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Why is std::move not [[nodiscard]] in C++20?
Why can templates only be implemented in the header file?Why is “using namespace std” considered bad practice?What are move semantics?Why are elementwise additions much faster in separate loops than in a combined loop?Why is reading lines from stdin much slower in C++ than Python?Why is it faster to process a sorted array than an unsorted array?Why should I use a pointer rather than the object itself?enum to string in modern C++11 / C++14 / C++17 and future C++20Why is `std::reference_wrapper` deprecated in c++17 and removed in c++20?Why use std::make_unique in C++17?
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I've recently read about [[nodiscard]] in C++17, and as far as I understand it's a new feature (design by contract?) which forces you to use the return value. This makes sense for controversial functions like std::launder (nodiscard since C++20), but I wonder why std::move isn't defined like so in C++17/20. Do you know a good reason or is it because C++20 isn't finalised yet?
c++ language-lawyer c++17 c++20
asked Apr 20 at 10:07
bbalchevbbalchev
3971416
|
show 6 more comments
I've recently read about [[nodiscard]] in C++17, and as far as I understand it's a new feature (design by contract?) which forces you to use the return value. This makes sense for controversial functions like std::launder (nodiscard since C++20), but I wonder why std::move isn't defined like so in C++17/20. Do you know a good reason or is it because C++20 isn't finalised yet?
c++ language-lawyer c++17 c++20
asked Apr 20 at 10:07
bbalchevbbalchev
3971416
1
I know a reason that makes sense to me, but unless there's an official document explicitly saying why it isn't, that just turns the question to be opinion-based.
– Sebastian Redl
Apr 20 at 10:24
8
Because absolutely nothing bad (or at all) happens when you don't use it.
– Sebastian Redl
Apr 20 at 10:33
6
@SebastianRedl: similarly, nothing useful happens. It's the same as writing an empty statement, so[[nodiscard]]would help diagnose bugs. Also, nothing bad happens whenvector::empty()is ignored, but that is marked[[nodiscard]]for obvious reasons.
– Vittorio Romeo
Apr 20 at 10:39
5
@SebastianRedl That sounds like a great reason to mark it[[nodiscard]]: "Hey, you did something completely pointless. Did you mean to do something else?"
– Barry
Apr 20 at 18:53
4
@bbalchevstd::movedoesn't move. Passing an object throughstd::moveand ignoring the result does absolutely nothing.
– tkausl
Apr 20 at 23:04
|
show 6 more comments
I've recently read about [[nodiscard]] in C++17, and as far as I understand it's a new feature (design by contract?) which forces you to use the return value. This makes sense for controversial functions like std::launder (nodiscard since C++20), but I wonder why std::move isn't defined like so in C++17/20. Do you know a good reason or is it because C++20 isn't finalised yet?
c++ language-lawyer c++17 c++20
asked Apr 20 at 10:07
bbalchevbbalchev
3971416
I've recently read about [[nodiscard]] in C++17, and as far as I understand it's a new feature (design by contract?) which forces you to use the return value. This makes sense for controversial functions like std::launder (nodiscard since C++20), but I wonder why std::move isn't defined like so in C++17/20. Do you know a good reason or is it because C++20 isn't finalised yet?
c++ language-lawyer c++17 c++20
c++ language-lawyer c++17 c++20
asked Apr 20 at 10:07
bbalchevbbalchev
3971416
asked Apr 20 at 10:07
bbalchevbbalchev
3971416
edited Apr 20 at 19:21
bbalchev
asked Apr 20 at 10:07
bbalchevbbalchev
3971416
asked Apr 20 at 10:07
bbalchevbbalchev
3971416
asked Apr 20 at 10:07
bbalchevbbalchev
3971416
3971416
1
I know a reason that makes sense to me, but unless there's an official document explicitly saying why it isn't, that just turns the question to be opinion-based.
– Sebastian Redl
Apr 20 at 10:24
8
Because absolutely nothing bad (or at all) happens when you don't use it.
– Sebastian Redl
Apr 20 at 10:33
6
@SebastianRedl: similarly, nothing useful happens. It's the same as writing an empty statement, so[[nodiscard]]would help diagnose bugs. Also, nothing bad happens whenvector::empty()is ignored, but that is marked[[nodiscard]]for obvious reasons.
– Vittorio Romeo
Apr 20 at 10:39
5
@SebastianRedl That sounds like a great reason to mark it[[nodiscard]]: "Hey, you did something completely pointless. Did you mean to do something else?"
– Barry
Apr 20 at 18:53
4
@bbalchevstd::movedoesn't move. Passing an object throughstd::moveand ignoring the result does absolutely nothing.
