Crthuky

PTIJ: Who should I vote for? (21st Knesset Edition)

how to write formula in word in latex

How can I track script which gives me "command not found" right after the login?

Does someone need to be connected to my network to sniff HTTP requests?

How to write cleanly even if my character uses expletive language?

Is it true that good novels will automatically sell themselves on Amazon (and so on) and there is no need for one to waste time promoting?

How to change two letters closest to a string and one letter immediately after a string using notepad++

How Could an Airship Be Repaired Mid-Flight

Look at your watch and tell me what time is it. vs Look at your watch and tell me what time it is

Why do passenger jet manufacturers design their planes with stall prevention systems?

Employee lack of ownership

If the DM rolls initiative once for a group of monsters, how do end-of-turn effects work?

Why Choose Less Effective Armour Types?

Should we release the security issues we found in our product as CVE or we can just update those on weekly release notes?

How to read the value of this capacitor?

How do I hide Chekhov's Gun?

Bach's Toccata and Fugue in D minor breaks the "no parallel octaves" rule?

Creature kill and resurrect effects on the stack interaction?

SOQL: Populate a Literal List in WHERE IN Clause

Interplanetary conflict, some disease destroys the ability to understand or appreciate music

Why did it take so long to abandon sail after steamships were demonstrated?

How to use deus ex machina safely?

Is this a real picture of Jordan Peterson in New Zealand with a fan wearing a shirt that says "I'm a Proud Islamaphobe"?

If I can solve Sudoku can I solve Travelling Salesman Problem(TSP)? If yes, how?

May I change the held type in a std::variant from within a call to std::visit

18

2

Does the following code invoke undefined behaviour?

std::variant<A,B> v = ...;

std::visit([&v](auto& e)
 if constexpr (std::is_same_v<std::remove_reference_t<decltype(e)>,A>)
 e.some_modifying_operation_on_A();
 else 
 int i = e.some_accessor_of_B();
 v = some_function_returning_A(i); 
 
, v);

In particular, when the variant does not contain an A,
this code re-assigns an A while still holding a reference to the previously held object of type B.
However, because the reference is not used anymore after the assignment,
I feel the code is fine.
However, would a standard-library be free to implement std::visit
in a way such that the above is undefined behaviour?

c++ c++17 std-variant

share|improve this question

edited 12 hours ago

Barry

184k21324595

asked 13 hours ago

burnpanckburnpanck

1,141622

• 
  
  
  

  
  5
  

  
  
  
  
  
  

  
  
  Do you want quotes from the standard to back up the answer(s) you get?
  
  – NathanOliver
  12 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  From looking at [variant.visit], I'm 99% sure this code is compliant and guaranteed not to have UB, since std::visit(vis, variant) should be equivalent to vis(get</* active member */>(variant)), but I'm not confident enough in reading the standard to be certain
  
  – Justin
  12 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @NathanOliver: I don't need actual quotes from the standard, as long as the experts here can agree on the answer:-).
  
  – burnpanck
  12 hours ago
  

  
  
  
  

add a comment | 

18

2

Does the following code invoke undefined behaviour?

std::variant<A,B> v = ...;

std::visit([&v](auto& e)
 if constexpr (std::is_same_v<std::remove_reference_t<decltype(e)>,A>)
 e.some_modifying_operation_on_A();
 else 
 int i = e.some_accessor_of_B();
 v = some_function_returning_A(i); 
 
, v);

In particular, when the variant does not contain an A,
this code re-assigns an A while still holding a reference to the previously held object of type B.
However, because the reference is not used anymore after the assignment,
I feel the code is fine.
However, would a standard-library be free to implement std::visit
in a way such that the above is undefined behaviour?

c++ c++17 std-variant

share|improve this question

edited 12 hours ago

Barry

184k21324595

asked 13 hours ago

burnpanckburnpanck

1,141622

• 
  
  
  

  
  5
  

  
  
  
  
  
  

  
  
  Do you want quotes from the standard to back up the answer(s) you get?
  
  – NathanOliver
  12 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  From looking at [variant.visit], I'm 99% sure this code is compliant and guaranteed not to have UB, since std::visit(vis, variant) should be equivalent to vis(get</* active member */>(variant)), but I'm not confident enough in reading the standard to be certain
  
  – Justin
  12 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @NathanOliver: I don't need actual quotes from the standard, as long as the experts here can agree on the answer:-).
  
