operator[] difference of const and non-const

**phantomotap** · 07-10-2012

And a function that returns an iterator creates a temporary.

I'm fairly certain that you misunderstood what laserlight said.

The standard containers return different types (conceptually) of iterators by value depending on whether or not the container is constant in the given situation.

That the block content are fundamentally different, not just superficial differences?

O_o

Being "const correct" isn't a superficial difference.

Soma

**DynV** · 07-10-2012

Originally Posted by phantomotap

Originally Posted by DynV

That the block content are fundamentally different, not just superficial differences?

O_o

Being "const correct" isn't a superficial difference.

This was the important part

block content

perhaps the compiled versions are (much) different but certainly not the blocks as far as what I mentioned in #12.

**whiteflags** · 07-10-2012

The const version of the operator is read only. It's a semantic difference. The const operator will be used unless it specifically cannot be used -- like when the object is an lvalue -- and your constant objects will still be able to use the operator without violating their constness. It should be pretty easy, if you know what the subscript operator does, to imagine a bad line of code that changes an object that isn't supposed to be changed. Writing two operators varying in constness will prevent such code from being compiled.

**DynV** · 07-10-2012

Originally Posted by whiteflags

It should be pretty easy, if you know what the subscript operator does, to imagine a bad line of code that changes an object that isn't supposed to be changed. Writing two operators varying in constness will prevent such code from being compiled.

Sorry but I don't get this. :|

Would someone please just make me an example (or give URLs which someone with no prior knowledge of const & non-const difference would understand), or a few if there's different types, where const & non-const version of operator[] would have different block content. It would be most helpful.

**phantomotap** · 07-10-2012

O_o

Dude, just stop trying to fight it!

The code between the braces is irrelevant; the issue of having both overloads is about "const correctness".

The code may or may not be different depending on how the facility is intended to behave, but that's a meaningless focus of your confusion because these differences would still be about making the code "const correct".

Consider the often implementation of the standard containers (as I've already referenced); the two overloads return either completely different or semantically (conceptually) different types. The code in the blocks may still look identical but because the functions are returning different types they behave differently. That is all that matters. The one overload returns an iterator which is itself mutable and which can be used to mutate the contents of the container. The other overload (the `const' one) returns an iterator which is itself mutable but which can not be used to mutate the contents of the container.

You are focusing on a non-issue; having both overloads is about being "const correct". If the code needs to be different to make that possible it certainly can be different. If the code doesn't need to be different to make that possible it certainly does not have to be different.

Soma

**DynV** · 07-10-2012

Originally Posted by phantomotap

You are focusing on a non-issue

Yeah I'm just like that and I'm still focused on it, just like my OP mentioned:

Originally Posted by DynV

my question will remain the same, is there a difference in the code between the 2 beside something no-so-important like one display an error [...] and the other make a log

I know what you're saying about const correctness, that the const version should be the most important version and only to add the non-const version if needed but I'm still wondering what kind of code could be different ; not the signature part, the block part. Because for now having 2 versions of the same block, in my disarranged mind, only has use if 1 of the 2 should be forbidden, ie:

Code:

/* ..*/ operator[](/* ..*/) {
    assert(false);
}

**whiteflags** · 07-10-2012

I am of the opinion that you should implement both versions, because you need them.

If you didn't have both versions, how wrong would this program be?

Code:

#include <vector>
#include <iostream>

int main()
{
    const std::vector<int> foo(10, 42); // create ten 42s
    foo[1] = 40;                        // can't change them!

    std::vector<int> bar(10, 42);
    bar[1] = 40;                   // except when they're not const
}

It is extremely likely that both versions of the subscript operator are apparently the same. This is because the subscript operator has a simple meaning.

**laserlight** · 07-10-2012

Originally Posted by King Mir

A temporary binds to a const reference, never a non-const reference. And a function that returns an iterator creates a temporary.

And the parameter of both overloads of operator[] is of const reference type, hence what you are talking about does not matter.

Originally Posted by King Mir

Member function lookup is a little different; you can call a non const method on a temporary.

Exactly.

EDIT:

Originally Posted by whiteflags

If you didn't have both versions, how wrong would this program be?

Also, if your container is designed to be immutable, then you might provide only the const overload. As you should have seen earlier in this thread, it would still work when called with a non-const container.

**DynV** · 07-10-2012

Originally Posted by whiteflags

t is extremely likely that both versions of the subscript operator are apparently the same. This is because the subscript operator has a simple meaning.

I will assume this is what I was looking for ; that it's extremely unlikely that what my OP suggested (block of const & non-const operator[] be different) happen. So I might was well put that in my mind for when I get very advanced in C++.

Thanks again

Although I'm not so happy about you because after watching your signature earlier, I spent 30 minutes listening to chiptune.

**iMalc** · 07-10-2012

Just remember though, that some classes will intentionally only have say the const version.
e.g. if you had a class that wrapped a read-only connection to a database.
So it's not about whether they have the same behaviour or not, it's about saying which of the two pieces of functionality is your class meant to provide. Unfortunately for those classes that intend to expose both, there can be some code duplication. It's usually small though.

**DynV** · 07-11-2012

Originally Posted by iMalc

Just remember though, that some classes will intentionally only have say the const version.
e.g. if you had a class that wrapped a read-only connection to a database.

That's what I understood from the concept right before creating this thread but the more examples, the merrier.

I was about to forget: I did search on the topic before creating a new thread but the search algorithm isn't what I'm used to

; there was ~1 related topic per result page, as seen in the attached image.

**whiteflags** · 07-11-2012

I'm not really sure how you searched. The search algorithm is actually pretty good. But you have to use advance search if you want to control where you search. In my experience, search by default looks for the keywords in the whole post, in all of the open forums. If you want to search by title in the C++ Programming forum or other things like that, go to advanced.

**DynV** · 07-11-2012

I used advanced, as the search criteria show on top of the image. I also use Search Titles Only, either one of us had luck going for or against, I'm missing something unusual (I'm not a stranger to such search) or the algorithm changed.

**whiteflags** · 07-11-2012

The forum software does change rather constantly. The search method that works best for me is in advanced like you've been using. I like to search single content type, too, because that makes sense to me. I haven't really figured out what multiple content type searches are supposed to do, unless you want to search in, like, visitor messages. And who cares about that. Anyway, the default is for multiple content type searches so you will either need to figure out the form on that page, or switch to single content type. From there you only need to figure out the form to search successfully. Maybe you've already done this

If I repeat your search on the single content type tab, I get this: http://cboard.cprogramming.com/searc...earchid=499297 -- four pages of operator overloading threads. If you continue to have significant trouble, you could always pm a moderator.