– tkausl
Apr 20 at 23:04
|
show 6 more comments
1
I know a reason that makes sense to me, but unless there's an official document explicitly saying why it isn't, that just turns the question to be opinion-based.
– Sebastian Redl
Apr 20 at 10:24
8
Because absolutely nothing bad (or at all) happens when you don't use it.
– Sebastian Redl
Apr 20 at 10:33
6
@SebastianRedl: similarly, nothing useful happens. It's the same as writing an empty statement, so[[nodiscard]]would help diagnose bugs. Also, nothing bad happens whenvector::empty()is ignored, but that is marked[[nodiscard]]for obvious reasons.
– Vittorio Romeo
Apr 20 at 10:39
5
@SebastianRedl That sounds like a great reason to mark it[[nodiscard]]: "Hey, you did something completely pointless. Did you mean to do something else?"
– Barry
Apr 20 at 18:53
4
@bbalchevstd::movedoesn't move. Passing an object throughstd::moveand ignoring the result does absolutely nothing.
– tkausl
Apr 20 at 23:04
1
1
I know a reason that makes sense to me, but unless there's an official document explicitly saying why it isn't, that just turns the question to be opinion-based.
– Sebastian Redl
Apr 20 at 10:24
I know a reason that makes sense to me, but unless there's an official document explicitly saying why it isn't, that just turns the question to be opinion-based.
– Sebastian Redl
Apr 20 at 10:24
8
8
Because absolutely nothing bad (or at all) happens when you don't use it.
– Sebastian Redl
Apr 20 at 10:33
Because absolutely nothing bad (or at all) happens when you don't use it.
– Sebastian Redl
Apr 20 at 10:33
6
6
@SebastianRedl: similarly, nothing useful happens. It's the same as writing an empty statement, so
[[nodiscard]] would help diagnose bugs. Also, nothing bad happens when vector::empty() is ignored, but that is marked [[nodiscard]] for obvious reasons.– Vittorio Romeo
Apr 20 at 10:39
@SebastianRedl: similarly, nothing useful happens. It's the same as writing an empty statement, so
[[nodiscard]] would help diagnose bugs. Also, nothing bad happens when vector::empty() is ignored, but that is marked [[nodiscard]] for obvious reasons.– Vittorio Romeo
Apr 20 at 10:39
5
5
@SebastianRedl That sounds like a great reason to mark it
[[nodiscard]]: "Hey, you did something completely pointless. Did you mean to do something else?"– Barry
Apr 20 at 18:53
@SebastianRedl That sounds like a great reason to mark it
[[nodiscard]]: "Hey, you did something completely pointless. Did you mean to do something else?"– Barry
Apr 20 at 18:53
4
4
@bbalchev
std::move doesn't move. Passing an object through std::move and ignoring the result does absolutely nothing.– tkausl
Apr 20 at 23:04
@bbalchev
std::move doesn't move. Passing an object through std::move and ignoring the result does absolutely nothing.– tkausl
Apr 20 at 23:04
|
show 6 more comments
2 Answers
2
active
oldest
votes
AFAIK P0600R1 is the only proposal for adding [[nodiscard]] to the standard library that was applied to C++20. From that paper:
We suggest a conservative approach:
[...]
It should not be added when:
- [...]
- not using the return value makes no sense but doesn’t hurt and is usually not an error
- [...]
So, [[nodiscard]] should not signal bad code if this
- [...]
- doesn’t hurt and probably no state change was meant that doesn’t happen
So the reason is that the standard library uses a conservative approach and a more aggresive one is not yet proposed.
answered Apr 20 at 13:05
cpplearnercpplearner
6,10622543
3
I dunno. Not using the return value ofstd::moveis always an error in my book, and should thus be nodiscard according to your citation: it either implies that the user forgot to use the return value, or that the call is unnecessary, since it has no effect.
– Konrad Rudolph
Apr 20 at 23:21
add a comment |
The MSVC standard library team went ahead and added several thousand instances of [[nodiscard]] since VS 2017 15.6, and have reported wild success with it (both in terms of finding lots of bugs and generating no user complaints). The criteria they described were approximately:
- Pure observers, e.g.
vector::size(),vector::empty, and evenstd::count_if() - Things that acquire raw resources, e.g.
allocate() - Functions where discarding the return value is extremely likely to lead to incorrect code, e.g.
std::remove()
MSVC does mark both std::move() and std::forward() as [[nodiscard]] following these criteria.
While it's not officially annotated as such in the standard, it seems to provide clear user benefit and it's more a question of crafting such a paper to mark all the right things [[nodiscard]] (again, several thousand instances from MSVC) and apply them -- it's not complex work per se, but the volume is large. In the meantime, maybe prod your favorite standard library vendor and ask them to [[nodiscard]] lots of stuff?
answered Apr 20 at 18:52
BarryBarry
188k21337618
add a comment |
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2 Answers
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active
oldest
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2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
AFAIK P0600R1 is the only proposal for adding [[nodiscard]] to the standard library that was applied to C++20. From that paper:
We suggest a conservative approach:
[...]
