Crthuky

Generating adjacency matrices from isomorphic graphs

Reply 'no position' while the job posting is still there

Can the Supreme Court overturn an impeachment?

Indicating multiple different modes of speech (fantasy language or telepathy)

Why has "pence" been used in this sentence, not "pences"?

Folder comparison

Should I install hardwood flooring or cabinets first?

Divine apple island

What is this type of notehead called?

Can I rely on this github repository files?

Structured binding on const

Flux received by a negative charge

How do ground effect vehicles perform turns?

How do I repair my stair bannister?

How much character growth crosses the line into breaking the character

Can a significant change in incentives void an employment contract?

Why is Arduino resetting while driving motors?

Greatest common substring

When quoting, must I also copy hyphens used to divide words that continue on the next line?

Is it possible to have a strip of cold climate in the middle of a planet?

What (else) happened July 1st 1858 in London?

Did US corporations pay demonstrators in the German demonstrations against article 13?

How do I extrude a face to a single vertex

Some numbers are more equivalent than others

Structured binding on const

What is the difference between const int*, const int * const, and int const *?String literal in templates - different behavior of compilersstructured binding with [[maybe_unused]]Capturing array of vectors in lambda makes elements constExplicit destructor call with decltypeWhy “int & const” compiles fine with MSVC?Discards qualifiers unknown cause (std::bind() / lambda)Visual accept std::string from std::byte iteratorC++17 - Binding rvalue reference to non-const lvalue refShall structured binding to a copy of a const c-array be const?

7

Is the following code supposed to compile?

void foo() 
 const std::pair<int, int> x = 1, 2;

 auto [a, b] = x;

 static_assert(std::is_const_v<decltype(a)>);
 static_assert(std::is_const_v<decltype(b)>);

• 
  MSVC says "yes!".


• 
  GCC says "oh no, man!".


• 
  Clang says "no way!".

---

So, is this an MSVC bug?

The standard is not straightforward here (I had a quick look), but considering the rules for auto, I suppose, a and b should be copied discarding cv-qualifier.

c++ c++17 structured-bindings

share|improve this question

asked 6 hours ago

Biagio FestaBiagio Festa

5,19321239

add a comment | 

7

Is the following code supposed to compile?

void foo() 
 const std::pair<int, int> x = 1, 2;

 auto [a, b] = x;

 static_assert(std::is_const_v<decltype(a)>);
 static_assert(std::is_const_v<decltype(b)>);

• 
  MSVC says "yes!".


• 
  GCC says "oh no, man!".


• 
  Clang says "no way!".

---

So, is this an MSVC bug?

The standard is not straightforward here (I had a quick look), but considering the rules for auto, I suppose, a and b should be copied discarding cv-qualifier.

c++ c++17 structured-bindings

share|improve this question

asked 6 hours ago

Biagio FestaBiagio Festa

5,19321239

add a comment | 

Is the following code supposed to compile?

void foo() 
 const std::pair<int, int> x = 1, 2;

 auto [a, b] = x;

 static_assert(std::is_const_v<decltype(a)>);
 static_assert(std::is_const_v<decltype(b)>);

• 
  MSVC says "yes!".


• 
  GCC says "oh no, man!".


• 
  Clang says "no way!".

---

So, is this an MSVC bug?

The standard is not straightforward here (I had a quick look), but considering the rules for auto, I suppose, a and b should be copied discarding cv-qualifier.

c++ c++17 structured-bindings

share|improve this question

asked 6 hours ago

Biagio FestaBiagio Festa

5,19321239

void foo() 
 const std::pair<int, int> x = 1, 2;

 auto [a, b] = x;

 static_assert(std::is_const_v<decltype(a)>);
 static_assert(std::is_const_v<decltype(b)>);

share|improve this question

asked 6 hours ago

Biagio FestaBiagio Festa

5,19321239

share|improve this question

asked 6 hours ago

Biagio FestaBiagio Festa

5,19321239

asked 6 hours ago

Biagio FestaBiagio Festa

5,19321239

asked 6 hours ago

Biagio FestaBiagio Festa

5,19321239

2 Answers
2

active

oldest

votes

8

Is the following code supposed to compile?

It is not. This is an MSVC bug.

A structured binding declaration introduces a new name (for specification only), e, that is declared like:

auto e = x;

The type of e is called E, and since the initializer is tuple-like, the types of the bindings are given by tuple_element_t<i, E>. In this case E is pair<int, int>, so the two types are just int. The rule for decltype of a structured binding is to give the referenced type, so decltype(a) and decltype(b) are both int.

The important part here is that a and b (the structured bindings) come from the invented variable (e), and not its initializer (x). e is not const because you just declared it auto. What we're doing is copying x, and then taking bindings into this (non-const) copy.

share|improve this answer

answered 6 hours ago

BarryBarry

185k21325600

add a comment | 

3

The static assertions should fail (so this would be an MSVC bug I guess). Why? Because it's basically the same as the case of:

void foo() 
 const int x_1 = 1;
 const int x_2 = 2;

 auto a = x_1;
 auto b = x_2;

 static_assert(std::is_const_v<decltype(a)>);
 static_assert(std::is_const_v<decltype(b)>);

which does indeed fail on MSVC as well.

