ISO/IEC JTC1 SC22 WG21 P0870R5 2023-02-10
Project: Programming Language C++
Audience: Library Working Group, Library Evolution Working Group, Study Group 6 (Numerics)
Giuseppe D'Angelo, giuseppe.dangelo@kdab.comThis paper proposes a new type trait for the C++ Standard Library,
is_convertible_without_narrowing
, to detect whether a type is
implictly convertible to another type without going through a narrowing
conversion.
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[variant.ctor], from [N4861]:
template<class T> constexpr variant(T&& t) noexcept(see below);
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template<class T> constexpr variant(T&& t) noexcept(see below);
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is_convertible_without_narrowing
following a poll on the mailing list;
the semantics have been adapted.void
, etc.).A narrowing conversion is formally defined by the C++ Standard in [dcl.init.list], with the intent of forbidding them from list-initializations.
It is useful in certain contexts to know whether a conversion between two types
would qualify as a narrowing conversion. For instance, it may be useful to
inhibit (via SFINAE) construction or conversion of "wrapper" types
(like std::variant
or std::optional
) when
a narrowing conversion is requested.
A use case the author has recently worked on was to prohibit narrowing conversions when establishing a connection in the Qt framework (see [Qt]), with the intent of making the type system more robust. Simplifying, a connection is established between a signal non-static member function of an object and a slot non-static member function of another object. The signal's and the slot's argument lists must have compatible arguments for the connection to be established: the signal must have at least as many arguments as the slot, and the each argument of the signal must be implictly convertible to the corresponding argument of the slot. In case of a mismatch, a compile-time error is generated. Bugs have been observed when connections were successfully established, but a narrowing conversion (and subsequent loss of data/precision) was happening. Detecting such narrowing conversions, and making the connection fail for such cases, would have prevented the bugs.
This proposal is a pure library extension.
It proposes changes to an existing header, <type_traits>
,
but it does not require changes to any standard classes or functions and
it does not require changes to any of the standard requirement tables.
This proposal does not require any changes in the core language, and it has been implemented in standard C++.
This proposal does not depend on any other library extensions.
This proposal may help with the implementation of
[P0608R3], the adopted resolution for
[LEWG 227]. In particular, in
[P0608R3]'s wording section, the "invented variable
x
" is used to detect a narrowing conversion. However, the same
paragraph is also broader than the current proposal, in the sense that it
special-cases boolean conversions [conv.bool], while the current proposal
does not handle them in any special way. The part about boolean conversions
has been anyhow removed by adoping [P1957R2].
The just mentioned [P1957R2], as a follow up of
[P0608R3], made conversions from pointers to
bool
narrowing. It was adopted in the Prague meeting,
therefore extending the definition of narrowing conversion in [dcl.init.list].
We do not see this as a problem: we aim to provide a trait to
detect narrowing conversions exactly as specified by the Standard. Should
the specification change, then the trait detection should also change
accordingly; cf. the design decisions below.
This proposal overlaps with [P1818R1], a proposal
that aims at introducing a distinction between narrowing and widening conversions.
[P1818R1] is much more ambitious than this proposal;
amongst other things, it aims at defining a widening
trait.
The wording of [P1818R1] is not complete, but we think
that such a trait somehow overlaps with the semantics of the trait proposed here;
it would ultimately depend on the exact definition of what constitutes a
"widening conversion". Anyhow, the adoption of [P1818R1]
is orthogonal to the present proposal.
[P1619R1] and [P1998R1] introduce
functions (called respectively can_convert
and
is_value_lossless_convertable
) that check whether a given value
of an integer type can be represented by another integer type. This is in line
with the spirit of detecting narrowing conversions, namely, preventing loss of
information and/or preventing undefined behavior. While this proposal works on
types, the functions examine specific values; we therefore think that
the proposals are somehow orthogonal to the current proposal.
Moreover, [P1619R1] and [P1998R1]'s
functions only deal with integer types, while the narrowing definition in
[dcl.init.list] (adopted by this proposal) also deals with floating point and
enumeration types (and, following [P1957R2]'s
adoption, also boolean and pointer types).
