char* to double

#include<iostream>
#include <string.h>
using namespace std;
double char_x_to_double(char* input_char)
{
	int flag = 0;
	double sum = 0.0;
    double divisor = 10.0;
	char *p = input_char;
	if (*input_char)
	{
	//所有输入的数值都必须在(0~9,+,-,.)之中
		while(*p)
		{
		if (!((*p=='+') || (*p=='-')||(*p=='.')|| ((*p>='0') && (*p<='9'))))	
		{
				cout<<"输入非法！请输入数值型参数！例如：12.3,125,-12.6等"<<endl;
			    return 0.0;
		    }
		p++;
		}

		//int j=0;
		if (*input_char == '+')
			{
				flag = 1;
		       input_char++; 
			   //j++;
			   //cout<<"(flag + )j="<<j<<endl;
		    }
		if (*input_char == '-')
			{
				flag = -1;
				input_char++;
				   //j++;
			   //cout<<"(flag - )j="<<j<<endl;
		    }
		
		//计算小数点前面的和
		while(*input_char != '.' && *input_char)
			{
				sum = sum*10 + (double(*input_char) - 48); //0的ascii码是48
				
			  // cout<<"j"<<*input_char<<endl;
			   input_char++;
				   //j++;
		    }
	
		if (*input_char=='.')
			{
				input_char++;
				//计算小数点后面的和
				while(*input_char)
				{
			
				sum = sum + ( double(*input_char)-48)/divisor;
				divisor *= 10;
				input_char++;
				//j++;
				//cout<<"j"<<j<<endl;
			}
		}
		if (flag == 1)
			return sum;
		if (flag == -1)
			return -sum;
		return sum;

	}
	else
	{
	cout<<"输入为空!"<<endl;;
	return 0.0;
	}

}
int main()
{
	double out_double;
	char s1[]="-12.34";
	cout<<"输入s1[]="<<s1<<endl;
	out_double=char_x_to_double(s1);
	cout<<"输出的double类型数为:"<<out_double<<endl;

	char s2[]="+23.45";
	cout<<"输入s2[]="<<s2<<endl;
	out_double=char_x_to_double(s2);
	cout<<"输出的double类型数为:"<<out_double<<endl;
	
	char s3[]="3456";
	cout<<"输入s3[]="<<s3<<endl;
	out_double=char_x_to_double(s3);
	cout<<"输出的double类型数为:"<<out_double<<endl;

	

	char s4[]="+3456789";
	cout<<"输入s4[]="<<s4<<endl;
	out_double=char_x_to_double(s4);
	cout<<"输出的double类型数为:"<<out_double<<endl;

	char s5[]="";
	cout<<"输入s5[]="<<s5<<endl;
	out_double=char_x_to_double(s5);
	cout<<"输出的double类型数为:"<<out_double<<endl;

	char s6[]="abcd";
	cout<<"输入s6[]="<<s6<<endl;
	out_double=char_x_to_double(s6);
	cout<<"输出的double类型数为:"<<out_double<<endl;
	return 0;
	
}