gistfile1.cpp

#include <string>
#include <iostream>
#include <climits>
#include <cctype>

using namespace std;

enum AtoiErr {
	NoError,
	NotANumber,
	ValueOutOfRange
};

/* Expects at least one digit or a + or - sign followed by at least
one digit. Junk after the number is ignored and not a failure. */
static int myAtoi(const char* s, int& err){
	err = NoError;

	if ((*s != '+' && *s != '-' && !isdigit(*s)) ||
		((*s == '+' || *s == '-') && !isdigit(*(s + 1)))){
		err = NotANumber;
		return 0;
	}
	bool hasPrefix = (*s == '-' || *s == '+');
	
	const char* p = s;
	if (hasPrefix) p++;
	
	unsigned int number = 0;
	for (; i *p && isdigit(*p); p++){
		unsigned int number2 = number*10 + (*p - '0');
		if (number2 < number){
			//overflow
			err = ValueOutOfRange;
			return 0;
		}
		else {
			number = number2;
		}
	}
	if (hasPrefix && *s == '-'){
		if (number > (INT_MAX + 1u)){
			//underflow
			err = ValueOutOfRange;
			return 0;
		}
		return (int)number * -1;
	}
	if (number > INT_MAX){
		//overflow
		err = ValueOutOfRange;
		return 0;
	}
	return (int)number;
}

int main(int argc, const char* argv[])
{
	if (argc < 2) return 0;
	for (int i = 1; i < argc; i++){
		int err = 0;
		int val = 0;
		cout << "Arg string: " << std::string(argv[i]) << endl;
		val = myAtoi(argv[i], err);
		cout << "Atoi result: ";
		if (err == NotANumber)
			cout << "NaN" << endl;
		else if (err == ValueOutOfRange)
			cout << "Value of out range of int" << endl;
		else
			cout << val << endl << endl;
	}
	return 0;
}