[P2509R0] proposes a type trait which complements the
present proposal, by detecting if a given target type can represent all the values
of a given source type. This is different from detecting a narrowing conversion
as defined by the core language; for instance, converting int
to double
is a narrowing conversion even on common platforms where double
can actually precisely represent any int
value.
The most natural place to add the trait presented by this proposal
is the already existing <type_traits>
header, which
collects most (if not all) of the type traits available from the Standard Library.
In the <type_traits>
header the is_convertible
type trait checks whether a type is implictly convertible to another type.
Building upon this established name, the proposed name for the trait
described by this proposal shall therefore be is_convertible_without_narrowing
,
with the corresponding is_convertible_without_narrowing_v
type trait helper
variable template.
is_convertible_v<From, To> == true
be
a precondition of trying to instantiate
is_convertible_without_narrowing<From, To>
?The author deems this unnecessary. Adding such a precondition would likely make the usage of the proposed trait more difficult and/or error prone.
As far as the author can see, the precondition is not going to dramatically simplify the specification for the proposed trait, or its implementation.
From
convertible to a type To
,
should is_convertible_without_narrowing_v<From, To>
yield
false
only for the combinations of types listed in [dcl.init.list]?
What about user-defined types?The R1 revision of this paper was limiting the possibility for
the trait to detect narrowing if and only if both From
and To
were one of the types listed in [dcl.init.list], which
are all builtin types.
Now, while we strongly want the type trait to match the language definition
of narrowing conversion, there are some objections to simply detecting
if we are in one of the those cases.
For instance, should a conversion from const double
to float
be considered narrowing or not? If we take the listing
in [dcl.init.list] literally, then the answer would be "no"
(that is, the trait would have to yield true
), because
const double
is not in the list.
However, this is not really useful, nor expected. If some code checks whether
there is a conversion between two types and the conversion isn't narrowing,
such code could erroneously infer that this is the case between
const double
and float
and do the conversion.
(This is in spite of the fact that list-initialization would not work,
because there is a narrowing conversion in the conversion sequence).
To elaborate on this last sentence: the definition of list-initialization
in [dcl.init.list] contains several cases where, if a narrowing
conversion takes place, then the program is ill-formed. We believe
that, more than detecting if a given conversion between two types is
a narrowing conversion (as strictly per the definition),
users want to detect if a conversion has a narrowing conversion,
and thus would be forbidden if used in a list-initialization.
As a consequence, we claim that
is_convertible_without_narrowing_v<const double, float>
should
yield false
.
This is not simply doable by stripping From
and To
of their cv-qualifiers (e.g. by applying std::decay_t
on them),
and then checking if the resulting types do not form a narrowing conversion. One
must also consider all the other cases for conversions, which necessarily
include user-defined types, as they may define implicit conversion operators
that can be used as part of a conversion sequence that also includes a narrowing
conversion. As an example:
struct S { operator double() const; };
S s;
float f1 = s; // OK
float f2 = { s }; // ERROR: narrowing
We therefore claim that is_convertible_without_narrowing_v<S, float>
should yield false
. A similar example is used in
[P0608R3] and
then [P1957R2]'s design. Slightly adapted:
struct Bad { operator char const*() &&; };
With the adoption of [P1957R2] in the Standard, we
claim that is_convertible_without_narrowing_v<Bad, bool>
should yield
false
. For completeness,
is_convertible_without_narrowing_v<Bad &, bool>
should yield
false
(there is no conversion), and
is_convertible_without_narrowing_v<Bad &&, bool>
should yield
false
as well (narrowing).
