of singleton classes, which use a static variable and thus the object is available for the duration of the program.

kamransoomro84 · 02-21-2007, 05:11 AM

Hmm. It makes sense now. Thanks a lot everyone.

xhi · 02-21-2007, 12:28 PM

Quote:

Originally Posted by dmail

Well if its a member then it is valid and the constructor will initialise it. A reference is like constant pointer to a type(on steroids); yet it does not use the pointer syntax.
http://www.parashift.com/c++-faq-lite/references.html

a reference has to be initialized. so even if it is a member, the initial post is still wrong.

the only way a ref can stand alone is as a parameter.

edit> i stand corrected. as dmail pointed out the ref type can be initialized in the initializer list of the constructor..

tuxdev · 02-21-2007, 12:40 PM

It is not wrong to have a member reference. The reference must be initialized by the constructor in the initializer list, which satisfies the "all references must be initialized" requirement.

xhi · 02-21-2007, 01:18 PM

Quote:

Originally Posted by tuxdev

It is not wrong to have a member reference. The reference must be initialized by the constructor in the initializer list, which satisfies the "all references must be initialized" requirement.

true true. thanks for pointing that out, Tux.