#include "ToBase64.h" // string, experimental/string_view

using std::length_error; // stdexcept
using std::string; // string
using std::experimental::string_view; // experimental/string_view
using std::uint8_t; // cinttypes

using namespace std;

string to_base64(const string_view input)

string to_base64(const string_view input);
string to_base64(string_view input);

auto remainder{ 0 };
uint8_t leftover{ 0 };

auto remainder = 0;
auto leftover = uint8_t(0);

int remainder = 0;
uint8_t leftover = 0;

switch ((remainder %= 3)++)

output += "==";
// ...
output += '=';

output += "==n";
// ...
output += '=n';  // oops