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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 7 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 7 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 7 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 7 hours ago

Biagio FestaBiagio Festa

5,19321239

share|improve this question

asked 7 hours ago

Biagio FestaBiagio Festa

5,19321239

asked 7 hours ago

Biagio FestaBiagio Festa

5,19321239

asked 7 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 | 

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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