Note that, although user-defined datatypes are involved, the detection of whether there is a narrowing conversion sticks to the list of narrowing conversions in [dcl.list.init], which deals with fixed number of cases: between certain integer and floating point types; from unscoped enumeration types to a integer or a floating point type; and from pointer types to boolean. We are not proposing to allow any customization to these cases (see below).
is_convertible_without_narrowing<From, To>
yield
false
for boolean conversions [conv.bool]?The author deems this to be out of scope for the proposed trait, which should have only the semantics defined by the language regarding narrowing conversion. Another trait should be therefore added to detect (implicit) boolean conversions, which, according to the current wording of [dcl.init.list], are not considered narrowing conversions. (Note that with the adoption of [P1957R2], conversions from pointer types to boolean are indeed considered narrowing.)
It is the author's opinion that, in general, implicit boolean conversions are undesirable for the same reasons that make narrowing conversions unwanted in certain contexts — in the sense that a conversion towards bool may discard important information. However, we recognize the importance of not imbuing a type trait with extra, not yet well-defined semantics, and therefore we are excluding boolean conversions from the present proposal.
is_convertible_without_narrowing<From, To>
use
some other semantics than the narrowing defined in [dcl.init.list]?Given that narrowing is formally defined in the core language, we are strongly against deviating from that definition. If anything, proposals aiming at marking as narrowing certain conversions between user defined types should also amend the core language definition; the type trait would still stick to that definition.
is_convertible_without_narrowing<From, To>
be allowed?We believe that specializations of the trait should be forbidden. This is consistent with the other type traits defined in [meta] (cf. [meta.rqmts]), and with the fact that the current wording of [dcl.init.list] defining narrowing conversions does not extend to user-defined types.
We are aware of broader work happening in the area, such as
[P1818R1]. That proposal introduces a keyword to mark
widening conversions, but one may think of an alternative approach where users
are expected to specialize is_convertible_without_narrowing
in order to
mark a conversion as non-narrowing. In any case, lifting the above limitation on
specializations would always be possible.
This objection has been raised during the LEWGI meeting in Prague. The objection is sound, in the sense that a future possible change to the definition of narrowing conversion will impact user code using the trait.
On the other hand, during the Prague meeting, EWGI and EWG did not raise this concern regarding this proposal.
First and foremost, we would like to remark that the trait is already
implementable — and has indeed already been implemented — using
standard C++ (e.g. via To{std::declval<From>()}
, cf. below).
Any change to the definition of narrowing would therefore already be impacting
user code. The impact would likely go beyond type traits / metaprogramming,
since initialization itself would change semantics.
An example of such a change affecting [dcl.init.list] comes from
[P1957R2], which has been adopted in Prague. That
adoption changed the semantics of code like bool b = {ptr};
(making
it ill-formed; was OK before).
In general, changes to the core language that affected type traits
have already happened. For instance, between C++17 and C++20 the definition
of "aggregate" in [dcl.init.aggr] has been changed, therefore changing the
semantics of the std::is_aggregate
type trait.
Similarly, the definitions of "trivial class" and of "trivially copyable class"
in [class.prop] have changed between C++14 and C++17;
meaning that also the type traits std::is_trivial
and
std::is_trivially_copy_constructible
have changed semantics.
We can therefore reasonably assume that the presence of a trait does not seem to preclude changes to the core language (or, if such an objection was raised when proposing these changes to the core language, it was also disregarded, as the changes did ultimately land).
Finally, it is the author's opinion that code that is using facilities to detect narrowing conversions would like to stick to the core language definition. If the definition changes, we foresee that the detection wants to change too, following the definition. Note that an ad-hoc solution (rather than a standardized trait) may fall out of sync with the core language definition, and thus start misdetecting some cases.
This point was raised during the LEWGI meeting in Prague, and it was deemed unnecessary at this point, because ultimately this proposal is just about a type trait. If it will be deemed useful, adding a concept could always be done at a later time, via another proposal.
The LEWGI review of the R3 revision of this paper discussed several options for the name of the trait.
In earlier versions we proposed the name is_narrowing_convertible
;
however, during the review of the paper, there was a general consensus
that the majority of uses is going to be about detecting whether a conversion
is possible without narrowing.
