Structured binding on const
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
edited 20 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
edited 20 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
edited 20 hours ago
einpoklum
36.2k28132260
asked yesterday
Biagio FestaBiagio Festa
5,21021240
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
c++ c++17 structured-bindings
edited 20 hours ago
einpoklum
36.2k28132260
asked yesterday
Biagio FestaBiagio Festa
5,21021240
edited 20 hours ago
einpoklum
36.2k28132260
asked yesterday
Biagio FestaBiagio Festa
5,21021240
edited 20 hours ago
einpoklum
36.2k28132260
edited 20 hours ago
einpoklum
36.2k28132260
edited 20 hours ago
einpoklum
36.2k28132260
36.2k28132260
asked yesterday
Biagio FestaBiagio Festa
5,21021240
asked yesterday
Biagio FestaBiagio Festa
5,21021240
asked yesterday
Biagio FestaBiagio Festa
5,21021240
5,21021240
add a comment |
add a comment |
2 Answers
2
active
oldest
votes
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.
answered yesterday
BarryBarry
185k21326601
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.
answered yesterday
einpoklumeinpoklum
36.2k28132260
add a comment |
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
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.
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.
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.
answered yesterday
BarryBarry
185k21326601
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.
answered yesterday
BarryBarry
185k21326601
answered yesterday
BarryBarry
185k21326601
answered yesterday
BarryBarry
185k21326601
answered yesterday
BarryBarry
185k21326601
185k21326601
add a comment |
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.
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.
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.
answered yesterday
einpoklumeinpoklum
36.2k28132260
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.
answered yesterday
einpoklumeinpoklum
36.2k28132260
edited 11 hours ago
answered yesterday
einpoklumeinpoklum
36.2k28132260
answered yesterday
einpoklumeinpoklum
36.2k28132260
answered yesterday
einpoklumeinpoklum
36.2k28132260
36.2k28132260
add a comment |
add a comment |
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