c++ int,double等数字类型与string的相互转换

unsigned long stringToNum(const string& str)  //string 到数字
    {  
        istringstream iss(str);  
        unsigned long num;  
        iss >> num;  
        return num;      
    } 
    string numToString(unsigned long d) {  //数字到string
        stringstream ss;
        ss <