In C++ expression types decay on assignment: the auto sees an int, not a const int. Structured binding simply lets you do more than a single auto binding at a time.

share|improve this answer

edited 6 hours ago

answered 6 hours ago

einpoklumeinpoklum

36.1k28132260

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%2f55329651%2fstructured-binding-on-const%23new-answer', 'question_page');

);

Post as a guest

Name

Email

Required, but never shown

8

Is the following code supposed to compile?

It is not. This is an MSVC bug.

A structured binding declaration introduces a new name (for specification only), e, that is declared like:

auto e = x;

The type of e is called E, and since the initializer is tuple-like, the types of the bindings are given by tuple_element_t<i, E>. In this case E is pair<int, int>, so the two types are just int. The rule for decltype of a structured binding is to give the referenced type, so decltype(a) and decltype(b) are both int.

The important part here is that a and b (the structured bindings) come from the invented variable (e), and not its initializer (x). e is not const because you just declared it auto. What we're doing is copying x, and then taking bindings into this (non-const) copy.

share|improve this answer

answered 6 hours ago

BarryBarry

185k21325600

add a comment | 

8

Is the following code supposed to compile?

It is not. This is an MSVC bug.

A structured binding declaration introduces a new name (for specification only), e, that is declared like:

auto e = x;

The type of e is called E, and since the initializer is tuple-like, the types of the bindings are given by tuple_element_t<i, E>. In this case E is pair<int, int>, so the two types are just int. The rule for decltype of a structured binding is to give the referenced type, so decltype(a) and decltype(b) are both int.

The important part here is that a and b (the structured bindings) come from the invented variable (e), and not its initializer (x). e is not const because you just declared it auto. What we're doing is copying x, and then taking bindings into this (non-const) copy.

share|improve this answer

answered 6 hours ago

BarryBarry

185k21325600

add a comment | 

Is the following code supposed to compile?

It is not. This is an MSVC bug.

A structured binding declaration introduces a new name (for specification only), e, that is declared like:

auto e = x;

The type of e is called E, and since the initializer is tuple-like, the types of the bindings are given by tuple_element_t<i, E>. In this case E is pair<int, int>, so the two types are just int. The rule for decltype of a structured binding is to give the referenced type, so decltype(a) and decltype(b) are both int.

The important part here is that a and b (the structured bindings) come from the invented variable (e), and not its initializer (x). e is not const because you just declared it auto. What we're doing is copying x, and then taking bindings into this (non-const) copy.

share|improve this answer

answered 6 hours ago

BarryBarry

185k21325600

auto e = x;

share|improve this answer

answered 6 hours ago

BarryBarry

185k21325600

answered 6 hours ago

BarryBarry

185k21325600

answered 6 hours ago

BarryBarry

185k21325600

3

The static assertions should fail (so this would be an MSVC bug I guess). Why? Because it's basically the same as the case of:

void foo() 
 const int x_1 = 1;
 const int x_2 = 2;

 auto a = x_1;
 auto b = x_2;

 static_assert(std::is_const_v<decltype(a)>);
 static_assert(std::is_const_v<decltype(b)>);

which does indeed fail on MSVC as well.

In C++ expression types decay on assignment: the auto sees an int, not a const int. Structured binding simply lets you do more than a single auto binding at a time.

share|improve this answer

edited 6 hours ago

answered 6 hours ago

einpoklumeinpoklum

36.1k28132260

add a comment | 

3

The static assertions should fail (so this would be an MSVC bug I guess). Why? Because it's basically the same as the case of:

void foo() 
 const int x_1 = 1;
 const int x_2 = 2;

 auto a = x_1;
 auto b = x_2;

 static_assert(std::is_const_v<decltype(a)>);
 static_assert(std::is_const_v<decltype(b)>);

which does indeed fail on MSVC as well.

In C++ expression types decay on assignment: the auto sees an int, not a const int. Structured binding simply lets you do more than a single auto binding at a time.

share|improve this answer

edited 6 hours ago

answered 6 hours ago

einpoklumeinpoklum

36.1k28132260

add a comment | 

The static assertions should fail (so this would be an MSVC bug I guess). Why? Because it's basically the same as the case of:

void foo() 
 const int x_1 = 1;
 const int x_2 = 2;

 auto a = x_1;
 auto b = x_2;

 static_assert(std::is_const_v<decltype(a)>);
 static_assert(std::is_const_v<decltype(b)>);

which does indeed fail on MSVC as well.

In C++ expression types decay on assignment: the auto sees an int, not a const int. Structured binding simply lets you do more than a single auto binding at a time.

share|improve this answer

edited 6 hours ago

answered 6 hours ago

einpoklumeinpoklum

36.1k28132260

void foo() 
 const int x_1 = 1;
 const int x_2 = 2;

 auto a = x_1;
 auto b = x_2;

 static_assert(std::is_const_v<decltype(a)>);
 static_assert(std::is_const_v<decltype(b)>);

share|improve this answer

edited 6 hours ago

answered 6 hours ago

einpoklumeinpoklum

36.1k28132260

answered 6 hours ago

einpoklumeinpoklum

36.1k28132260

answered 6 hours ago

einpoklumeinpoklum

36.1k28132260