C++11-style [[unused]] attribute in gcc?

Question

Under gcc/g++ 4.9 I can write:

int x __attribute__((unused)) = f();

to indicate that x is intentionally unused.

Is it possible to do this with the C++11 [[]] attribute notation somehow?

I tried:

int x [[unused]] = f();

but it doesn't work.

(Yes, I know it is an implementation-defined attribute.)

Why would you think it would work? Did you see any language specifications? — SwiftMango, Aug 09 '15 at 23:05
And no it doesn't work. It's gcc extension only. Not language specs — SwiftMango, Aug 09 '15 at 23:05
It does not perfectly fit your question as an answer but [my comment below](http://stackoverflow.com/questions/31909631/c11-style-unused-attribute-in-gcc#comment51735300_31909665) may interest you. — Shafik Yaghmour, Aug 10 '15 at 02:21

Alan Kazbekov · Answer 1 · 2017-11-18T19:54:29.430

28

There is [[maybe_unused]] attribute in C++17. It's implemented in GCC 7, see C++ Standards Support in GCC .

Example from P0212R1 proposal:

[[maybe_unused]] void f([[maybe_unused]] bool thing1,
                        [[maybe_unused]] bool thing2) {
    [[maybe_unused]] bool b = thing1 && thing2;
    assert(b);
}

edited Nov 18 '17 at 19:54

answered Nov 17 '17 at 12:53

Alan Kazbekov

1,105
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AliciaBytes · Accepted Answer · 2015-08-09T23:17:03.013

Yes, use [[gnu::unused]]

Like already said unused isn't part of the standard attributes specified by the standard.

The standard allows implementation defined attributes too like the __attribute__ and __declspec ones to be used with the new syntax. If a compiler doesn't recognize an attribute (a gcc attribute when compiling on MSVC as example) it'll simply be ignored. (probably with a warning)

For gcc you can use the gnu prefix and the C++11 attribute syntax: [[gnu::unused]] instead of __attribute__((unused)) the same should apply for the other gcc attributes too.

example without gnu prefix

example with gnu prefix

Great, with this gcc allows syntax: X x [[gnu::unused]] = y; — Aelian, Sep 20 '18 at 18:07

Mateusz Grzejek · Answer 3 · 2023-04-14T15:44:46.433

The thing you are referring to is known as attribute specifiers. It is an attempt to standardize various, platform dependent, specifiers:

__attribute__ in case of GCC / ICC (Linux)
__declspec on MSVC / ICC (Windows)

As you can see in attached doc link, the only specifiers supported in C++11 are:

[[noreturn]]
[[carries_dependency]]

and in C++14:

[[deprecated]] (also supported as: [[deprecated("reason")]])

C++ 17 is the version that introduces the required feature:

[[maybe_unused]] (cppreference.com)

Example from the link:

#include <cassert>
 
[[maybe_unused]] void f([[maybe_unused]] bool thing1,
                        [[maybe_unused]] bool thing2)
{
    [[maybe_unused]] bool b = thing1 && thing2;
    assert(b); // in release mode, assert is compiled out, and b is unused
               // no warning because it is declared [[maybe_unused]]
} // parameters thing1 and thing2 are not used, no warning

So the answer is: no, it's not possible, using only C++11 features - the required C++ version to get this in a portable way is C++ 17.

If you are not interested only in portable solutions, there might be a way. C++ standard does not limit this list:

Only the following attributes are defined by the C++ standard. All other attributes are implementation-specific.

Various compilers can support some non-standard specifiers. For example, you can read this page in order to find out, that Clang supports:

[[gnu::unused]]

Perhaps your version of GCC also supports this specifier. This page contains a bug report referring to generalized attributes support. [[gnu::unused]] is also mentioned.

Ben Deane came up with an interesting C++11 solution which I added as an [answer here](http://stackoverflow.com/a/31654792/1708801), it obviously relies on the optimizer but I have not seen one that does not do the right thing yet. — Shafik Yaghmour, Aug 10 '15 at 02:20
@ShafikYaghmour Interesting idea, although I don't think it is a good solution. It may *or may not* be optimized out in such case. `unused` attribute is more explicit and predictable. Also, every known platform has some form of `unused`, so the best *multiplatform* solution would be an `#ifdef` and common macro for all platforms - it's probably the best possible solution (until more attributes will be standardized). — Mateusz Grzejek, Aug 10 '15 at 02:39
I would probably file a bug report if it was not properly optimized away, I am not sure I would use that trick but it is worth documenting it may end up very useful to someone else. — Shafik Yaghmour, Aug 10 '15 at 02:43
The edit queue's full so I can't do anything, but the MSDN link currently redirects to https://learn.microsoft.com/en-us/previous-versions/dabb5z75(v=vs.140)?redirectedfrom=MSDN and should be updated when editing is possible again. — AJM, Jun 01 '22 at 16:27

C++11-style [[unused]] attribute in gcc?

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