An efficient and macro-less way to convert wchar_t to UTF-8

to_utf8.cpp

/*
 * Look Ma, no macros!
 */

/**
 * Append the UTF-8 encoding of @a ch to @a cont.
 */
template <class Container>
void to_utf8(Container& cont, wchar_t ch)
{
    /*
     * Define the bit-twiddling lambdas for the first and any continuation bytes.
     *
     * The optimizer will inline these and the whole template at the call site.
     * See http://godbolt.org/g/ab5zsi for an example.
     */
    auto f = [&](int mask, int shift) -> char {
        return mask | ((ch >> shift) & 0x3F);
    };
    auto c = [&](int shift) {
        return f(0x80, shift);
    };

    auto bi = std::back_inserter(cont);

    if (ch <= 0x7F)
        bi = static_cast<char>(ch);
    else if (ch <= 0x7FF)
        bi = f(0xC0, 6), bi = c(0);
    else if (ch <= 0xFFFF)
        bi = f(0xE0, 12), bi = c(6), bi = c(0);
    else if (ch <= 0x10FFFF)
        bi = f(0xF0, 18), bi = c(12), bi = c(6), bi = c(0);
    else
        bi = '?';  // error handling is not the point here ;-)
}