Functional languages with pattern matching (sometimes?) have the possibility to ignore some bound values, but with C++17 structured bindings there seem to be no way to do that (std::ignore with structured bindings?). The advice is to use a dummy name, but then we'll get warnings about unused variables.
With the latest heads of both clang and gcc, this does the expected thing, which is nice and useful,
[[maybe_unused]] auto x =4 ; // fine, no warning
[[maybe_unused]] auto [a,dummyb,dummyc] = std::tuple<int,int,float>(1,1,1.0f);
but I would also have hoped this would work:
auto [g,[[maybe_unused]]dummyh,[[maybe_unused]]dymmyi] =
std::tuple<int,int,float>(1,1,1.0f);
is there a specific reason attributes can not be used here? (in the standard as well as technically). Neither gcc or clang accepts this.
Edit, collecting the support status: (thanks to godbolt/compiler explorer). It works as expected in (could be earlier also):
gcc 8.0 trunk (g++ 8.0.0 20171015 experimental)
clang 4.0.0
icc 18 (not tested, according to specs)
msvc 19.22 (probably earlier) (Fixed, according to bug report)
Try it out in godbolt at https://gcc.godbolt.org/z/H2duYd
In the structure bindings paper:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0144r2.pdf
they discuss their reasoning:
3.8 Should there be a way to explicitly ignore components?
The motivation would be to silence compiler warnings about unused names.
We think the answer should be “not yet.” This is not motivated by use
cases (silencing compiler warnings is a motivation, but it is not a
use case per se), and is best left until we can revisit this in the
context of a more general pattern matching proposal where this should
fall out as a special case.
Symmetry with std::tie would suggest using
something like a std::ignore:
tuple<T1,T2,T3> f();
auto [x, std::ignore, z] = f(); // NOT proposed: ignore second element
However, this feels awkward.
Anticipating pattern matching in the language
could suggest a wildcard like _ or *, but since we do not yet have
pattern matching it is premature to pick a syntax that we know will be
compatible. This is a pure extension that can wait to be considered
with pattern matching.
Although this does not explicitly address [[maybe_unused]], I assume the reasoning might be the same. Stopping compiler warnings is not a use-case.
As a resolution to CWG 2360, the working draft of the standard gained the following wording ([dcl.attr.unused]):
The attribute may be applied to the declaration of a class, a typedef-name, a variable (including a structured binding declaration), a non-static data member, a function, an enumeration, or an enumerator.
For an entity marked maybe_unused, implementations should not emit a warning that the entity or its structured bindings (if any) are used or unused. For a structured binding declaration not marked maybe_unused, implementations should not emit such a warning unless all of its structured bindings are unused.
Structured binding declarations were previously not explicitly mentioned.
Related
I have a function I will need to leave partially implemented for a variety of reasons and I want to prevent future users (read as me in the future when I have forgotten that I did this) to know the function is incomplete, buggy and untested.
Option n1 is merely adding a comment // Warning this thing is partially implemented and will break randomly
This however won't create compile time warnings so, I am not a fan.
Option n2 is to use [[deprecated("reason")]] which has the advantage of raising compile warnings but its misleading, the function wasn't deprecated it's actually the opposite of deprecation, it's a WIP and will perhaps one day be fully implemented.
Are there alternatives?
The [[deprecated]] attribute is exactly what this is for (emphasis mine) :
https://en.cppreference.com/w/cpp/language/attributes
[deprecated]
[deprecated("reason")]
indicates that the use of the name or entity declared with this attribute is allowed, but discouraged for some reason
You can still use the function, you just get a warning message that you shouldn't rely on its use.
Caveat: MSVC breaks the standard and emits a compiler error (due to SDL flag being turned on by default) instead of a warning.
The only thing in standard C++ for this is the [[deprecated("message")]] attribute.
GNU has a non-standard Function Attribute for warning messages:
warning ("message")
If this attribute is used on a function declaration and a call to such a function is not eliminated through dead code elimination or other optimizations, a warning which will include message will be diagnosed. This is useful for compile time checking, especially together with __builtin_constant_p and inline functions. While it is possible to define the function with a message in .gnu.warning* section, when using this attribute the problem will be diagnosed earlier and with exact location of the call even in presence of inline functions or when not emitting debugging information.
