Hidden features of C++

fcarney

I find it amazing that hidden gems exist like this in C++.
Coding for dyslexics:

#include <QCoreApplication>
#include <QDebug>
#include <cstring>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    // reverse indexing
    char str[]  = "Hello World";
    char str2[] = "Beano Could";

    qInfo() << str;

    for(int i=0; i<strlen(str); ++i){
        int i2 = strlen(str) - 1;
        i[str] = (i2-i)[str2];
    }

    qInfo() << str;

    return a.exec();
}

VRonin

For the curious and lazy like me: http://cpp.sh/6jyb

Now you have to explain why though

fcarney

I wonder if the operator [] somehow distinguishes which parameter is which when called. I have no idea if it produces the same code when indexing str[i] versus indexing i[str].

kshegunov

@fcarney said in Hidden features of C++:

I find it amazing that hidden gems exist like this in C++

Why do you find it amazing, and what's gemmy about it?

Have fun with it: https://godbolt.org/z/gPiYxP
The compiler just treats everything as pointer(s) and does the pointer addition as needed.

fcarney

@kshegunov said in Hidden features of C++:

Why do you find it amazing, and what's gemmy about it?

Because its interesting and fun. It took a bit, but I found this explanation:

What square brackets really mean

Accessing an element of an array via ptr[3] is actually just short for *(ptr + 3). This can be equivalently written as *(3 + ptr) and therefore as 3[ptr], which turns out to be completely valid code.

Once its explained it makes complete sense. I was imagining some operator overloading function juxtaposition of terms nonsense.

fcarney

@kshegunov said in Hidden features of C++:

https://godbolt.org/z/gPiYxP

Ooh, that is a cool website. Assembler right in a browser. Thanks!

KroMignon

@fcarney that's nothing special, it just simple pointer operation/arithmetic

@VRonin cool website, didn't know it!

kshegunov

The guy with the site: https://www.youtube.com/watch?v=bSkpMdDe4g4 (also a very good talk).

VRonin

If you want to have more fun: https://www.ioccc.org/years.html
For example: https://www.ioccc.org/2018/burton1/prog.nowarn.c

fcarney

Here is another interesting site:
http://madebyevan.com/obscure-cpp-features/

JonB

i[str] = (i2-i)[str2];

I'm going to throw this in here, and doubtless wait for others to disagree: I think the compiler should generate a warning on i[str]. Do I realize it's the same as str[i] --- yes, I do. Do I realize it's just the same as *(i + str), which I wouldn't complain about --- yes, I do. Bit I'd still like to see a friendly, 2019, open source compiler warning about i not being indexable....

EDIT "I think the compiler should": maybe I meant "I expected the compiler to"....

fcarney

@JonB said in Hidden features of C++:

doubtless wait for others to disagree

Or the obligatory "My <insert language> doesn't do this. That is why it is superior."
cough cough rust cough cough... ;)

(disclaimer: I have never programmed rust)

fcarney

Now I am kinda scared:

qInfo() << 3<:"Foobar"];

Outputs 'b'.
But this:

std::cout << 3<:"Foobar"];

Crashes

I am really struggling as to how that translates to an index in the first case, but crashes in the second case.

fcarney

@fcarney said in Hidden features of C++:

std::cout << 3<:"Foobar"];

Nevermind, it needs "\n" to output. It was crashing cause I was killing it... doh! I rarely use cout anymore.

fcarney

Also,
digraphs

Why? Why?!

kshegunov

@fcarney said in Hidden features of C++:

digraphs

That's some bag-o-worms. My advice - don't dig there.

Kent-Dorfman

I think the take-away from this discussion is that if you want to write code to really piss off someone else who has to read or maintain it then use C++.

JonB

@Kent-Dorfman
No, C++ readability/maintenance has nothing on this little gem language, which has always been a favorite of mine: https://en.wikipedia.org/wiki/Brainfuck. Here, for example, is Hello World! in its entirety:

++++++++[>++++[>++>+++>+++>+<<<<-]>+>+>->>+[<]<-]>>.>---.+++++++..+++.>>.<-.<.+++.------.--------.>>+.>++.

And you can go to https://sange.fi/esoteric/brainfuck/impl/interp/i.html to paste it in and run :)

ODБOï

@JonB there is also Malbolge, but i don't thik this languages are designed to write real programs.

hello world

('&%:9]!~}|z2Vxwv-,POqponl$Hjig%eB@@>}=<M:9wv6WsU2T|nm-,jcL(I&%$#"
`CB]V?Tx<uVtT`Rpo3NlF.Jh++FdbCBA@?]!~|4XzyTT43Qsqq(Lnmkj"Fhg${z@>