字符串(char*)转化为int

一个精简版本：

#include <ctype.h>
int my_atoi(const char* p)
{
	assert(p != NULL);
	bool neg_flag = false;// 符号标记
	int res = 0;// 结果
	if(p[0] == '+' || p[0] == '-')
		neg_flag = (*p++ != '+');
	while(isdigit(*p)) res = res*10 + (*p++ - '0');
	return neg_flag ?0 -res : res;
}

long strtoint(char *str,int length)
{
	if(length > 1) 
		return str[0]=='-' ? strtoint(str, length-1)*10-(str[length-1]-'0') : strtoint(str, length-1)*10+str[length-1]-'0';
 
	else 
		return str[0]=='-' ? 0 : str[0]-'0';
}

一个较完善的版本：

#include <iostream>
#include <cctype>
#include <cassert>
using namespace std;

#define MAX_INT ((unsigned)(-1) >> 1)
//#define MIN_INT (MAX_INT + 1)

int Str2Int(const char* pStr)
{
	assert( pStr != NULL );
	bool negFlag=false;
	int ret=0;
	if (pStr[0]=='+' || pStr[0]=='-')
		negFlag=(*(pStr++) != '+');

	bool flag=true;

	while(flag)
	{
		if(isdigit(*pStr)) 
		{
			if (!negFlag)
			{
				if ( ret*10.0 > MAX_INT -(*(pStr)-'0'))
				{
					cout<< "Overflow!"<<endl;
					assert(false);
				}
			}
			else
			{
				if ( ret*10.0 >MAX_INT -(*(pStr)-'0')+1 )
				{
					cout<< "Underflow!"<<endl;
					assert(false);
				}
			}
		
			ret= ret*10 + (*(pStr++)-'0');
		}
		
		else
		{
			if (*pStr== '.' || *pStr== '\0')
				flag=false;
			
			else
			{
				cout<< "Illegal character!"<<endl;
				assert(false);
			}
				
		}
	}
	return negFlag==true ? (-ret): ret;

}

int main()
{
	char a[10]="-123.4";
	cout<<Str2Int(a)<<endl;

	char *b="1234";
	cout<<Str2Int(b)<<endl;

	char *d="2147483647";
	cout<<Str2Int(d)<<endl;

	char *e="-2147483648";
	cout<<Str2Int(e)<<endl;

 	char *f="-2147483649";
 	cout<<Str2Int(f)<<endl;

	char *g="2147483648";
	cout<<Str2Int(g)<<endl;

	char *c=NULL;
	cout<<Str2Int(c)<<endl;
	return 0;
}

32位机 int型有符号整数的范围 -2147483648 ~ 2147483647