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Subject: Anonymous unions with non-trivial member types
From: Yongwei Wu (wuyongwei_at_[hidden])
Date: 2020-08-28 06:35:26


Since C++11, unions may have non-trivial member types, as long as custom
non-static special member functions are provided by the user to remedy the
deleted special functions. So far, I have not found clear evidence from the
Standard whether anonymous unions may have non-trivial member types. The
following code may be compiled by GCC 8+ and MSVC, but is rejected by Clang
10 (interestingly, Clang 7 accepts it):

#include <iostream> // std::cout/endl
#include <string> // std::string

using namespace std;

struct StringIntChar {
    enum { String, Int, Char } type;
    ~StringIntChar()
    {
        if (type == String) {
            string_value.~string();
        }
    }
    union {
        string string_value;
        int int_value;
        char char_value;
    };
};

int main()
{
    StringIntChar obj{.type = StringIntChar::String,
                      .string_value = "Hello world"s};
    cout << obj.string_value << endl;
}

I am aware I am using some C++20 features here. The key question is only
about anonymous unions. Can they have non-trivial member types, as in the
code? Is the compilation problem a regression bug in Clang?

Thanks in advance.

Best regards,

Yongwei

-- 
Yongwei Wu
URL: http://wyw.dcweb.cn/


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