As such, is_narrowing_convertible
(which detects if a conversion
does involve a narrowing conversion) would always need to be combined with
is_convertible
to achieve the desidered result, for instance like this:
~~~
// detect if From is convertible to To without a narrowing conversion
requires (is_convertible_v<From, To> && !is_narrowing_convertible_v<From, To>)
~~~
Another concern was raised about the very usage of "narrowing convertible", which is not currently a term used anywhere in the Standard.
Therefore, a poll was taken on the lib-ext mailing list, trying to find a better name, offering options for both a positive name (conversion sequence involves a narrowing conversion) or a negative one (conversion sequence does not involve a narrowing conversion). The proposed options were:
is_narrowing_convertible
is_convertible_with_narrowing
is_narrowing
is_non_narrowing_convertible
is_nonarrowing_convertible
is_nonnarrowing_convertible
is_convertible_without_narrowing
is_convertible_no_narrowing
is_non_narrowing
The poll had very scarce participation, but
is_convertible_without_narrowing
emerged as the preferred option.
The R4 revision of this paper renamed the trait, and, consequently,
adapted the discussion regarding its semantics —
that is, to detect types that are convertible through a conversion
sequence that does not require a narrowing conversion.
This required some rewording in a few places.
Many of the cases for narrowing conversions listed in [dcl.init.list] have an exception that states that a given conversion is not narrowing if the source is a constant expression, and the converted value satisfies certain criteria. For instance, consider this example:
int from = 42;
float to{from}; // ill-formed, narrowing
The initialization of to
is ill-formed because of narrowing, although 42 is representable by a float
. This is because the source is not a constant expression, so the relative provision in [dcl.init.list]/7.3 does not apply. Specifically, a lvalue-to-rvalue conversion is required for the initialization, and [expr.const]/5.9 makes the expression not a core constant expression.
It's however sufficient to change the snippet to this:
const int from = 42; // add const here
float to{from}; // OK
and now there is no longer a narrowing conversion, because the source is a constant expression, and 42 can be represented exactly by both float
and int
.
A similar example is this one:
std::integral_constant<int, 42> from; // not const
float to{from}; // OK
Here the initialization of to
is again well-formed. The source now is a constant expression, and 42 can be represented exactly. Although from
is not "an object that is usable in constant expressions" ([expr.const]/4), there is nothing that disqualifies the expression from being a core constant expression.
(Again, specifically: the aforementioned lvalue-to-rvalue conversion does not apply here. The list-initialization of an object of type float
([dcl.init.general]/16.1) will resolve into direct-initialization ([dcl.init.list]/3.9); then, according to [dcl.init.general]/16.7, a user-defined conversion function is called and that "converts the initializer expression into the object being initialized". For the std::integral_constant
class template, the conversion function (in the example, from.operator int()
) is constexpr
. Then, the prvalue int
obtained as a result is converted to float
using a floating-integral conversion ([conv.fpint]/2). In conclusion, this means that the source is a constant expression, and there is not a narrowing conversion in the initialization of to
.)
It may appear that these considerations need to affect the design of the trait: should is_convertible_without_narrowing_v<int, float>
yield true
or false
? What about is_convertible_without_narrowing_v<const int, float>
, or is_convertible_without_narrowing_v<std::integral_constant<int, 42>, float>
?
The answer to these questions lies in the fact that the "exceptions" to the narrowing conversion rules are all based on the source being a constant expression, and that is a quality that is not reflected in any way in the type system.
For instance, a const int
variable may or may not be (usable in) a constant expression depending on how and where it gets initialized ([expr.const]/2, 3, 4). Converting such a variable to a float
may or may not be a narrowing conversion. We do not have any means to know if it's the case from the types alone — not to mention the values.
For this reason, a type trait like the one that we are proposing will never be able to provide a detection that is complete, but only a correct one (no false positives). This does not undermine the usefulness of the trait, which is first and foremost to avoid conversions that may result in accidental loss of data.
