braced-init-list and unsigned types
gcc 8 and clang 7 do not accept the following code, which should default-construct a temporary of type unsigned int:
unsigned int ui = unsigned int{};
clang 7 reports an error such as
<source>:6:22: error: expected primary-expression before 'unsigned'
Visual C++ 2015 and 2017 accept this.
Obviously, this works with a type such as int, or any default-constructible class type.
Is this correct C++14 code (and in that case a bug of clang and gcc)? If not, why not? Which types other than unsigned types would suffer from the same restriction?
c++ initialization c++14 language-lawyer
edited Dec 5 '18 at 1:35
songyuanyao
93.9k11182249
asked Nov 16 '18 at 10:49
user2019765user2019765
1367
add a comment |
gcc 8 and clang 7 do not accept the following code, which should default-construct a temporary of type unsigned int:
unsigned int ui = unsigned int{};
clang 7 reports an error such as
<source>:6:22: error: expected primary-expression before 'unsigned'
Visual C++ 2015 and 2017 accept this.
Obviously, this works with a type such as int, or any default-constructible class type.
Is this correct C++14 code (and in that case a bug of clang and gcc)? If not, why not? Which types other than unsigned types would suffer from the same restriction?
c++ initialization c++14 language-lawyer
edited Dec 5 '18 at 1:35
songyuanyao
93.9k11182249
asked Nov 16 '18 at 10:49
user2019765user2019765
1367
3
Try(unsigned int){};. Think of the {} applied only tointand then there'sunsignedhanging where it shouldn't, hence the error message.
– DeiDei
Nov 16 '18 at 10:54
@DeiDei that is a syntax error in Standard C++ (parenthesized typename followed by{does not match any grammar rule)
– M.M
Dec 5 '18 at 2:17
add a comment |
gcc 8 and clang 7 do not accept the following code, which should default-construct a temporary of type unsigned int:
unsigned int ui = unsigned int{};
clang 7 reports an error such as
<source>:6:22: error: expected primary-expression before 'unsigned'
Visual C++ 2015 and 2017 accept this.
Obviously, this works with a type such as int, or any default-constructible class type.
Is this correct C++14 code (and in that case a bug of clang and gcc)? If not, why not? Which types other than unsigned types would suffer from the same restriction?
c++ initialization c++14 language-lawyer
edited Dec 5 '18 at 1:35
songyuanyao
93.9k11182249
asked Nov 16 '18 at 10:49
user2019765user2019765
1367
gcc 8 and clang 7 do not accept the following code, which should default-construct a temporary of type unsigned int:
unsigned int ui = unsigned int{};
clang 7 reports an error such as
<source>:6:22: error: expected primary-expression before 'unsigned'
Visual C++ 2015 and 2017 accept this.
Obviously, this works with a type such as int, or any default-constructible class type.
Is this correct C++14 code (and in that case a bug of clang and gcc)? If not, why not? Which types other than unsigned types would suffer from the same restriction?
c++ initialization c++14 language-lawyer
c++ initialization c++14 language-lawyer
edited Dec 5 '18 at 1:35
songyuanyao
93.9k11182249
asked Nov 16 '18 at 10:49
user2019765user2019765
1367
edited Dec 5 '18 at 1:35
songyuanyao
93.9k11182249
asked Nov 16 '18 at 10:49
user2019765user2019765
1367
edited Dec 5 '18 at 1:35
songyuanyao
93.9k11182249
edited Dec 5 '18 at 1:35
songyuanyao
93.9k11182249
edited Dec 5 '18 at 1:35
songyuanyao
93.9k11182249
93.9k11182249
asked Nov 16 '18 at 10:49
user2019765user2019765
1367
asked Nov 16 '18 at 10:49
user2019765user2019765
1367
asked Nov 16 '18 at 10:49
user2019765user2019765
1367
1367
3
Try(unsigned int){};. Think of the {} applied only tointand then there'sunsignedhanging where it shouldn't, hence the error message.
