Document number | P3122R1 |
Date | 2024-03-12 |
Project | Programming Language C++, Library Working Group |
Reply-to | Jonathan Wakely <cxx@kayari.org> |
std::remove_if
.Instead of sprinkling [[nodiscard]]
in hundreds of places in the library
clauses, there should be a single normative recommendation to use it wherever
it's appropriate. LEWG policy should be to avoid spending time discussing it.
This paper proposes a policy for LEWG and new wording for the standard.
It's not a good use of time to discuss adding [[nodiscard]]
to individual
function declarations. That requires LEWG time, LWG time, WG21 votes,
editorial changes, and reviews to ensure those changes were correctly applied.
For example,
P2377R0 [[nodiscard]] in the Standard Library: Clause 23 Iterators Library
only covers a single clause and is still nearly ten pages of wording.
And it has a mistake in the very first change it proposes to a declaration,
which would need to be reviewed and corrected.
What we want is for real implementations to give real warnings to users writing real code. Whether the attribute is literally present on individual declarations in the PDF standard is not important.
P2377R0 argued that it's a good use of committee time to add it to the library because it reminds implementors to use the attribute and improve the quality of their implementations. I feel that a single normative recommendation to use it is a far better use of time than micro-managing how implementations declare individual functions.
Implementers are best placed to decide on which functions to add the attribute
for their specific implementation. If a compiler has built-in warnings
for side-effect-free expressions, e.g. it can determine that it == end;
doesn't modify its arguments and so issues a warning, then there is no need
for that implementation to explicitly add the [[nodiscard]]
attribute to
every inline operator==
definition.
We should aim for good quality diagnostics,
not worry about how that is achieved.
Implementors are also more able to respond promptly to changing needs or to user feedback regarding nodiscard diagnostics. New compiler releases happen more frequently than revisions of the C++ standard, and compiler vendors can trial new nodiscard warnings under an optional setting to gather usage experience. This is more flexible than spending WG21 time to discuss an addition that is then enshrined in the standard for at least three years until the next revision (or until more committee time is spent to revise it via a defect report).
The wording below only talks about functions. The valid uses for nodiscard
on types are less common, and it might be worth discussing individual cases,
and adding the attribute to declarations in the standard if implementors
need extra guidance there. Alternatively, maybe blanket wording can be
produced that accurately describes the cases that should produce warnings.
The wording does not cover functions which return a pointer to allocated
storage, such as operator new
and std::allocator<T>::allocate
.
All such functions are marked nodiscard
already. If the group prefers it,
we could remove those existing nodiscard
attributes and add a new bullet like
"Functions that return a pointer to allocated storage. [Example:
operator new
returns memory which could not be deallocated if discarded.
— end example]"
The wording does not cover functions that are considered likely to be confused
with another function, due to ambiguous meaning of English nouns vs verbs,
such as std::vector<T, A>::empty()
which could be misunderstood to
empty the container (which is what the clear
function does). It is not easy
to specify generic wording that clearly defines what might or might not be
confusing, and the particular example of vector::clear
is already covered
because it's a pure function that is only called for its return value. The
more general policy already solves the problem, without a special case based
on potential confusion around the name.
Policy: Implementors are encourage to diagnose discarded-value expressions via blanket wording in the library introduction clause, so the explicit use of
nodiscard
in library wording should be avoided. Subclause [lib.nodiscard] covers the cases that implementors are encouraged to diagnose. Anything covered there does not need to be markednodiscard
in library wording. New proposals should not usenodiscard
in proposed wording unless it's a case not covered by [lib.nodiscard], and then the paper should provide rationale for not following the policy.
Rational: Implementers are best placed to decide on which functions to add the attribute for their specific implementation. They do not need the standard to specify where it must be used.
Policy paper: P3122
This wording is relative to N4971.
Add a new subclause after 16.4.6.15 [lib.types.movedfrom]:
16.4.6.?? Discarded calls [lib.nodiscard]
Except where shown otherwise, it is unspecified which functions defined
in the C++ standard library are marked with a nodiscard
attribute.
Recommended practice: Implementations should issue a warning for potentially-evaluated discarded-value expressions ([expr.context]) where the expression is a call to any of the following:
operator==
and operator<=>
functions. const
member functions and explicit object member functions
that do not directly or indirectly modify objects accessible via their
arguments.vector::empty() const
and
unordered_map::bucket_size(size_type) const
do not modify any objects.
— end example]
vector::begin()
and
set::find(const key_type&)
are not const
so that they can return a mutable iterator,
but they do not modify the container or its elements.
basic_string::operator[](size_type)
does not modify the
string or its contents.
— end example]as_const(T&)
behaves like a cast, it has no side effects.
back_inserter(Container&)
takes a reference to a non-const
object but does not modify it.
ranges::next(I)
increments its by-value parameter,
which does not modify any other objects.
— end example]
void
.copy_if
returns an iterator pointing to the end of the result range,
which depends on how many elements of the input range match the predicate.
remove
and unique
return an iterator pointing to the end of the result
range, which depends on how many elements from the input range were not
included in the output range.
— end example]Thanks to Nicolai Josuttis, Hana Dusíková, and Christopher Di Bella
for the past work identifying where nodiscard
is useful.
P0600R1, [[nodiscard]] in the Library, Nicolai Josuttis, 2017.
P2351R0, Mark all library static cast wrappers as [[nodiscard]]
, Hana Dusíková, 2020.
P2377R0, [[nodiscard]] in the Standard Library: Clause 23 Iterators Library, Christopher Di Bella, 2021.
N4971, Working Draft - Programming Languages -- C++, Thomas Köppe, 2023.