  – burnpanck
  12 hours ago
  

  
  
  
  

add a comment | 

Does the following code invoke undefined behaviour?

std::variant<A,B> v = ...;

std::visit([&v](auto& e)
 if constexpr (std::is_same_v<std::remove_reference_t<decltype(e)>,A>)
 e.some_modifying_operation_on_A();
 else 
 int i = e.some_accessor_of_B();
 v = some_function_returning_A(i); 
 
, v);

In particular, when the variant does not contain an A,
this code re-assigns an A while still holding a reference to the previously held object of type B.
However, because the reference is not used anymore after the assignment,
I feel the code is fine.
However, would a standard-library be free to implement std::visit
in a way such that the above is undefined behaviour?

c++ c++17 std-variant

share|improve this question

edited 12 hours ago

Barry

184k21324595

asked 13 hours ago

burnpanckburnpanck

1,141622

std::variant<A,B> v = ...;

std::visit([&v](auto& e)
 if constexpr (std::is_same_v<std::remove_reference_t<decltype(e)>,A>)
 e.some_modifying_operation_on_A();
 else 
 int i = e.some_accessor_of_B();
 v = some_function_returning_A(i); 
 
, v);

share|improve this question

edited 12 hours ago

Barry

184k21324595

asked 13 hours ago

burnpanckburnpanck

1,141622

share|improve this question

edited 12 hours ago

Barry

184k21324595

asked 13 hours ago

burnpanckburnpanck

1,141622

edited 12 hours ago

Barry

184k21324595

edited 12 hours ago

Barry

184k21324595

asked 13 hours ago

burnpanckburnpanck

1,141622

asked 13 hours ago

burnpanckburnpanck

1,141622

1 Answer
1

active

oldest

votes

11

The code is fine.

There is no requirement in the specification of std::visit that the visitor not change the alternative of any of the variants it is invoked on. The only requirement is:

Requires: For each valid pack m, e(m) shall be a valid expression. All such expressions shall be of the same type and value category; otherwise, the program is ill-formed.

Your visitor is a valid expression for each m and always returns void, so it satisfies the requirements and has well-defined behavior.

share|improve this answer

answered 12 hours ago

BarryBarry

184k21324595

add a comment | 

Your Answer

StackExchange.ifUsing("editor", function ()
StackExchange.using("externalEditor", function ()
StackExchange.using("snippets", function ()
StackExchange.snippets.init();
);
);
, "code-snippets");

StackExchange.ready(function()
var channelOptions =
tags: "".split(" "),
id: "1"
;
initTagRenderer("".split(" "), "".split(" "), channelOptions);

StackExchange.using("externalEditor", function()
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled)
StackExchange.using("snippets", function()
createEditor();
);

else
createEditor();

);

function createEditor()
StackExchange.prepareEditor(
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: true,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: 10,
bindNavPrevention: true,
postfix: "",
imageUploader:
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
,
onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
);



);

draft saved

draft discarded

Sign up or log in

StackExchange.ready(function ()
StackExchange.helpers.onClickDraftSave('#login-link');

var $window = $(window),
onScroll = function(e)
var $elem = $('.new-login-left'),
docViewTop = $window.scrollTop(),
docViewBottom = docViewTop + $window.height(),
elemTop = $elem.offset().top,
elemBottom = elemTop + $elem.height();
if ((docViewTop elemBottom))
StackExchange.using('gps', function() StackExchange.gps.track('embedded_signup_form.view', location: 'question_page' ); );
$window.unbind('scroll', onScroll);

;
$window.on('scroll', onScroll);

);

Sign up using Google

Sign up using Facebook

Sign up using Email and Password

Post as a guest

Name

Email

Required, but never shown

StackExchange.ready(
function ()
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fstackoverflow.com%2fquestions%2f55187548%2fmay-i-change-the-held-type-in-a-stdvariant-from-within-a-call-to-stdvisit%23new-answer', 'question_page');

);

Post as a guest

Name

Email

Required, but never shown

11

The code is fine.

There is no requirement in the specification of std::visit that the visitor not change the alternative of any of the variants it is invoked on. The only requirement is:

Requires: For each valid pack m, e(m) shall be a valid expression. All such expressions shall be of the same type and value category; otherwise, the program is ill-formed.

Your visitor is a valid expression for each m and always returns void, so it satisfies the requirements and has well-defined behavior.

share|improve this answer

answered 12 hours ago

BarryBarry

184k21324595

add a comment | 

11

The code is fine.

There is no requirement in the specification of std::visit that the visitor not change the alternative of any of the variants it is invoked on. The only requirement is:

Requires: For each valid pack m, e(m) shall be a valid expression. All such expressions shall be of the same type and value category; otherwise, the program is ill-formed.

Your visitor is a valid expression for each m and always returns void, so it satisfies the requirements and has well-defined behavior.

share|improve this answer

answered 12 hours ago

BarryBarry

184k21324595

add a comment | 

The code is fine.

There is no requirement in the specification of std::visit that the visitor not change the alternative of any of the variants it is invoked on. The only requirement is:

Requires: For each valid pack m, e(m) shall be a valid expression. All such expressions shall be of the same type and value category; otherwise, the program is ill-formed.

Your visitor is a valid expression for each m and always returns void, so it satisfies the requirements and has well-defined behavior.

share|improve this answer

answered 12 hours ago

BarryBarry

184k21324595

share|improve this answer

answered 12 hours ago

BarryBarry

184k21324595

answered 12 hours ago

BarryBarry

184k21324595

answered 12 hours ago

BarryBarry

184k21324595