Shall structured binding to a copy of a const c-array be const?
Consider this code (demo):
#include <tuple>
#include <type_traits>
struct Ag{int i;int j;};
using T = std::tuple<int,int>;
using Ar = int[2];
const Ag ag {};
const T t {};
const Ar ar {};
void bind_ag(){
auto [i,j] = ag;
static_assert(std::is_same_v<decltype((i)),int&>);
}
void bind_t(){
auto [i,j] = t;
static_assert(std::is_same_v<decltype((i)),int&>);
}
void bind_ar(){
auto [i,j] = ar;
static_assert(std::is_same_v<decltype((i)),int&>); //For GCC
static_assert(std::is_same_v<decltype((i)),const int&>); //For Clang (and standard?)
}
A structured binding to a copy of a const c-array are declared const by Clang and non-const by GCC.
The behavior of GCC for c-array is consistent with the behavior observed for aggregate or tuple-like types.
On the other hand from my reading of the standard, I suppose Clang follows what is written. In [dcl.struct.bind]/1 e has type cv A where A is the type of the initializer expression and the cv is the cv-qualifier of the structured binding declaration. And the type of the initializer expression ar is accordingly to [expr.type]/1 const int[2].
What should be expected? My opinion is that Clang follows the standard. On the other hand I feel the intent was that the behaviors for array, aggregate and tuple-like types were equivalent.
c++ language-lawyer c++17 structured-bindings
asked Dec 11 '18 at 14:21
Oliv
8,2601956
add a comment |
Consider this code (demo):
#include <tuple>
#include <type_traits>
struct Ag{int i;int j;};
using T = std::tuple<int,int>;
using Ar = int[2];
const Ag ag {};
const T t {};
const Ar ar {};
void bind_ag(){
auto [i,j] = ag;
static_assert(std::is_same_v<decltype((i)),int&>);
}
void bind_t(){
auto [i,j] = t;
static_assert(std::is_same_v<decltype((i)),int&>);
}
void bind_ar(){
auto [i,j] = ar;
static_assert(std::is_same_v<decltype((i)),int&>); //For GCC
static_assert(std::is_same_v<decltype((i)),const int&>); //For Clang (and standard?)
}
A structured binding to a copy of a const c-array are declared const by Clang and non-const by GCC.
The behavior of GCC for c-array is consistent with the behavior observed for aggregate or tuple-like types.
On the other hand from my reading of the standard, I suppose Clang follows what is written. In [dcl.struct.bind]/1 e has type cv A where A is the type of the initializer expression and the cv is the cv-qualifier of the structured binding declaration. And the type of the initializer expression ar is accordingly to [expr.type]/1 const int[2].
What should be expected? My opinion is that Clang follows the standard. On the other hand I feel the intent was that the behaviors for array, aggregate and tuple-like types were equivalent.
c++ language-lawyer c++17 structured-bindings
asked Dec 11 '18 at 14:21
Oliv
8,2601956
1
Isn't this note related: eel.is/c++draft/dcl.struct.bind#3.note-1?
– Daniel Langr
Dec 11 '18 at 14:44
3
gcc.gnu.org/bugzilla/show_bug.cgi?id=86049
– cpplearner
Dec 11 '18 at 14:59
add a comment |
Consider this code (demo):
#include <tuple>
#include <type_traits>
struct Ag{int i;int j;};
using T = std::tuple<int,int>;
using Ar = int[2];
const Ag ag {};
const T t {};
const Ar ar {};
void bind_ag(){
auto [i,j] = ag;
static_assert(std::is_same_v<decltype((i)),int&>);
}
void bind_t(){
auto [i,j] = t;
static_assert(std::is_same_v<decltype((i)),int&>);
}
void bind_ar(){
auto [i,j] = ar;
static_assert(std::is_same_v<decltype((i)),int&>); //For GCC
static_assert(std::is_same_v<decltype((i)),const int&>); //For Clang (and standard?)
}
A structured binding to a copy of a const c-array are declared const by Clang and non-const by GCC.
The behavior of GCC for c-array is consistent with the behavior observed for aggregate or tuple-like types.
On the other hand from my reading of the standard, I suppose Clang follows what is written. In [dcl.struct.bind]/1 e has type cv A where A is the type of the initializer expression and the cv is the cv-qualifier of the structured binding declaration. And the type of the initializer expression ar is accordingly to [expr.type]/1 const int[2].
