jeudi 21 décembre 2017

Do the c++11 strict alias rules allow accessing uint64_t via char *, char(&)[N],even std::array

According to this stackoverflow answer about C++11/14 strict alias rules:

If a program attempts to access the stored value of an object through a glvalue of other than one of the following types the behavior is undefined:

  • the dynamic type of the object,

  • a cv-qualified version of the dynamic type of the object,

  • a type similar (as defined in 4.4) to the dynamic type of the object,
  • a type that is the signed or unsigned type corresponding to the dynamic type of the object,
  • a type that is the signed or unsigned type corresponding to a cv-qualified version of the dynamic type of the object,
  • an aggregate or union type that includes one of the aforementioned types among its elements or non-static data members (including, recursively, an element or non-static data member of a subaggregate or contained union),
  • a type that is a (possibly cv-qualified) base class type of the dynamic type of the object,
  • a char or unsigned char type.

can we access the storage of other type using

(1) char *

(2) char(&)[N]

(3) std::array<char, N>

without depending on undefined behavior?

constexpr uint64_t lil_endian = 0x65'6e'64'69'61'6e; 
    // a.k.a. Clockwise-Rotated Endian which allocates like
    // char[8] = { n,a,i,d,n,e,\0,\0 }

const auto& arr =   // std::array<char,8> &
    reinterpret_cast<const std::array<char,8> &> (lil_endian);

const auto& carr =  // char(&)[8]>
    reinterpret_cast<const char(&)[8]>           (lil_endian);

const auto* p =     // char *
    reinterpret_cast<const char *>(std::addressof(lil_endian));

int main()
{
    const auto str1  = std::string(arr.crbegin()+2, arr.crend() );

    const auto str2  = std::string(std::crbegin(carr)+2, std::crend(carr) );

    const auto sv3r  = std::string_view(p, 8);
    const auto str3  = std::string(sv3r.crbegin()+2, sv3r.crend() );

    auto lam = [](const auto& str) {
        std::cout << str << '\n'
                  << str.size() << '\n' << '\n' << std::hex;
        for (const auto ch : str) {
            std::cout << ch << " : " << static_cast<uint32_t>(ch) << '\n';
        }
        std::cout << '\n' << '\n' << std::dec;
    };

    lam(str1);
    lam(str2);
    lam(str3);
}

all lambda invocations produce:

endian
6

e : 65
n : 6e
d : 64
i : 69
a : 61
n : 6e

godbolt.org/g/cdDTAM (enable -fstrict-aliasing -Wstrict-aliasing=2 )

http://ift.tt/2BSOf9C

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