Safe Endian Union

What is it?

SafeEndianUnion is a class written in C++20, that using in the underlying implementation a class similar to C union with endianness and alignment safety, that doesn't matter what you do, it will work on Little Endian and Big Endian machines.

Many times people are avoiding to use union since their endianness problem, and using bitwise operations to not rely on the endianness of the targeted machine, but these bitwise operations are very ugly most of the cases, and can make developers hard to understand it, union solves the bitwise ugliness, and make the code more elegant, and now it solves also the endianness issue.

Usage Examples:

Converting an RGBA value to hex and hex to RGBA:

struct RGBA { 
    uint8_t r, g, b, a;
};

int main()
{
    // First parameter: specify the order of the struct.
    // Second parameter: expanding the union to:
    // union { unsigned int; RGBA; };
    evi::SafeEndianUnion<evi::ByteOrder::Big, evi::Union<uint32_t, RGBA>> uni;
    uni = 0xAABBCCFF;
    
    auto as_struct = uni.get<RGBA>();
    std::cout << "R = " << (int)as_struct.r << "\nG = " << (int)as_struct.g << "\nB = " << (int)as_struct.b << "\nA = " << (int)as_struct.a << "\n";

    auto as_hex = uni.get<0>();
    std::cout << "HEX = " << std::hex << +as_hex << "\n";
}

And the output on both endianness is:

R = 170
G = 187
B = 204
A = 255
HEX = aabbccff

Test it yourself on godbolt!

String Manipulation

struct data
{
    char a[1];
    char b[2];
    char c[3];
    char d[4];
    char e[5];
};

int main()
{
    using array = std::array<char, sizeof(data)>;
    evi::SafeEndianUnion<evi::ByteOrder::Little, evi::Union<array, data>> uni;
    uni.set(array{"abcdefghi"});

    auto as_struct = uni.get<data>();
    std::cout << std::string(as_struct.a, sizeof(as_struct.a)) << "\n";
    std::cout << std::string(as_struct.b, sizeof(as_struct.b)) << "\n";
    std::cout << std::string(as_struct.c, sizeof(as_struct.c)) << "\n";
    std::cout << std::string(as_struct.d, sizeof(as_struct.d)) << "\n";
    std::cout << std::string(as_struct.e, sizeof(as_struct.e)) << "\n";
}

And the output on both endianness is:

a
bc
def
ghi

Try it yourself on godbolt!

What you can and can't do

• You cannot use pointers or references in your structs.
• You cannot have different types in your struct, stick to only one type, this may change the size of the struct due to alignment and padding.
• You cannot use bitfields.
• The struct must be a POD.
• evi::Union<...> accepts only arithmetic types and a stack allocated arrays ( either std::array<T, N> or array[N] ).
• Your struct is limited only up to 32 fields.

How to use?

It's just a simple header to drop into your project, and just run.

Here are a few fair points:

• It does not use any external libraries ( like Boost ), so you don't have to link anything.
• Make sure you enable concepts and constraints in your compilers. ( in GCC it's -fconcepts ).
• Make sure the compiler is using C++20. ( in GCC and Clang it's -std=++2a or -std=++20 ).