Switching byte order without type punning

I Need to switch the order of bytes so that an int16 with contents (byte1, byte2) -> (byte2, byte1). I did this using a union:

union ConversionUnion
{
    uint8_t m_8[4];
    uint16_t m_16[2];
    uint32_t m_32;
};

//use
uint16_t example = 0xFFDE
ConversionUnion converter;
converter.m_16[0] = example;
std::swap(converter.m_8[0], converter.m_8[1]);
example = converter.m_16[0]; //0xDEFF

Now this does work on gcc, but i have been informed that this is undefined behavior (gcc 6.3, C++11).

Questions:

1) Is this really undefined behavior, I ask because i've seen this before in embedded code. Other stackoverflow questions seem to debate this, who's actually correct (for C++11 & C++14).

2) If this is undefined behavior, can byte order swapping be done without a bunch of bit shifting in a portable way. I really hate bit shifting, its horribly ugly.