What should be expected? My opinion is that Clang follows the standard. On the other hand I feel the intent was that the behaviors for array, aggregate and tuple-like types were equivalent.
c++ language-lawyer c++17 structured-bindings
asked Dec 11 '18 at 14:21
Oliv
8,2601956
Consider this code (demo):
#include <tuple>
#include <type_traits>
struct Ag{int i;int j;};
using T = std::tuple<int,int>;
using Ar = int[2];
const Ag ag {};
const T t {};
const Ar ar {};
void bind_ag(){
auto [i,j] = ag;
static_assert(std::is_same_v<decltype((i)),int&>);
}
void bind_t(){
auto [i,j] = t;
static_assert(std::is_same_v<decltype((i)),int&>);
}
void bind_ar(){
auto [i,j] = ar;
static_assert(std::is_same_v<decltype((i)),int&>); //For GCC
static_assert(std::is_same_v<decltype((i)),const int&>); //For Clang (and standard?)
}
A structured binding to a copy of a const c-array are declared const by Clang and non-const by GCC.
The behavior of GCC for c-array is consistent with the behavior observed for aggregate or tuple-like types.
On the other hand from my reading of the standard, I suppose Clang follows what is written. In [dcl.struct.bind]/1 e has type cv A where A is the type of the initializer expression and the cv is the cv-qualifier of the structured binding declaration. And the type of the initializer expression ar is accordingly to [expr.type]/1 const int[2].
What should be expected? My opinion is that Clang follows the standard. On the other hand I feel the intent was that the behaviors for array, aggregate and tuple-like types were equivalent.
c++ language-lawyer c++17 structured-bindings
c++ language-lawyer c++17 structured-bindings
asked Dec 11 '18 at 14:21
Oliv
8,2601956
asked Dec 11 '18 at 14:21
Oliv
8,2601956
edited Dec 11 '18 at 14:27
asked Dec 11 '18 at 14:21
Oliv
8,2601956
asked Dec 11 '18 at 14:21
Oliv
8,2601956
asked Dec 11 '18 at 14:21
Oliv
8,2601956
8,2601956
1
Isn't this note related: eel.is/c++draft/dcl.struct.bind#3.note-1?
– Daniel Langr
Dec 11 '18 at 14:44
3
gcc.gnu.org/bugzilla/show_bug.cgi?id=86049
– cpplearner
Dec 11 '18 at 14:59
add a comment |
1
Isn't this note related: eel.is/c++draft/dcl.struct.bind#3.note-1?
– Daniel Langr
Dec 11 '18 at 14:44
3
gcc.gnu.org/bugzilla/show_bug.cgi?id=86049
– cpplearner
Dec 11 '18 at 14:59
1
1
Isn't this note related: eel.is/c++draft/dcl.struct.bind#3.note-1?
– Daniel Langr
Dec 11 '18 at 14:44
Isn't this note related: eel.is/c++draft/dcl.struct.bind#3.note-1?
– Daniel Langr
Dec 11 '18 at 14:44
3
3
gcc.gnu.org/bugzilla/show_bug.cgi?id=86049
– cpplearner
Dec 11 '18 at 14:59
gcc.gnu.org/bugzilla/show_bug.cgi?id=86049
– cpplearner
Dec 11 '18 at 14:59
add a comment |
1 Answer
1
active
oldest
votes
The wording of the standard in [dcl.struct.bind] says:
If the assignment-expression in the initializer has array type
Aand no ref-qualifier is present,ehas type cvAand each element is copy-initialized or direct-initialized from the corresponding element of the assignment-expression as specified by the form of the initializer.
We have auto [i,j] = ar;, ar has array type const int[2], and the wording of the standard makes it clear that e has type const int[2]. Thus, per the wording, each binding references the element type - which is const int. Clang is technically correct.
However, as Richard Smith points out in gcc bug 80649:
I think this is a bug in the standard. The cv-qualifiers of the array type should be discarded, as they would be for any normal auto deduction.
That seems right. When you write auto x = y; you'd certainly expect x to not be top-level const, but here we have a situation where it still is. I don't think there's a Core issue open for this yet, but there should be.
answered Dec 11 '18 at 15:32
Barry
177k18304558
The cv-qualification cv comes from the decl-specifier-seq ("auto") not the initialiser ("ar"), no? Unless it's saying that the qualifiers onAare merged into cv. But I don't follow why that would be the case; without a ref-qualifier, why would it want to maintain constness? It doesn't do that anywhere else.
