How to fix the "'alignas' attribute cannot be applied to types" warning?

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Hi Guys,

For the below code, when I am running static code analysis, I'm getting the "'alignas' attribute cannot be applied to types" warning. How should I fix this warning? Please let me know if missing anything...

C++

template<size_t n, size_t a = alignof(size_t)>
struct test
{   
    struct test2
    {
        uint8_t d[n] alignas(a);
    };
};

Thanks in advance!!!

What I have tried:

I referred all articles regarding this, but only minimal information is exist apart from the concept of Memory Alignment.

Posted 6-Dec-21 1:03am

Piraisudan

Updated 6-Dec-21 1:40am

v2

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11917640 Member 6-Dec-21 8:59am

cppreference contains similar example: alignas(64) char cacheline[64]; Did you try to set alignas in the beginning?

Piraisudan 6-Dec-21 9:19am

Hi @User-11884187Watch, thanks for the hint...

I changed from "uint8_t d[n] alignas(a)" to "alignas(a) uint8_t d[n]", it seems the warning got resolved.

Let me comeback, if anything.

Thanks!!!

Stefan_Lang 7-Dec-21 9:46am

This seems a bit odd, but I remember compilers behaving oddly around any kind of template arguments other than types or int - unsigned int or size_t caused trouble or even compilation errors. Have you tried int a instead of size_t?

Piraisudan 7-Dec-21 10:18am

Hi @Stefan_Lang, thank you for the input, let me check with int.

Meanwhile, could you please explain little bit more on this. I'm still couldn't understand what exactly happening.

Thanks!!!

Stefan_Lang 7-Dec-21 11:28am

I can offer precious little more info, only that with some versions of C++ compilers I've had difficulty when using specific numeric type as template arguments.

Typically the cause of the problem was a template instantiation with an integral literal that was then interpreted as an int, whereas the template used some other type, such as unsigned int, long, or char.

In such cases, the error messages were typically not very helpful. The fastest way to find out if it was related to the template specification, was change the integral type to int: if that resolved the problem, it meant I needed to find a wonky template instantiation that accidentally instantiates with the wrong integral type.

Stefan_Lang 7-Dec-21 12:15pm

I only now realized that you actually posted a solution in a comment yesterday. After checking the documentation at https://en.cppreference.com/w/cpp/language/alignas I can only agree.

Forget my comments on size_t vs int - that was really a shot in the dark based on some bad experience in an unrelated case.

Piraisudan 7-Dec-21 23:23pm

@Stefan63, thank you for your prompt response and detailed explanation.

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