Functional languages with pattern matching (sometimes?) have the possibility to ignore some bound values, but with C++17 structured bindings there seem to be no way to do that (std::ignore with structured bindings?). The advice is to use a dummy name, but then we'll get warnings about unused variables.
With the latest heads of both clang and gcc, this does the expected thing, which is nice and useful,
[[maybe_unused]] auto x =4 ; // fine, no warning
[[maybe_unused]] auto [a,dummyb,dummyc] = std::tuple<int,int,float>(1,1,1.0f);
but I would also have hoped this would work:
auto [g,[[maybe_unused]]dummyh,[[maybe_unused]]dymmyi] =
std::tuple<int,int,float>(1,1,1.0f);
is there a specific reason attributes can not be used here? (in the standard as well as technically). Neither gcc or clang accepts this.
Edit, collecting the support status: (thanks to godbolt/compiler explorer). It works as expected in (could be earlier also):
gcc 8.0 trunk (g++ 8.0.0 20171015 experimental)
clang 4.0.0
icc 18 (not tested, according to specs)
msvc 19.22 (probably earlier) (Fixed, according to bug report)
Try it out in godbolt at https://gcc.godbolt.org/z/H2duYd
In the structure bindings paper:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0144r2.pdf
they discuss their reasoning:
3.8 Should there be a way to explicitly ignore components?
The motivation would be to silence compiler warnings about unused names.
We think the answer should be “not yet.” This is not motivated by use
cases (silencing compiler warnings is a motivation, but it is not a
use case per se), and is best left until we can revisit this in the
context of a more general pattern matching proposal where this should
fall out as a special case.
Symmetry with std::tie would suggest using
something like a std::ignore:
tuple<T1,T2,T3> f();
auto [x, std::ignore, z] = f(); // NOT proposed: ignore second element
However, this feels awkward.
Anticipating pattern matching in the language
could suggest a wildcard like _ or *, but since we do not yet have
pattern matching it is premature to pick a syntax that we know will be
compatible. This is a pure extension that can wait to be considered
with pattern matching.
Although this does not explicitly address [[maybe_unused]], I assume the reasoning might be the same. Stopping compiler warnings is not a use-case.
As a resolution to CWG 2360, the working draft of the standard gained the following wording ([dcl.attr.unused]):
The attribute may be applied to the declaration of a class, a typedef-name, a variable (including a structured binding declaration), a non-static data member, a function, an enumeration, or an enumerator.
For an entity marked maybe_unused, implementations should not emit a warning that the entity or its structured bindings (if any) are used or unused. For a structured binding declaration not marked maybe_unused, implementations should not emit such a warning unless all of its structured bindings are unused.
Structured binding declarations were previously not explicitly mentioned.
Could C++ standards gurus please enlighten me:
Since which C++ standard version has this statement failed because (v) seems to be equivalent to (*&v)?
I.e. for example the code:
#define DEC(V) ( ((V)>0)? ((V)-=1) : 0 )
...{...
register int v=1;
int r = DEC(v) ;
...}...
This now produces warnings under -std=c++17 like:
cannot take address of register variable
left hand side of operand must be lvalue
Many C macros enclose ALL macro parameters in parentheses, of which the above is meant only to be a representative example.
The actual macros that produce warnings are for instance
the RTA_* macros in /usr/include/linux/rtnetlink.h.
Short of not using/redefining these macros in C++, is there any workaround?
If you look at the revision summary of the latest C++1z draft, you'd see this in [diff.cpp14.dcl.dcl]
[dcl.stc]
Change: Removal of register storage-class-specifier.
Rationale: Enable repurposing of deprecated keyword in future
revisions of this International Standard.
Effect on original feature: A valid C++ 2014 declaration utilizing the register
storage-class-specifier is ill-formed in this International Standard.
The specifier can simply be removed to retain the original meaning.
The warning may be due to that.
register is no longer a storage class specifier, you should remove it. Compilers may not be issuing the right error or warnings but your code should not have register to begin with
The following is a quote from the standard informing people about what they should do with regards to register in their code (relevant part emphasized), you probably have an old version of that file
C.1.6 Clause 10: declarations [diff.dcl]
Change: In C++, register is not a storage class specifier.