– Lightness Races in Orbit
Dec 11 '18 at 16:21
5
@LightnessRacesinOrbit It does - butAis an array of const, socv Ais still an array of const. I don't think we want to maintain constness, hence Core issue.
– Barry
Dec 11 '18 at 16:25
Mm, I could be persuaded by that.
– Lightness Races in Orbit
Dec 11 '18 at 16:56
add a comment |
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The wording of the standard in [dcl.struct.bind] says:
If the assignment-expression in the initializer has array type
Aand no ref-qualifier is present,ehas type cvAand each element is copy-initialized or direct-initialized from the corresponding element of the assignment-expression as specified by the form of the initializer.
We have auto [i,j] = ar;, ar has array type const int[2], and the wording of the standard makes it clear that e has type const int[2]. Thus, per the wording, each binding references the element type - which is const int. Clang is technically correct.
However, as Richard Smith points out in gcc bug 80649:
I think this is a bug in the standard. The cv-qualifiers of the array type should be discarded, as they would be for any normal auto deduction.
That seems right. When you write auto x = y; you'd certainly expect x to not be top-level const, but here we have a situation where it still is. I don't think there's a Core issue open for this yet, but there should be.
answered Dec 11 '18 at 15:32
Barry
177k18304558
The cv-qualification cv comes from the decl-specifier-seq ("auto") not the initialiser ("ar"), no? Unless it's saying that the qualifiers onAare merged into cv. But I don't follow why that would be the case; without a ref-qualifier, why would it want to maintain constness? It doesn't do that anywhere else.
– Lightness Races in Orbit
Dec 11 '18 at 16:21
5
@LightnessRacesinOrbit It does - butAis an array of const, socv Ais still an array of const. I don't think we want to maintain constness, hence Core issue.
– Barry
Dec 11 '18 at 16:25
Mm, I could be persuaded by that.
– Lightness Races in Orbit
Dec 11 '18 at 16:56
add a comment |
The wording of the standard in [dcl.struct.bind] says:
If the assignment-expression in the initializer has array type
Aand no ref-qualifier is present,ehas type cvAand each element is copy-initialized or direct-initialized from the corresponding element of the assignment-expression as specified by the form of the initializer.
We have auto [i,j] = ar;, ar has array type const int[2], and the wording of the standard makes it clear that e has type const int[2]. Thus, per the wording, each binding references the element type - which is const int. Clang is technically correct.
However, as Richard Smith points out in gcc bug 80649:
I think this is a bug in the standard. The cv-qualifiers of the array type should be discarded, as they would be for any normal auto deduction.
That seems right. When you write auto x = y; you'd certainly expect x to not be top-level const, but here we have a situation where it still is. I don't think there's a Core issue open for this yet, but there should be.
answered Dec 11 '18 at 15:32
Barry
177k18304558
The cv-qualification cv comes from the decl-specifier-seq ("auto") not the initialiser ("ar"), no? Unless it's saying that the qualifiers onAare merged into cv. But I don't follow why that would be the case; without a ref-qualifier, why would it want to maintain constness? It doesn't do that anywhere else.
– Lightness Races in Orbit
Dec 11 '18 at 16:21
5
@LightnessRacesinOrbit It does - butAis an array of const, socv Ais still an array of const. I don't think we want to maintain constness, hence Core issue.
– Barry
Dec 11 '18 at 16:25
Mm, I could be persuaded by that.
– Lightness Races in Orbit
Dec 11 '18 at 16:56
add a comment |
The wording of the standard in [dcl.struct.bind] says:
If the assignment-expression in the initializer has array type
Aand no ref-qualifier is present,ehas type cvAand each element is copy-initialized or direct-initialized from the corresponding element of the assignment-expression as specified by the form of the initializer.
We have auto [i,j] = ar;, ar has array type const int[2], and the wording of the standard makes it clear that e has type const int[2]. Thus, per the wording, each binding references the element type - which is const int. Clang is technically correct.
However, as Richard Smith points out in gcc bug 80649:
I think this is a bug in the standard. The cv-qualifiers of the array type should be discarded, as they would be for any normal auto deduction.
