random access iterator

[George2]

Hello everyone,


For random access iterator, operator[] is supported. Mentioned in Bjarne's book, Chapter 19 (Iterators and Allocators).

I have not used operator[] on random access iterator before and I have not found a good and simple sample either. :-)

Could anyone show its usage with some pseudo code please?


thanks in advance,
George

[laserlight]

For random access iterator, operator[] is supported. Mentioned in Bjarne's book, Chapter 19 (Iterators and Allocators).

What exactly did Stroustrup write? Are you sure he did not say "for random access, operator[] is provided"?

[George2]

Hi laserlight,


If you have the book at hand, you can find section 19.2.1, there is a table about iterator operations and categories. For random access iterator, -> and [] are supported.

What exactly did Stroustrup write? Are you sure he did not say "for random access, operator[] is provided"?

regards,
George

[Raigne]

"supported" not provided. You can overload them to allow random access. They are not made that way.

[CornedBee]

There's no difference. Random access iterators have to implement [] with the logical semantics. The semantics are equivalent to those of a pointer.

As such, it[3] and *(it+3) are functionally equivalent.

[George2]

Thanks CornedBee,


Your description is clear.

There's no difference. Random access iterators have to implement [] with the logical semantics. The semantics are equivalent to those of a pointer.

As such, it[3] and *(it+3) are functionally equivalent.

Hi Raigne,


I have no idea what you mentioned. Could you provide some pseudo code please?


regards,
George

[CornedBee]

Pseudo code? For what? it[3] and *(it+3) are equivalent. You should know what it+3 does. You should know what dereferencing the result does. Thus, you should know what the whole right expression does, and thus you know what the indexing operator does.

[George2]

Hi CornedBee,


I do not mean pseudo code for your reply. I mean pseudo code for Raigne's reply in post #4. I do not know what he means. :-)

Pseudo code? For what? it[3] and *(it+3) are equivalent. You should know what it+3 does. You should know what dereferencing the result does. Thus, you should know what the whole right expression does, and thus you know what the indexing operator does.

regards,
George

[jian2587]

Raigne meant you have to DIY (he/she probably meant override not overload too. correct me if i'm wrong). i.e. we can't just call [] on an object and expect it to do what we want. for example you're creating a string class. you probably want [] to access the individual characters in your string. the compiler doesn't know how to do that, because there can be many different implementations of a string.

i've a question here too. if i have a class A, and I override [], and then I instantiate an array of As, how do I access the individual A instances, and how do I access the [] operator I overrided?

[CornedBee]

He meant overload, which is what you do with operators. Overriding is what you do with virtual functions you inherited.

Regarding your question, the [] applied to the array will get you a specific instance. The overloaded [] only comes into play when you apply it to a specific instance.

[jian2587]

that makes perfect sense! thanks cornedbee!
though, what if i want to access the overloaded [] from a specific instance of an array?
is this what i do?

myclasses[a][b] = something; // myclasses is an array of objects

[CornedBee]

Yes. The first [] applies to the array, the second to the instance.

[George2]

Thanks CornedBee,


Do you mean in the sample code,

myclasses[a][b] = something;

operator[] is invoked by the a-th instance of the array, and b and something are two input parameters for operator[] for the a-th element of the instance array?

Yes. The first [] applies to the array, the second to the instance.

Hi jian2587,

for example you're creating a string class. you probably want [] to access the individual characters in your string. the compiler doesn't know how to do that, because there can be many different implementations of a string.

I do not know what you mean above. Could you show some pseude code please?

BTW: operator[] on string works good. Here is my test code, I do not know why you say it can not work.

#include <string>

using namespace std;

int main()
{
	string str ("abcdefg");

	char ch = str[3]; // ch is 'd'

	return 0;
}

regards,
George

[jian2587]

oh george2, i actually meant your OWN implementation of a string class. sorry for the confusion.

[George2]

Hi CornedBee,


Any comments to my confusions in the following part in post #13?

--------------------
Do you mean in the sample code,

myclasses[a][b] = something;

operator[] is invoked by the a-th instance of the array, and b and something are two input parameters for operator[] for the a-th element of the instance array?
--------------------

oh george2, i actually meant your OWN implementation of a string class. sorry for the confusion.

regards,
George