Therefore, as a design decision, we are going to assume that the source expression is never a constant expression, and have the trait give the corresponding answer. Practically speaking, we believe this matches the main use case of this trait: constraining functions from being called with parameters of types that may narrow when converted to some other type. These parameters are usually not going to be (usable in) constant expressions anyways:
template <typename T>
struct my_optional
{
template <typename U = T>
requires std::is_convertible_without_narrowing_v<U, T>
my_optional(U&& u) // `u` not usable in constant expression
: data{std::forward<U>(u)}, loaded{true} {}
};
This design decision is reflected in the proposed wording. This stems from the fact that definition is built on top of the one for std::is_convertible
, and that one uses a call to std::declval<From>(), which is not a constexpr
function and therefore not a constant expression.
The proposed type trait has already been implemented in various codebases using ad-hoc solutions (depending on the quality of the compiler, etc.), using standard C++ and without the need of any special compiler hooks.
One possible implementation is to exhaustively check whether a conversion
between two types fall in one of the narrowing cases listed in [dcl.list.init].
This particular implementation has been done in
[Qt 5's connect()
]
(see this proposal's motivation about why this
detection has been added to Qt in the first place);
it has obviously maintenance and complexity shortcomings. The most important
part is:
template<typename From, typename To, typename Enable = void>
struct AreArgumentsNarrowedBase : std::false_type
{
};
template<typename From, typename To>
struct AreArgumentsNarrowedBase<From, To, typename std::enable_if<sizeof(From) && sizeof(To)>::type>
: std::integral_constant<bool,
(std::is_floating_point<From>::value && std::is_integral<To>::value) ||
(std::is_floating_point<From>::value && std::is_floating_point<To>::value && sizeof(From) > sizeof(To)) ||
((std::is_integral<From>::value || std::is_enum<From>::value) && std::is_floating_point<To>::value) ||
(std::is_integral<From>::value && std::is_integral<To>::value
&& (sizeof(From) > sizeof(To)
|| (std::is_signed<From>::value ? !std::is_signed<To>::value
: (std::is_signed<To>::value && sizeof(From) == sizeof(To))))) ||
(std::is_enum<From>::value && std::is_integral<To>::value
&& (sizeof(From) > sizeof(To)
|| (IsEnumUnderlyingTypeSigned<From>::value ? !std::is_signed<To>::value
: (std::is_signed<To>::value && sizeof(From) == sizeof(To)))))
>
{
};
(Historical note: at the time this implementation was done, the codebase could only use C++11 features, and some compilers were unable to cope with the SFINAE solution presented below.).
The Qt implementation strictly checks for the one of narrowing cases listed in [dcl.init.list], which is not the aim of the current proposal (cf. design decisions above). Also, the snippet does not yet implement the change introduced by [P1957R2], showing an inherent limitation to such an "exhaustive" solution: it may fall out of sync with the core language.
Another, much simpler implementation is to rely on SFINAE around an expression
such as To{std::declval<From>()}
. However, this form is also
error-prone: it may accidentally select aggregate initialization for To
,
or a constructor taking a std::initializer_list
, and so on.
If for any reason these make the expression ill-formed, then the trait is
going to report that there is a narrowing conversion between the types, even
when there actually isn't one (or vice-versa, depending on how one builds
the trait).
[P0608R3] uses a clever
workaround to avoid falling into these traps: declaring an array, that is,
building an expression like To t[] = { std::declval<From>() };
.
This solution is now used in the Standard (in [variant.ctor] and
[variant.assign]), amongst other things, in order to detect and prevent a
narrowing conversion. The trait that we are proposing could be used to replace
the ad-hoc solution, and clarify the intent of it.
A possible implementation that uses the just mentioned workaround is the following one (courtesy of Zhihao Yuan; received via private communication, adapted):
template<class From, class To>
concept is_convertible_without_narrowing = std::is_convertible_v<From, To> &&
requires (From&& x) {
{ std::type_identity_t<To[]>{std::forward<From>(x)} } -> std::same_as<To[1]>;
};
(A C++17-compatible version of the above implementation has been implemented in Qt 6.)