– DeiDei
Nov 16 '18 at 10:54
@DeiDei that is a syntax error in Standard C++ (parenthesized typename followed by{does not match any grammar rule)
– M.M
Dec 5 '18 at 2:17
add a comment |
3
Try(unsigned int){};. Think of the {} applied only tointand then there'sunsignedhanging where it shouldn't, hence the error message.
– DeiDei
Nov 16 '18 at 10:54
@DeiDei that is a syntax error in Standard C++ (parenthesized typename followed by{does not match any grammar rule)
– M.M
Dec 5 '18 at 2:17
3
3
Try
(unsigned int){};. Think of the {} applied only to int and then there's unsigned hanging where it shouldn't, hence the error message.– DeiDei
Nov 16 '18 at 10:54
Try
(unsigned int){};. Think of the {} applied only to int and then there's unsigned hanging where it shouldn't, hence the error message.– DeiDei
Nov 16 '18 at 10:54
@DeiDei that is a syntax error in Standard C++ (parenthesized typename followed by
{ does not match any grammar rule)– M.M
Dec 5 '18 at 2:17
@DeiDei that is a syntax error in Standard C++ (parenthesized typename followed by
{ does not match any grammar rule)– M.M
Dec 5 '18 at 2:17
add a comment |
2 Answers
2
active
oldest
votes
new_type { expression-list(optional) } like unsigned int{} fits the syntax of explicit type conversion, which allows only single-word type name.
A single-word type name followed by a braced-init-list is a prvalue of the specified type
designating a temporary (until C++17)whose result object is (since C++17)direct-list-initialized with the specified braced-init-list.
Note that unsigned int is not a single-word type name, while int is. So int {} works fine.
This is same for functional cast expression,
The functional cast expression consists of a simple type specifier or a typedef specifier (in other words, a single-word type name:
unsigned int(expression)orint*(expression)are not valid),
As a workaround, you can apply type alias, e.g.
using type = unsigned int;
type ui = type{};
answered Nov 16 '18 at 10:55
songyuanyaosongyuanyao
93.9k11182249
Thanks. Obviously, it is not a single word, I wasn't aware of this constraint.
– user2019765
Nov 16 '18 at 11:09
@user2019765 I agree this is not easy to notice; even it's obvious if you realize it.
– songyuanyao
Nov 16 '18 at 11:11
Interestingly, adding parentheses works with clang, but not with Visual C++, there it yields an error C4576: a parenthesized type followed by an initializer list is a non-standard explicit type conversion syntax
– user2019765
Nov 16 '18 at 11:12
1
@user2019765 I'm not sure, but I think both acceptingunsigned int{}and rejecting(unsigned int){}deosn't conform to the standard.
– songyuanyao
Nov 16 '18 at 11:14
(unsigned int){}is not a C-style cast: a C-style cast has form(type)expression, and{}is not an expression. With-pedantic, g++ warnsISO C++ forbids compound-literalsand clang warnscompound literals are a C99-specific feature.
– cpplearner
Nov 16 '18 at 13:57
|
show 8 more comments
"Int" is optional in "unsigned int".
Try simply:
unsigned ui = unsigned{};
It works.
Edit:
I found this answer:
Standard behavior for direct initialization of unsigned short
It's just because only single-word type name could be used in
functional cast expression, while unsigned short is not a single-word
type name; short is.
answered Nov 16 '18 at 10:55
artonaartona
71248
2
Thanks for your suggestion, but my question was not how to fix the expression, but if it is correct.
– user2019765
Nov 16 '18 at 11:03
add a comment |
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2 Answers
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active
oldest
votes
2 Answers
2
active
oldest
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active
oldest
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oldest
votes
new_type { expression-list(optional) } like unsigned int{} fits the syntax of explicit type conversion, which allows only single-word type name.
A single-word type name followed by a braced-init-list is a prvalue of the specified type
designating a temporary (until C++17)whose result object is (since C++17)direct-list-initialized with the specified braced-init-list.
