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Structured binding on const

How to convert a std::string to const char* or char*?What is the difference between const int*, const int * const, and int const *?const pointers in overload resolutionString literal in templates - different behavior of compilersConfirm This Standard Library Bug Relating to Allocators in MSVC 2015 RCCapturing array of vectors in lambda makes elements constExplicit destructor call with decltypeWhy “int & const” compiles fine with MSVC?Visual accept std::string from std::byte iteratorShall structured binding to a copy of a const c-array be const?

10

Is the following code supposed to compile?

#include <type_traits>



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

edited 15 hours ago

einpoklum

36.2k28132260

asked yesterday

Biagio FestaBiagio Festa

5,21021240

add a comment | 

10

Is the following code supposed to compile?

#include <type_traits>



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

edited 15 hours ago

einpoklum

36.2k28132260

asked yesterday

Biagio FestaBiagio Festa

5,21021240

add a comment | 

Is the following code supposed to compile?

#include <type_traits>



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

edited 15 hours ago

einpoklum

36.2k28132260

asked yesterday

Biagio FestaBiagio Festa

5,21021240

#include <type_traits>



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

edited 15 hours ago

einpoklum

36.2k28132260

asked yesterday

Biagio FestaBiagio Festa

5,21021240

share|improve this question

edited 15 hours ago

einpoklum

36.2k28132260

asked yesterday

Biagio FestaBiagio Festa

5,21021240

edited 15 hours ago

einpoklum

36.2k28132260

edited 15 hours ago

einpoklum

36.2k28132260

asked yesterday

Biagio FestaBiagio Festa

5,21021240

asked yesterday

Biagio FestaBiagio Festa

5,21021240

2 Answers
2

active

oldest

votes

12

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 yesterday

BarryBarry

185k21326601

add a comment | 

7

The static assertions in your code should fail. Why? Because your code is basically the same as the case of:

#include <type_traits>



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.

... and so the fact that MSVC doesn't fail on the assertions in your code seems to be a bug.

share|improve this answer

edited 6 hours ago

answered yesterday

einpoklumeinpoklum

36.2k28132260

add a comment | 

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12

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 yesterday

BarryBarry

185k21326601

add a comment | 

12

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 yesterday

BarryBarry

185k21326601

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 yesterday

BarryBarry

185k21326601

auto e = x;

share|improve this answer

answered yesterday

BarryBarry

185k21326601

answered yesterday

BarryBarry

185k21326601

answered yesterday

BarryBarry

185k21326601

7

The static assertions in your code should fail. Why? Because your code is basically the same as the case of:

#include <type_traits>



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.

... and so the fact that MSVC doesn't fail on the assertions in your code seems to be a bug.

share|improve this answer

edited 6 hours ago

answered yesterday

einpoklumeinpoklum

36.2k28132260

add a comment | 

7

The static assertions in your code should fail. Why? Because your code is basically the same as the case of:

#include <type_traits>



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.

... and so the fact that MSVC doesn't fail on the assertions in your code seems to be a bug.

share|improve this answer

edited 6 hours ago

answered yesterday

einpoklumeinpoklum

36.2k28132260

add a comment | 

The static assertions in your code should fail. Why? Because your code is basically the same as the case of:

#include <type_traits>



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.

... and so the fact that MSVC doesn't fail on the assertions in your code seems to be a bug.

share|improve this answer

edited 6 hours ago

answered yesterday

einpoklumeinpoklum

36.2k28132260

#include <type_traits>



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 yesterday

einpoklumeinpoklum

36.2k28132260

answered yesterday

einpoklumeinpoklum

36.2k28132260

answered yesterday

einpoklumeinpoklum

36.2k28132260