[[nodiscard]] attribute different compilation result for GCC and Clang

Question

#include <iostream>
#include <memory>

class A
{
public:
    static [[nodiscard]] std::unique_ptr<A> create();
    virtual int get_version() = 0;
    virtual ~A() = default;
};

class B : public A
{
public:
    [[nodiscard]] int get_version() override
    {
        return 20;
    }
};

std::unique_ptr<A>
A::create()
{
    return std::make_unique<B>();
}

int main()
{
    auto a = A::create();
    [[maybe_unused]] int v  = a->get_version();
}

I tried to use [[nodiscard]] to not allow ignoring the return value of A::create(). But, I get different compilation output in GCC and Clang.

Tried with

GCC: 8.5
Clang: 15.0.0
Compilation options: -O3 -std=c++17
Godbolt link: https://godbolt.org/z/qa7TfcK9f

GCC:

<source>:7:12: warning: attribute ignored [-Wattributes]
     static [[nodiscard]] std::unique_ptr<A> create();
            ^
<source>:7:12: note: an attribute that appertains to a type-specifier is ignored
ASM generation compiler returned: 0
<source>:7:12: warning: attribute ignored [-Wattributes]
     static [[nodiscard]] std::unique_ptr<A> create();
            ^
<source>:7:12: note: an attribute that appertains to a type-specifier is ignored
Execution build compiler returned: 0
Program returned: 0

Clang:

<source>:7:14: error: 'nodiscard' attribute cannot be applied to types
    static [[nodiscard]] std::unique_ptr<A> create();
             ^
1 error generated.
ASM generation compiler returned: 1
<source>:7:14: error: 'nodiscard' attribute cannot be applied to types
    static [[nodiscard]] std::unique_ptr<A> create();
             ^
1 error generated.
Execution build compiler returned: 1

Am I doing something wrong? And why does these compilers have different behavior?

This code works with MSVC v19.33 properly without any errors or warnings: https://godbolt.org/z/dWsv4jTo5

does it matter? I mean `static [[nodiscard]] std::unique_ptr create();` is wrong, should be `[[nodiscard]] static std::unique_ptr create();`. Does it really matter if it is reported as error or warning? fwiw, the message is more clear with never version of gcc https://godbolt.org/z/o4Ybj5v4a — 463035818_is_not_an_ai, Nov 16 '22 at 08:13
[A later version of GCC gives you an *error*](https://godbolt.org/z/z75P97PTG) that should be very informative. In short, you placed the `[[nodiscard]]` attribute in the wrong place. — Some programmer dude, Nov 16 '22 at 08:13
It works with MSVC 19.33 with compilation options /std:c++17 and /Os. It doesn't throw errors. So does MSVC have a bug? — kiner_shah, Nov 16 '22 at 08:15
sorry, you dont understand. Your code has a typo. You have the `[[nodiscard]]` in the wrong place. Ignoring this is the worst of all 3 — 463035818_is_not_an_ai, Nov 16 '22 at 08:21
@463035818_is_not_a_number, yes I get that. But why does it work for MSVC? — kiner_shah, Nov 16 '22 at 08:22
Never mind, I got the link which specifies it's some issue in MSVC: https://developercommunity.visualstudio.com/t/nodiscard-between-static-and-return-/1338052 — kiner_shah, Nov 16 '22 at 08:23
@kiner_shah Because all compilers are written differently. MSVC has a lot of quirks and weird behaviour patterns — SimonC, Nov 16 '22 at 08:23
Thanks guys for pointing out the problem. The issue is now solved. — kiner_shah, Nov 16 '22 at 08:24
"But why does it work for MSVC?" It doesnt work at all. Seems like msvc simply ignores the attribute, and issues no warning when you remove the [[maybe_unused]] https://godbolt.org/z/7d411Yz3n — 463035818_is_not_an_ai, Nov 16 '22 at 08:33

score 4 · Accepted Answer · answered Nov 16 '22 at 08:29

4

You can see here https://eel.is/c++draft/class.mem#general that the attribute can only appear first in a member declaration. Hence this

static [[nodiscard]] std::unique_ptr<A> create();

is wrong. And should be

[[nodiscard]] static std::unique_ptr<A> create();

Your code has a typo.

Newever versions of gcc report a more clear error:

source>:7:12: error: standard attributes in middle of decl-specifiers
    7 |     static [[nodiscard]] std::unique_ptr<A> create();
      |            ^
<source>:7:12: note: standard attributes must precede the decl-specifiers to apply to the declaration, or follow them to apply to the type
<source>:7:12: warning: attribute ignored [-Wattributes]
<source>:7:12: note: an attribute that appertains to a type-specifier is ignored

Actually I do not know if it has to be reported as error or if a warning is ok too. I don't think it relly matters, because you need to fix it anyhow, and unless you ignore warnings you cannot miss it. MSCV not diagnosing the issue is not nice. However, you should also keep in mind that [[nodiscard]] like similar attributes merely encourage the compiler to issue a warning. There is no gurantee to get a warning when it is discarded.

answered Nov 16 '22 at 08:29

463035818_is_not_an_ai

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Yes, unfortunately I observed that GCC doesn't give a warning when the return value is discarded. Clang gives proper warning. – kiner_shah Nov 16 '22 at 10:07
@kiner_shah not sure what you mean. Gcc warns as expected here https://godbolt.org/z/qM3rj3z4e – 463035818_is_not_an_ai Nov 16 '22 at 10:08
@kiner_shah fwiw, I saw this misunderstanding before `auto x = foo();` is already sufficient to not discard the value returned from `foo`. `x` being unused is a different story – 463035818_is_not_an_ai Nov 16 '22 at 10:09
@kiner_shah so in your original code you can remove the `[[maybe_unused]]` in `main` and no warning is to be expected from the `nodiscard` attribute, because you are not discarding the value returned – 463035818_is_not_an_ai Nov 16 '22 at 10:10
Ok, I tried it for get_version(). Like `auto b = A::create(); b->get_version();`. Since get_version is marked as `[[nodiscard]]`, it should throw a warning but it doesn't. https://godbolt.org/z/jr3Pda5Th – kiner_shah Nov 16 '22 at 10:31
@kiner_shah as I wrote: There is no guarantee, compilers are only "encouraged" to issue a warning – 463035818_is_not_an_ai Nov 16 '22 at 10:35
Yeah, I understand. It would be great to have the warning in GCC though. Thanks :-) – kiner_shah Nov 16 '22 at 10:36

[[nodiscard]] attribute different compilation result for GCC and Clang

1 Answers1