That seems right. When you write auto x = y; you'd certainly expect x to not be top-level const, but here we have a situation where it still is. I don't think there's a Core issue open for this yet, but there should be.
answered Dec 11 '18 at 15:32
Barry
177k18304558
The wording of the standard in [dcl.struct.bind] says:
If the assignment-expression in the initializer has array type
Aand no ref-qualifier is present,ehas type cvAand each element is copy-initialized or direct-initialized from the corresponding element of the assignment-expression as specified by the form of the initializer.
We have auto [i,j] = ar;, ar has array type const int[2], and the wording of the standard makes it clear that e has type const int[2]. Thus, per the wording, each binding references the element type - which is const int. Clang is technically correct.
However, as Richard Smith points out in gcc bug 80649:
I think this is a bug in the standard. The cv-qualifiers of the array type should be discarded, as they would be for any normal auto deduction.
That seems right. When you write auto x = y; you'd certainly expect x to not be top-level const, but here we have a situation where it still is. I don't think there's a Core issue open for this yet, but there should be.
answered Dec 11 '18 at 15:32
Barry
177k18304558
answered Dec 11 '18 at 15:32
Barry
177k18304558
answered Dec 11 '18 at 15:32
Barry
177k18304558
answered Dec 11 '18 at 15:32
Barry
177k18304558
177k18304558
The cv-qualification cv comes from the decl-specifier-seq ("auto") not the initialiser ("ar"), no? Unless it's saying that the qualifiers onAare merged into cv. But I don't follow why that would be the case; without a ref-qualifier, why would it want to maintain constness? It doesn't do that anywhere else.
– Lightness Races in Orbit
Dec 11 '18 at 16:21
5
@LightnessRacesinOrbit It does - butAis an array of const, socv Ais still an array of const. I don't think we want to maintain constness, hence Core issue.
– Barry
Dec 11 '18 at 16:25
Mm, I could be persuaded by that.
– Lightness Races in Orbit
Dec 11 '18 at 16:56
add a comment |
The cv-qualification cv comes from the decl-specifier-seq ("auto") not the initialiser ("ar"), no? Unless it's saying that the qualifiers onAare merged into cv. But I don't follow why that would be the case; without a ref-qualifier, why would it want to maintain constness? It doesn't do that anywhere else.
– Lightness Races in Orbit
Dec 11 '18 at 16:21
5
@LightnessRacesinOrbit It does - butAis an array of const, socv Ais still an array of const. I don't think we want to maintain constness, hence Core issue.
– Barry
Dec 11 '18 at 16:25
Mm, I could be persuaded by that.
– Lightness Races in Orbit
Dec 11 '18 at 16:56
The cv-qualification cv comes from the decl-specifier-seq ("
auto") not the initialiser ("ar"), no? Unless it's saying that the qualifiers on A are merged into cv. But I don't follow why that would be the case; without a ref-qualifier, why would it want to maintain constness? It doesn't do that anywhere else.– Lightness Races in Orbit
Dec 11 '18 at 16:21
The cv-qualification cv comes from the decl-specifier-seq ("
auto") not the initialiser ("ar"), no? Unless it's saying that the qualifiers on A are merged into cv. But I don't follow why that would be the case; without a ref-qualifier, why would it want to maintain constness? It doesn't do that anywhere else.– Lightness Races in Orbit
Dec 11 '18 at 16:21
5
5
@LightnessRacesinOrbit It does - but
A is an array of const, so cv A is still an array of const. I don't think we want to maintain constness, hence Core issue.– Barry
Dec 11 '18 at 16:25
@LightnessRacesinOrbit It does - but
A is an array of const, so cv A is still an array of const. I don't think we want to maintain constness, hence Core issue.– Barry
Dec 11 '18 at 16:25
Mm, I could be persuaded by that.
– Lightness Races in Orbit
Dec 11 '18 at 16:56
Mm, I could be persuaded by that.
– Lightness Races in Orbit
Dec 11 '18 at 16:56
add a comment |
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1
Isn't this note related: eel.is/c++draft/dcl.struct.bind#3.note-1?
– Daniel Langr
Dec 11 '18 at 14:44
3
gcc.gnu.org/bugzilla/show_bug.cgi?id=86049
– cpplearner
Dec 11 '18 at 14:59