java:

public static void writeVarInt(int value) {
    do {
        byte temp = (byte)(value & 0b01111111);
        // Note: >>> means that the sign bit is shifted with the rest of the number rather than being left alone
        value >>>= 7;
        if (value != 0) {
            temp |= 0b10000000;
        }
        writeByte(temp);
    } while (value != 0);
}

c++:

void PacketWriterNoCopy::writeVarInt(std::int32_t v)
{
  writeVarInt(*reinterpret_cast<std::uint32_t*>(std::addressof(v)));
}

void PacketWriterNoCopy::writeVarInt(std::uint32_t v)
{
  do
  {
    std::int8_t temp = static_cast<std::uint8_t>(v & 0b01111111);
    v >>= 7;
    if (v != 0)
    {
      temp |= 0b10000000;
    }
    writeByte(temp);
  } while (v != 0);
}

http://wiki.vg/Protocol#VarInt_and_VarLong