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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.)

Andrew Tomazos
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3 Answers3

25

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

AliciaBytes
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21

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);
}
Alan Kazbekov
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12

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

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")]])

So the answer is: no, it's not possible, using only C++11 features.

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.

Mateusz Grzejek
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  • 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
  • Are there any changes for C++17 (C++1z)? – Macxx Aug 08 '17 at 12:06
  • The edit queue's full so I can't do anything, but the MSDN link currently redirects to https://docs.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