char-range to integer

main.cpp

#include <array>
#include <cassert>
#include <chrono>
#include <iostream>
#include <limits>
#include <random>
#include <sstream>
#include <string>
#include <tuple>
#include <type_traits>
#include <vector>


template<typename CharIterator>
std::tuple<CharIterator, char> drop_spaces(CharIterator it, CharIterator end) {
	char c;
	while(it != end && std::isspace((c = *it))) {
		++it;
	}
	return std::make_tuple(it, c);
}

template<typename Integer, typename CharIterator>
std::tuple<Integer, CharIterator, char> fast_str_to_integer(CharIterator it, const CharIterator end) {
	//static_assert(std::is_arithmetic<Integer>::value, "fast_str_to<T> only works for integers");
	
	char c;
	std::tie(it, c) = drop_spaces(it, end);
	
	bool is_negative = false;
	if(std::is_signed<Integer>::value) { // this should be optimized away entirely
		switch (c) {
			case '-':
				is_negative = true;
				// fallthrough
			case '+':
				++it;
				if (it == end) {
					throw std::invalid_argument{"string contains no digits"};
				}
				c = *it;
				break;
			default:
				break;
		}
	}
	
	if(!isdigit(c)) {
		throw std::invalid_argument{"string contains no digits"};
	}
	
	Integer val = 0;
	while (true) {
		assert(std::numeric_limits<Integer>::max() / 10 >= val);
		
		val *= 10;
		
		c -= '0';
		assert(std::numeric_limits<Integer>::max() - c >= val);
		
		val += c;
		
		++it;
		if(it == end) {
			break;
		}
		c = *it;
		if(!isdigit(c)) {
			break;
		}
	}
	
	if(is_negative) {
		val = -val;
	}
	
	std::tie(it, c) = drop_spaces(it, end);
	
	return std::make_tuple(val, it, c);
}