While an array declaration can be used as the main part of detection,
a complete implementation also needs handling of the "corner cases". Types such
as cv void
, reference types, function types,
arrays of unknown bounds, etc., require special care because they cannot
be used to declare the array of To
, and this may give wrong
results. Note that we propose that it is legal to specialize
is_convertible_without_narrowing
with these types, following
the precedent set by is_convertible
.
This handling of "corner cases" is not much different anyhow from
the practical implementations of other type traits: for instance, in
both [libstdc++]
and [libc++] implementations of
is_convertible
there is code that deals with some types in a
special way.
In conclusion, it is the author's opinion that
is_convertible_without_narrowing
can be implemented in standard C++
without placing an unreasonable burden on implementors.
Author's note: given the amount of subtleties involved, we claim
that it is extremely likely that non-experts would go for a "naïve",
wrong solution (e.g. To{std::declval<From>()}
). This further
underlines the necessity of providing detection of narrowing conversions
in the Standard Library, rather than having users reinventing it with
ad-hoc solutions.
We propose the usage of the __cpp_lib_is_convertible_without_narrowing
macro, following the pre-existing nomenclature for the other traits in
<type_traits>
.
All changes are relative to [N4928].
In [meta.type.synop], add to the header synopsis:
template <class From, class To> struct is_convertible; template <class From, class To> struct is_nothrow_convertible; template <class From, class To> struct is_convertible_without_narrowing;
template <class From, class To> constexpr bool is_convertible_v = is_convertible<From, To>::value; template <class From, class To> constexpr bool is_nothrow_convertible_v = is_nothrow_convertible<From, To>::value; template <class From, class To> constexpr bool is_convertible_without_narrowing_v = is_convertible_without_narrowing<From, To>::value;
In [meta.rel], insert a new row to the Type relationship predicates table (Table 50) after the row for is_nothrow_convertible
, with these contents in each column:
- Template
template <class From, class To> struct is_convertible_without_narrowing;
- Condition
is_convertible_v<From, To>
istrue
and the conversion, as defined byis_convertible
, does not require a narrowing conversion ([dcl.init.list]).- Comments
From
andTo
shall be complete types, cvvoid
, or arrays of unknown bound.
In [version.syn], add to the list of feature testing macros in paragraph 2:
#define __cpp_lib_is_convertible_without_narrowing TBD
// also in<type_traits>
The actual value is left to be determined (TBD
);
it should be replaced by yyyymmL
,
matching the year and month of adoption of this proposal.
Thanks to KDAB for supporting this work.
Thanks to Marc Mutz for his feedback and championing this paper in Prague; to Zhihao Yuan for the discussion and some sample code; to Walter E. Brown and André Somers for their feedback.
All remaining errors are ours and ours only.
[Qt] Qt version 5.15, QT_NO_NARROWING_CONVERSIONS_IN_CONNECT, QObject class documentation
[Qt connect()
] Qt version 5.15, traits for detecting narrowing conversions
[Qt connect()
] Qt version 6.4.2, traits for detecting narrowing conversions
[P0608R3] Zhihao Yuan, A sane variant converting constructor (LEWG 227)
[LEWG 227] Zhihao Yuan, Bug 227 - variant converting constructor allows unintended conversions
[P1957R2] Zhihao Yuan, Converting from T*
to bool
should be considered narrowing (re: US 212)
[P1818R1] Lawrence Crowl, Narrowing and Widening Conversions
[P1619R1] Lisa Lippincott, Functions for Testing Boundary Conditions on Integer Operations
[P1998R1] Ryan McDougall, Simple Facility for Lossless Integer Conversion
[libstdc++] libstdc++, implementation of is_convertible
[libc++] libc++, implementation of is_convertible
[N4928] Thomas Koeppe, Working Draft, Standard for Programming Language C++
[P1467R4] David Olsen, Michał Dominiak, Extended floating-point types and standard names
[P2509R0] Giuseppe D'Angelo, A proposal for a type trait to detect value-preserving conversions