Note that unsigned int is not a single-word type name, while int is. So int {} works fine.
This is same for functional cast expression,
The functional cast expression consists of a simple type specifier or a typedef specifier (in other words, a single-word type name:
unsigned int(expression)orint*(expression)are not valid),
As a workaround, you can apply type alias, e.g.
using type = unsigned int;
type ui = type{};
answered Nov 16 '18 at 10:55
songyuanyaosongyuanyao
93.9k11182249
Thanks. Obviously, it is not a single word, I wasn't aware of this constraint.
– user2019765
Nov 16 '18 at 11:09
@user2019765 I agree this is not easy to notice; even it's obvious if you realize it.
– songyuanyao
Nov 16 '18 at 11:11
Interestingly, adding parentheses works with clang, but not with Visual C++, there it yields an error C4576: a parenthesized type followed by an initializer list is a non-standard explicit type conversion syntax
– user2019765
Nov 16 '18 at 11:12
1
@user2019765 I'm not sure, but I think both acceptingunsigned int{}and rejecting(unsigned int){}deosn't conform to the standard.
– songyuanyao
Nov 16 '18 at 11:14
(unsigned int){}is not a C-style cast: a C-style cast has form(type)expression, and{}is not an expression. With-pedantic, g++ warnsISO C++ forbids compound-literalsand clang warnscompound literals are a C99-specific feature.
– cpplearner
Nov 16 '18 at 13:57
|
show 8 more comments
new_type { expression-list(optional) } like unsigned int{} fits the syntax of explicit type conversion, which allows only single-word type name.
A single-word type name followed by a braced-init-list is a prvalue of the specified type
designating a temporary (until C++17)whose result object is (since C++17)direct-list-initialized with the specified braced-init-list.
Note that unsigned int is not a single-word type name, while int is. So int {} works fine.
This is same for functional cast expression,
The functional cast expression consists of a simple type specifier or a typedef specifier (in other words, a single-word type name:
unsigned int(expression)orint*(expression)are not valid),
As a workaround, you can apply type alias, e.g.
using type = unsigned int;
type ui = type{};
answered Nov 16 '18 at 10:55
songyuanyaosongyuanyao
93.9k11182249
Thanks. Obviously, it is not a single word, I wasn't aware of this constraint.
– user2019765
Nov 16 '18 at 11:09
@user2019765 I agree this is not easy to notice; even it's obvious if you realize it.
– songyuanyao
Nov 16 '18 at 11:11
Interestingly, adding parentheses works with clang, but not with Visual C++, there it yields an error C4576: a parenthesized type followed by an initializer list is a non-standard explicit type conversion syntax
– user2019765
Nov 16 '18 at 11:12
1
@user2019765 I'm not sure, but I think both acceptingunsigned int{}and rejecting(unsigned int){}deosn't conform to the standard.
– songyuanyao
Nov 16 '18 at 11:14
(unsigned int){}is not a C-style cast: a C-style cast has form(type)expression, and{}is not an expression. With-pedantic, g++ warnsISO C++ forbids compound-literalsand clang warnscompound literals are a C99-specific feature.
– cpplearner
Nov 16 '18 at 13:57
|
show 8 more comments
new_type { expression-list(optional) } like unsigned int{} fits the syntax of explicit type conversion, which allows only single-word type name.
A single-word type name followed by a braced-init-list is a prvalue of the specified type
designating a temporary (until C++17)whose result object is (since C++17)direct-list-initialized with the specified braced-init-list.
Note that unsigned int is not a single-word type name, while int is. So int {} works fine.
This is same for functional cast expression,
The functional cast expression consists of a simple type specifier or a typedef specifier (in other words, a single-word type name:
unsigned int(expression)orint*(expression)are not valid),
As a workaround, you can apply type alias, e.g.
using type = unsigned int;
type ui = type{};
answered Nov 16 '18 at 10:55
songyuanyaosongyuanyao
93.9k11182249
new_type { expression-list(optional) } like unsigned int{} fits the syntax of explicit type conversion, which allows only single-word type name.
