str to int 轉換

#include "stdafx.h"
#include "iostream"
#include "cassert"
using namespace std;

long int str_to_int(char * str)
{
	assert(str != NULL);
	long int sum = 0;
	char flag = '0';
	if(*str == '+')
	{
		flag = '+';
		str++;
	}
	else if (*str == '-')
	{
		flag = '-';
		str++;
	}
	
	while (*str != '\0' && *str >= '0' && *str <= '9')
	{
		if((sum -(*str - '0'))> std::numeric_limits<long int>::max()/10)//溢出檢查
		{
			sum = 0;
			cout<<"overflow!"<<endl;
			return 0;
		}
		sum = 10 * sum + (*str - '0');
		str++;
		cout<<std::numeric_limits<long int>::max()<<endl;
	}
	if(*str != '\0')
		cout<<"str include non number char!"<<endl;
	else 
	{
		if (flag == '-')
			sum = sum * (-1);
		return sum;
	}

	return 0;
}

int _tmain(int argc, _TCHAR* argv[])
{
	char *str = "123456789000000000";
	cout<<str_to_int(str)<<endl;
	return 0;
}