Back to Basics: Understanding Value Categories - Ben Saks - CppCon 2019

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Discussion & Comments: / cpp
Presentation Materials: github.com/CppCon/CppCon2019
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Back to Basics: Understanding Value Categories
You may be familiar with the terms lvalue, rvalue, and xvalue, if only because they occasionally appear in compiler error messages. However, if you have only a vague understanding of what they mean, you're not alone. Lvalues, rvalues, and xvalues aren't actually language features, but rather semantic properties of expressions and sub-expressions. Understanding these properties provides valuable insights into the behavior of built-in operators, the code generated to execute those operators, and the meaning of some otherwise cryptic compiler error messages. Understanding these properties is also essential for working effectively with lvalue and rvalue reference types, as well as overloaded operators.
This session explains why the language distinguishes among lvalues, rvalues, and xvalues. It describes how the semantic properties of lvalues, rvalues, and xvalues are different and how those properties are reflected in the behavior of lvalue and rvalue references.
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Ben Saks
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Videos Filmed & Edited by Bash Films: www.BashFilms.com
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[@lsmgeb89]

After I have been reading some other materials about value categories, this lecture really helps me to understand them. Ben explained these concepts in a crystal clear way. Every modern C++ books should have this chapter.

[@me5ng3]

What materials were you reading about value categories? I'd love to read more on the subject

[@pmcgee003]

TFW your Basics lecture includes the term Temporary Materialisation Conversion.

[@Possseidon]

Ahhh! I knew what lvalue and rvalue where, but never got my head around glvalue, prvalues and xvalues. Now I finally got it! Absolutely great talk!

[@gustafbstrom]

This talk made how to think about C++ value catogories so much more clear to me. Thanks sir!

[@orlandoboffill7448]

Mr Ben Saks is a gifted teacher!

[@mikkqu]

This guy is awesome, I hope to see more of him in the future!

[@YourCRTube]

Great talk. Both approachable and exhaustive.

[@kamilziemian995]

Saks family is an family of experts in explaining C++ in a clear way.

[@DominoPivot]

I've been working on highly abstract languages for years so that was an excellent refresher.

[@saeedmahmoodi7211]

after 2 months of consusions i finally got it Geart job!

[@RyutlisWang]

Thank you Ben for such clear and thorough explanation of value categories, the best I have seen.

[@collapsingspace]

<a href="#" class="seekto" data-time="2894">48:14</a> For people wondering if std::move(T &&a) has rvalue reference to T as parameter how does it work with something like std::move(str) because str is here an lvalue and it's clearly told in the talk that references to rvalue types only bind to rvalues and not lvalues.. There's a deeper concept at play here note that std::move is templatized and the T&& a parameter actually becomes T&a when called like std::move(str)... you can find videos on type deduction on youtube.

[@toolmanp3885]

This lecture is truly insightful and easy to follow after I tries to study the value categories in cpp references. Huge thanks!

[@kamilziemian995]

Every C++ programmer should watch this video. With exception to Ben Saks and other superprogrammers that already know it content. 😉

[@jiaweihe1244]

I cannot find the slides file in the github link provided, is there any other place to get the slides?

[@TernaryHound]

Great lecture. I have used these types and move semantics for years and yet this gave me such a more comfortable understanding of the motivations behind these concepts. Well done!

[@ra1n_]

1 hour just flew by Whatta teacher !

[@SirToxe]

Excellent talk by Ben.

[@JasonMelton1]

Very helpful in untangling these concepts!

[@bboysil]

Perfect introduction to value categories.

[@tourdesource]

Clear as water now, thank you Ben!

[@dickpiano1802]

When people, who want to make things more complex, so others will be in awe get together you get the C++ universe.

[@zehaia]

Best explanation of this subject out there.

[@RahulRahul-pi5fm]

Thank you Ben for the presentation.

[@marketqueue8562]

Thanks.... Cleared all my confusion...

[@kamilziemian995]

I should watch all Ben Saks talks avaliable online.

[@guykeren9666]

Gifted! great talk, Helps to understand the basics (how much I didn't know)

[@anatheistsopinion9974]

Beyond excellent!

[@qasimijaz1664]

what a great talk, thanks Ben

[@pawanadhikari110]

The presentation content isn't available in the github .

[@unclechaelsneckvein]

Exceptionally good presentation.

[@konradkomisarczyk396]

Loved the talk

[@manuvaad]

Great talk!

[@aftostok6080]

This is great.

[@Skotopes666]

Great talk. Very helpful

[@paulozhang9346]

Amazing, thanks man.

[@CppCon]

Glad you liked it!

[@hanzheng173]

very helpful, thanks.

[@janpapaj4373]

Superb lecture!

[@CppCon]

Glad it was helpful!

[@Sabinagirl164]

Great talk

[@shoulderstack5527]

I didn't know std::thread had the ++ operator. If C++ didn't exist, and someone said they had a great idea for a language, and described C++ as it is today, no one would take it seriously. This is no criticism of the speaker, who has done a great job.

[@NonTwinBrothers]

<a href="#" class="seekto" data-time="2925">48:45</a> Alright I'll admit. Intentional or not, that one made me laugh

[@syedimadhaqqi4340]

I cannot find the presentation material. Can you please advise a direct link. Thanks

[@nguyendaison27]

Thank you.

[@CppCon]

You're welcome!

[@liveonphoenix5045]

@<a href="#" class="seekto" data-time="2588">43:08</a>, I don't understand the 'move' assignment overloaded operator that accepts 'rval' or 'temp-val', it is said that it will delete both previous 'lval' that became 'rval' or 'temp-val', whereupon both 's2' and 's3' become 'nullptr'? By moving, do they also mean emptying those two objects?

[@liveonphoenix5045]

Oh, I see, only use move semantic if we know the `src` is no longer required. As a reminder, never miss an excellent talk/seminar/lecture, Keep watching until the end.

[@abdelrhmanahmed1378]

int x = 6; int z = 10; int& p =x; p = 100; cout

[@D0Samp]

Because the target of a reference is defined at assignment and does not change afterwards. "p = z" does not change the reference from x to z, it just assigns the value of z similar to "p = 100". The pointer equivalent looks like this: int * const p = &x; *p = 100; *p = z; // "p = &z" does not work, since the pointer address is const *p = 200;