A single-word type name followed by a braced-init-list is a prvalue of the specified type
designating a temporary (until C++17)whose result object is (since C++17)direct-list-initialized with the specified braced-init-list.
Note that unsigned int is not a single-word type name, while int is. So int {} works fine.
This is same for functional cast expression,
The functional cast expression consists of a simple type specifier or a typedef specifier (in other words, a single-word type name:
unsigned int(expression)orint*(expression)are not valid),
As a workaround, you can apply type alias, e.g.
using type = unsigned int;
type ui = type{};
answered Nov 16 '18 at 10:55
songyuanyaosongyuanyao
93.9k11182249
edited Nov 16 '18 at 15:46
answered Nov 16 '18 at 10:55
songyuanyaosongyuanyao
93.9k11182249
answered Nov 16 '18 at 10:55
songyuanyaosongyuanyao
93.9k11182249
answered Nov 16 '18 at 10:55
songyuanyaosongyuanyao
93.9k11182249
93.9k11182249
Thanks. Obviously, it is not a single word, I wasn't aware of this constraint.
– user2019765
Nov 16 '18 at 11:09
@user2019765 I agree this is not easy to notice; even it's obvious if you realize it.
– songyuanyao
Nov 16 '18 at 11:11
Interestingly, adding parentheses works with clang, but not with Visual C++, there it yields an error C4576: a parenthesized type followed by an initializer list is a non-standard explicit type conversion syntax
– user2019765
Nov 16 '18 at 11:12
1
@user2019765 I'm not sure, but I think both acceptingunsigned int{}and rejecting(unsigned int){}deosn't conform to the standard.
– songyuanyao
Nov 16 '18 at 11:14
(unsigned int){}is not a C-style cast: a C-style cast has form(type)expression, and{}is not an expression. With-pedantic, g++ warnsISO C++ forbids compound-literalsand clang warnscompound literals are a C99-specific feature.
– cpplearner
Nov 16 '18 at 13:57
|
show 8 more comments
Thanks. Obviously, it is not a single word, I wasn't aware of this constraint.
– user2019765
Nov 16 '18 at 11:09
@user2019765 I agree this is not easy to notice; even it's obvious if you realize it.
– songyuanyao
Nov 16 '18 at 11:11
Interestingly, adding parentheses works with clang, but not with Visual C++, there it yields an error C4576: a parenthesized type followed by an initializer list is a non-standard explicit type conversion syntax
– user2019765
Nov 16 '18 at 11:12
1
@user2019765 I'm not sure, but I think both acceptingunsigned int{}and rejecting(unsigned int){}deosn't conform to the standard.
– songyuanyao
Nov 16 '18 at 11:14
(unsigned int){}is not a C-style cast: a C-style cast has form(type)expression, and{}is not an expression. With-pedantic, g++ warnsISO C++ forbids compound-literalsand clang warnscompound literals are a C99-specific feature.
– cpplearner
Nov 16 '18 at 13:57
Thanks. Obviously, it is not a single word, I wasn't aware of this constraint.
– user2019765
Nov 16 '18 at 11:09
Thanks. Obviously, it is not a single word, I wasn't aware of this constraint.
– user2019765
Nov 16 '18 at 11:09
@user2019765 I agree this is not easy to notice; even it's obvious if you realize it.
– songyuanyao
Nov 16 '18 at 11:11
@user2019765 I agree this is not easy to notice; even it's obvious if you realize it.