It should not be added when:
- [...]
- not using the return value makes no sense but doesn’t hurt and is usually not an error
- [...]
So, [[nodiscard]] should not signal bad code if this
- [...]
- doesn’t hurt and probably no state change was meant that doesn’t happen
So the reason is that the standard library uses a conservative approach and a more aggresive one is not yet proposed.
answered Apr 20 at 13:05
cpplearnercpplearner
6,10622543
3
I dunno. Not using the return value ofstd::moveis always an error in my book, and should thus be nodiscard according to your citation: it either implies that the user forgot to use the return value, or that the call is unnecessary, since it has no effect.
– Konrad Rudolph
Apr 20 at 23:21
add a comment |
AFAIK P0600R1 is the only proposal for adding [[nodiscard]] to the standard library that was applied to C++20. From that paper:
We suggest a conservative approach:
[...]
It should not be added when:
- [...]
- not using the return value makes no sense but doesn’t hurt and is usually not an error
- [...]
So, [[nodiscard]] should not signal bad code if this
- [...]
- doesn’t hurt and probably no state change was meant that doesn’t happen
So the reason is that the standard library uses a conservative approach and a more aggresive one is not yet proposed.
answered Apr 20 at 13:05
cpplearnercpplearner
6,10622543
3
I dunno. Not using the return value ofstd::moveis always an error in my book, and should thus be nodiscard according to your citation: it either implies that the user forgot to use the return value, or that the call is unnecessary, since it has no effect.
– Konrad Rudolph
Apr 20 at 23:21
add a comment |
AFAIK P0600R1 is the only proposal for adding [[nodiscard]] to the standard library that was applied to C++20. From that paper:
We suggest a conservative approach:
[...]
It should not be added when:
- [...]
- not using the return value makes no sense but doesn’t hurt and is usually not an error
- [...]
So, [[nodiscard]] should not signal bad code if this
- [...]
- doesn’t hurt and probably no state change was meant that doesn’t happen
So the reason is that the standard library uses a conservative approach and a more aggresive one is not yet proposed.
answered Apr 20 at 13:05
cpplearnercpplearner
6,10622543
AFAIK P0600R1 is the only proposal for adding [[nodiscard]] to the standard library that was applied to C++20. From that paper:
We suggest a conservative approach:
[...]
It should not be added when:
- [...]
- not using the return value makes no sense but doesn’t hurt and is usually not an error
- [...]
So, [[nodiscard]] should not signal bad code if this
- [...]
- doesn’t hurt and probably no state change was meant that doesn’t happen
So the reason is that the standard library uses a conservative approach and a more aggresive one is not yet proposed.
answered Apr 20 at 13:05
cpplearnercpplearner
6,10622543
answered Apr 20 at 13:05
cpplearnercpplearner
6,10622543
answered Apr 20 at 13:05
cpplearnercpplearner
6,10622543
answered Apr 20 at 13:05
cpplearnercpplearner
6,10622543
6,10622543
3
I dunno. Not using the return value ofstd::moveis always an error in my book, and should thus be nodiscard according to your citation: it either implies that the user forgot to use the return value, or that the call is unnecessary, since it has no effect.
– Konrad Rudolph
Apr 20 at 23:21
add a comment |
3
I dunno. Not using the return value ofstd::moveis always an error in my book, and should thus be nodiscard according to your citation: it either implies that the user forgot to use the return value, or that the call is unnecessary, since it has no effect.
– Konrad Rudolph
Apr 20 at 23:21
3
3
I dunno. Not using the return value of
std::move is always an error in my book, and should thus be nodiscard according to your citation: it either implies that the user forgot to use the return value, or that the call is unnecessary, since it has no effect.– Konrad Rudolph
Apr 20 at 23:21
I dunno. Not using the return value of
std::move is always an error in my book, and should thus be nodiscard according to your citation: it either implies that the user forgot to use the return value, or that the call is unnecessary, since it has no effect.– Konrad Rudolph
Apr 20 at 23:21
add a comment |
The MSVC standard library team went ahead and added several thousand instances of [[nodiscard]] since VS 2017 15.6, and have reported wild success with it (both in terms of finding lots of bugs and generating no user complaints). The criteria they described were approximately:
- Pure observers, e.g.
vector::size(),vector::empty, and evenstd::count_if() - Things that acquire raw resources, e.g.
allocate() - Functions where discarding the return value is extremely likely to lead to incorrect code, e.g.
std::remove()
MSVC does mark both std::move() and std::forward() as [[nodiscard]] following these criteria.
