Is it possible to initialize reference variables later?

Question

Best explained with an example:

Class banana {
    int &yumminess;
    banana::banana() {
        //load up a memory mapped file, create a view
        //the yumminess value is the first thing in the view so
        yumminess = *((int*)view);
    }
}

But that doesn't work :/ there is no way I can know where the view is going to be when I dreclare the "yumminess" reference variable. Right now i just use a pointer and dereference it all the time, is there any way to bring this little extra bit of convenience to my class?

declare yumminess as `int* yumminess` and initialise it to zero in the constructor — Richard Hodges, Jul 07 '14 at 16:22
A reference must be initialized to something from the moment it starts existing. You would have to initialize it in the constructor initialization list. — juanchopanza, Jul 07 '14 at 16:22
You can make a function to load up the file and create the view. — chris, Jul 07 '14 at 16:29
You could also have a dummy pointer and a reference to it. `int* dummy = nullptr; int &ref = *dummy;` Though, it's extra work and memory. — Brandon, Jul 07 '14 at 16:49
Related: https://stackoverflow.com/questions/14882995/declare-a-reference-and-initialize-later — Ciro Santilli OurBigBook.com, Jul 08 '20 at 11:12

πάντα ῥεῖ · Accepted Answer · 2014-07-07T18:21:41.343

6

In short: No, it's intentionally not possible.

Think twice: Something like uninitialized references cannot really exist; such wouldn't make sense at all.
Thus they'll need to be set at the time of construction of the enclosing class, or at a point of static initialization.

You'll need to use pointers for such case.

Besides note that

 yumminess = (int*)view;

would be wrongly casted (to a pointer) anyway.

"Right now i just use a pointer and dereference it all the time ..."

That's also easy to overcome writing an appropriate member function to access the reference.

int* yumminess;

// ...

int& yumminessRef() {
    if(!yumminess) { 
        throw some_appropriate_exception("`yumminess` not initialized properly.");
    }
    return *yumminess;
}

edited Jul 07 '14 at 18:21

answered Jul 07 '14 at 16:23

πάντα ῥεῖ

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That's an entirely _inappropriate_ getter [member function]. – Lightness Races in Orbit Jul 07 '14 at 16:37
yea just noticed that. I think I got an idea though, I'll try make another class "convenientBanana" that inherits the regular banana and gets the addresses necessary for ref initialization from there. – Jake Freelander Jul 07 '14 at 16:37
@LightnessRacesinOrbit Yes in sense of a _getter_ method that was wrong. Reworded a bit. – πάντα ῥεῖ Jul 07 '14 at 16:39
I still wish you'd stop misusing the word "method". – Lightness Races in Orbit Jul 07 '14 at 16:44
@LightnessRacesinOrbit Sorry, I was taught so over 25 years ago, and still tend to use the term (what you've once learned). I think they used the term, to not confuse us with what we already were knowing about c functions at that time. C++ and OOP was all pretty new those days ;) ... – πάντα ῥεῖ Jul 07 '14 at 16:56
@πάνταῥεῖ Well so was I and so were many of us; doesn't mean you have to keep repeating the wrong stuff and teaching it to the next generation. :) – Lightness Races in Orbit Jul 07 '14 at 17:06
@LightnessRacesinOrbit I'll try to keep my wording cleaner in the future, THX for helping improving myself. – πάντα ῥεῖ Jul 07 '14 at 17:53
@KarliRaudsepp Interestingly enough, 2 of my colleagues asked me about almost the same thing today. There are ways, to initialize references from inherited classes of course. You can pass a reference to a member instantiated in an inherited class to a base class constructor in the member initializer list. But to clarify about your obstacles doing this in particular, I'd recommend to ask another question. – πάντα ῥεῖ Jul 07 '14 at 17:59

score 2 · Answer 2 · answered Jul 07 '14 at 16:23

2

No, not directly.

If you think the pointer is inconvenient, have a look at std::optional.

answered Jul 07 '14 at 16:23

Christian Hackl

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Is it possible to initialize reference variables later?

2 Answers2