– songyuanyao
Nov 16 '18 at 11:11
Interestingly, adding parentheses works with clang, but not with Visual C++, there it yields an error C4576: a parenthesized type followed by an initializer list is a non-standard explicit type conversion syntax
– user2019765
Nov 16 '18 at 11:12
Interestingly, adding parentheses works with clang, but not with Visual C++, there it yields an error C4576: a parenthesized type followed by an initializer list is a non-standard explicit type conversion syntax
– user2019765
Nov 16 '18 at 11:12
1
1
@user2019765 I'm not sure, but I think both accepting
unsigned int{} and rejecting (unsigned int){} deosn't conform to the standard.– songyuanyao
Nov 16 '18 at 11:14
@user2019765 I'm not sure, but I think both accepting
unsigned int{} and rejecting (unsigned int){} deosn't conform to the standard.– songyuanyao
Nov 16 '18 at 11:14
(unsigned int){} is not a C-style cast: a C-style cast has form (type)expression, and {} is not an expression. With -pedantic, g++ warns ISO C++ forbids compound-literals and clang warns compound literals are a C99-specific feature.– cpplearner
Nov 16 '18 at 13:57
(unsigned int){} is not a C-style cast: a C-style cast has form (type)expression, and {} is not an expression. With -pedantic, g++ warns ISO C++ forbids compound-literals and clang warns compound literals are a C99-specific feature.– cpplearner
Nov 16 '18 at 13:57
|
show 8 more comments
"Int" is optional in "unsigned int".
Try simply:
unsigned ui = unsigned{};
It works.
Edit:
I found this answer:
Standard behavior for direct initialization of unsigned short
It's just because only single-word type name could be used in
functional cast expression, while unsigned short is not a single-word
type name; short is.
answered Nov 16 '18 at 10:55
artonaartona
71248
2
Thanks for your suggestion, but my question was not how to fix the expression, but if it is correct.
– user2019765
Nov 16 '18 at 11:03
add a comment |
"Int" is optional in "unsigned int".
Try simply:
unsigned ui = unsigned{};
It works.
Edit:
I found this answer:
Standard behavior for direct initialization of unsigned short
It's just because only single-word type name could be used in
functional cast expression, while unsigned short is not a single-word
type name; short is.
answered Nov 16 '18 at 10:55
artonaartona
71248
2
Thanks for your suggestion, but my question was not how to fix the expression, but if it is correct.
– user2019765
Nov 16 '18 at 11:03
add a comment |
"Int" is optional in "unsigned int".
Try simply:
unsigned ui = unsigned{};
It works.
Edit:
I found this answer:
Standard behavior for direct initialization of unsigned short
It's just because only single-word type name could be used in
functional cast expression, while unsigned short is not a single-word
type name; short is.
answered Nov 16 '18 at 10:55
artonaartona
71248
"Int" is optional in "unsigned int".
Try simply:
unsigned ui = unsigned{};
It works.
Edit:
I found this answer:
Standard behavior for direct initialization of unsigned short
It's just because only single-word type name could be used in
functional cast expression, while unsigned short is not a single-word
type name; short is.
answered Nov 16 '18 at 10:55
artonaartona
71248
edited Nov 16 '18 at 11:05
answered Nov 16 '18 at 10:55
artonaartona
71248
answered Nov 16 '18 at 10:55
artonaartona
71248
answered Nov 16 '18 at 10:55
artonaartona
71248
71248
2
Thanks for your suggestion, but my question was not how to fix the expression, but if it is correct.
– user2019765
Nov 16 '18 at 11:03
add a comment |
2
Thanks for your suggestion, but my question was not how to fix the expression, but if it is correct.
– user2019765
Nov 16 '18 at 11:03
2
2
Thanks for your suggestion, but my question was not how to fix the expression, but if it is correct.
– user2019765
Nov 16 '18 at 11:03
Thanks for your suggestion, but my question was not how to fix the expression, but if it is correct.
– user2019765
Nov 16 '18 at 11:03
add a comment |
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3
Try
(unsigned int){};. Think of the {} applied only tointand then there'sunsignedhanging where it shouldn't, hence the error message.– DeiDei
Nov 16 '18 at 10:54
@DeiDei that is a syntax error in Standard C++ (parenthesized typename followed by
{does not match any grammar rule)– M.M
Dec 5 '18 at 2:17