Rationale: The storage class specifier had no effect in C++.
Effect on original feature: Deletion of semantically well-defined feature.
Difficulty of converting: Syntactic transformation.
How widely used: Common.
Your worry is unwarranted since the file in question does not actually contain the register keyword:
grep "register" /usr/include/linux/rtnetlink.h
outputs nothing. Either way, you shouldn't be receiving the warning since:
System headers don't emit warnings by default, at least in GCC
It isn't wise to try to compile a file that belongs to a systems project like the linux kernel in C++ mode, as there may be subtle and nasty breaking changes
Just include the file normally or link the C code to your C++ binary. Report a bug if you really are getting a warning that should normally be suppressed to your compiler vendor.
I recall someone once telling me,
"there is no need for auto inside range-based for loops. It would
not be ambiguous in the language if we were to remove it."
Is that a true statement?
Is the folowing code valid C++ syntax?
for (elem : range){...}
I had assumed this was already valid syntax, but when I went to compile with
clang++ --std=c++1z, I was shown the following error:
range-based for loop requires type for loop variable
for (elem: range){
The compiler still recognizes this as a range-based for loop, so why can't it also derive the type?
The:
for (elem : range){...}
syntax is not currently valid but there was a proposal to make this valid syntax and the syntax is supported in gcc 5.2 (see it live):
#include <vector>
int main()
{
std::vector<int> v ;
for( elem : v )
{
}
}
and if we try this in C++14 mode it says:
warning: range-based for loop without a type-specifier only available
with -std=c++1z or -std=gnu++1z
So this would clearly work and has been implemented in gcc. It looks like this feature was removed in gcc 6.0.
As far as I can tell this was implemented in gcc with the expectation that proposal N3853: Range-Based For-Loops: The Next Generation would be accepted but it was rejected and the updated version N3994 says:
This updates N3853 (see [1]) which proposed the syntax "for (elem :
range)", by adding support for attributes and answering additional
questions. Please see the original proposal for the rationale behind
this feature, which is not repeated here.
We can see it was rejected from the EWG issue 81 and we can also see this from the Urbana meeting minutes. Although there are many issues with the proposal I believe STL made a convincing set of arguments in the Question and Answers section of the proposal and I was disappointed that the proposal was rejected.
The syntax requires a type for a range-for statement, even if it's auto. for (elem : range) {...} is a syntax error, so technically, yes, it's true, the language could state that this is equivalent to for (auto elem : range) {...}.
There are at least two major problems with that, though:
The first is that for (T elem : range) doesn't require T to use auto. If the compiler sees for (elem, it wouldn't yet have any way of knowing whether elem is some typedef from an outer scope, or a newly declared variable. It's not ambiguous, but it's a bit complicated for compilers to handle correctly.
The second is that you then get the question of what the default should be. Legitimate arguments could be made for auto. Legitimate arguments could be made for auto &. Legitimate arguments could be made for const auto &. The current approach just lets the programmer choose. Legitimate arguments could probably (I'm not entirely sure) also be made for some of those with auto replaced by decltype(auto).
This is really a question for ELL...
The sentence is written in the subjunctive case, meaning that it is talking about a purely hypothetical situation, not the actual language rules. Subjunctive is used for counterfactual situations.
While qualifying an enumeration value with the name of the enumeration is not valid C++03, it is valid C++11, from what I understand. Despite this, MSVC 10 generates warning C4482 for the following:
enum E { A, B };
int i = E::A; // warning C4482 (but valid C++11?)
Since much of our code uses C++11 features (especially lambdas), it seems safe to disable this warning. Am I right that the code is valid C++11?
Note: I did not write the code in question, and I would prefer to not go through and change every occurrence of this.
Edit: Added some relevant links.
MSDN page for the warning.
Another question about the warning. The question and answers all seem to reference C++03.
Since much of our code uses C++11 features (especially lambdas), it seems safe to disable this warning.
If you're already relying on C++11 features, then yes. C++11 does allow you to use regular enums scoped by the enumeration's name. Microsoft had this as an extension for some time, so they issued a warning about the non-standard behavior.
So you can disable it.
Note that older compilers like VC2010, instead of warning, did raise compile error C2653 (with message "... is not a class or namespace name").