While it's not officially annotated as such in the standard, it seems to provide clear user benefit and it's more a question of crafting such a paper to mark all the right things [[nodiscard]] (again, several thousand instances from MSVC) and apply them -- it's not complex work per se, but the volume is large. In the meantime, maybe prod your favorite standard library vendor and ask them to [[nodiscard]] lots of stuff?
answered Apr 20 at 18:52
BarryBarry
188k21337618
add a comment |
The MSVC standard library team went ahead and added several thousand instances of [[nodiscard]] since VS 2017 15.6, and have reported wild success with it (both in terms of finding lots of bugs and generating no user complaints). The criteria they described were approximately:
- Pure observers, e.g.
vector::size(),vector::empty, and evenstd::count_if() - Things that acquire raw resources, e.g.
allocate() - Functions where discarding the return value is extremely likely to lead to incorrect code, e.g.
std::remove()
MSVC does mark both std::move() and std::forward() as [[nodiscard]] following these criteria.
While it's not officially annotated as such in the standard, it seems to provide clear user benefit and it's more a question of crafting such a paper to mark all the right things [[nodiscard]] (again, several thousand instances from MSVC) and apply them -- it's not complex work per se, but the volume is large. In the meantime, maybe prod your favorite standard library vendor and ask them to [[nodiscard]] lots of stuff?
answered Apr 20 at 18:52
BarryBarry
188k21337618
add a comment |
The MSVC standard library team went ahead and added several thousand instances of [[nodiscard]] since VS 2017 15.6, and have reported wild success with it (both in terms of finding lots of bugs and generating no user complaints). The criteria they described were approximately:
- Pure observers, e.g.
vector::size(),vector::empty, and evenstd::count_if() - Things that acquire raw resources, e.g.
allocate() - Functions where discarding the return value is extremely likely to lead to incorrect code, e.g.
std::remove()
MSVC does mark both std::move() and std::forward() as [[nodiscard]] following these criteria.
While it's not officially annotated as such in the standard, it seems to provide clear user benefit and it's more a question of crafting such a paper to mark all the right things [[nodiscard]] (again, several thousand instances from MSVC) and apply them -- it's not complex work per se, but the volume is large. In the meantime, maybe prod your favorite standard library vendor and ask them to [[nodiscard]] lots of stuff?
answered Apr 20 at 18:52
BarryBarry
188k21337618
The MSVC standard library team went ahead and added several thousand instances of [[nodiscard]] since VS 2017 15.6, and have reported wild success with it (both in terms of finding lots of bugs and generating no user complaints). The criteria they described were approximately:
- Pure observers, e.g.
vector::size(),vector::empty, and evenstd::count_if() - Things that acquire raw resources, e.g.
allocate() - Functions where discarding the return value is extremely likely to lead to incorrect code, e.g.
std::remove()
MSVC does mark both std::move() and std::forward() as [[nodiscard]] following these criteria.
While it's not officially annotated as such in the standard, it seems to provide clear user benefit and it's more a question of crafting such a paper to mark all the right things [[nodiscard]] (again, several thousand instances from MSVC) and apply them -- it's not complex work per se, but the volume is large. In the meantime, maybe prod your favorite standard library vendor and ask them to [[nodiscard]] lots of stuff?
answered Apr 20 at 18:52
BarryBarry
188k21337618
edited Apr 20 at 21:44
answered Apr 20 at 18:52
BarryBarry
188k21337618
answered Apr 20 at 18:52
BarryBarry
188k21337618
answered Apr 20 at 18:52
BarryBarry
188k21337618
188k21337618
add a comment |
add a comment |
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1
I know a reason that makes sense to me, but unless there's an official document explicitly saying why it isn't, that just turns the question to be opinion-based.
– Sebastian Redl
Apr 20 at 10:24
8
Because absolutely nothing bad (or at all) happens when you don't use it.
– Sebastian Redl
Apr 20 at 10:33
6
@SebastianRedl: similarly, nothing useful happens. It's the same as writing an empty statement, so
[[nodiscard]]would help diagnose bugs. Also, nothing bad happens whenvector::empty()is ignored, but that is marked[[nodiscard]]for obvious reasons.– Vittorio Romeo
Apr 20 at 10:39
5
@SebastianRedl That sounds like a great reason to mark it
[[nodiscard]]: "Hey, you did something completely pointless. Did you mean to do something else?"– Barry
Apr 20 at 18:53
4
@bbalchev
std::movedoesn't move. Passing an object throughstd::moveand ignoring the result does absolutely nothing.– tkausl
Apr 20 at 23:04