Each deserialized 64 bit integer number should be converted to bit wise equivalent 64 bit floating number

Question

I have above statement in file I am refering . Expected output is double. I could not find anything relevant to my problem. I found this Passing a structure through Sockets in C but dont know if its relevant. I am not reading that int64 value. I am getting it from other process and that is the way it is designed.

Does anyone have any theory about serialization and deserialization of ints?

std::memcpy will do it for you. union of int64 and float is UB, — Richard Critten, Mar 02 '17 at 12:57
Thanks Nathan for update.I am not reading that one. I am getting it from other process and that is the way it is designed. — hemant bendale, Mar 02 '17 at 12:58
@RichardCritten void * comp = &(x_double); double component = (*(F64 *)comp); i guess this does the same it did not work — hemant bendale, Mar 02 '17 at 13:06
Is also UB. memcpy is now the only standard defined way of copying data between different types (iirc there are few edge cases with char / unsigned char). — Richard Critten, Mar 02 '17 at 13:08
If it didn't work, what was the expected output and what did you get? Is it just the old endianness problem? — harold, Mar 04 '17 at 16:58

score 1 · Answer 1 · answered Mar 02 '17 at 13:24

1

There is exactly one defined way to bitwise-copy one type into another in c++ - memcpy.

template<class Out, class In, std::enable_if_t<(sizeof(In) == sizeof(Out))>* = nullptr>
Out mangle(const In& in)
{
    Out result;
    std::memcpy(std::addressof(result), std::addressof(in), sizeof(Out));
    return result;
}


int main()
{
    double a = 1.1;
    auto b = mangle<std::uint64_t>(a);
    auto c = mangle<double>(b);

    std::cout << a << " " << std::hex << b << " " << c << std::endl;
}

example output:

1.1 3ff199999999999a 1.1

answered Mar 02 '17 at 13:24

Richard Hodges

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void * comp = &(x_double); double component = (*(F64 *)comp); I get the same answer which is not corerct. but thanks for your help – hemant bendale Mar 03 '17 at 09:08
you may well get the same answer, but the behaviour when using c-style casts is not defined by the standard – Richard Hodges Mar 03 '17 at 10:14

vre · Answer 2 · 2017-03-02T13:23:32.197

-3

How about reading that 64-bit number and using reinterpret_cast to convert it to bitwise equivalent floating point number.

int64_t a = 121314;
double b = *reinterpret_cast<double*>(&a);
int64_t c = *reinterpret_cast<int64_t*>(&b);
assert(a==c);

edited Mar 02 '17 at 13:23

answered Mar 02 '17 at 12:58

vre

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thanks but it does not build error C2440: 'reinterpret_cast': cannot convert from 'int64_t' to 'double' – hemant bendale Mar 02 '17 at 13:01

Each deserialized 64 bit integer number should be converted to bit wise equivalent 64 bit floating number

2 Answers2