需要更改c++代码

我写这段代码是为了进行日期验证。。它很好用。。但是我需要发送一个字符串，并且应该进入这个函数，验证字符串并将其返回。如何更改代码。。。

如果我删除静态并使用原型变量，我就不会得到所需的输出。。例如

main()
{
dobvalidation(b);
}
void dobvalidation(string b)
{
//validates
}

我需要上面的格式。。这是我的代码

#include <iostream>
#include <string>
#include <cstring>
#include <stdlib.h>
#include <ctime>
using namespace std;
void checkFormat();
void dobValidation();
static string input;
int main()
{
    cout<<"Enter date of birth (dd-mm-yyyy)n";
    getline(cin,input,'n');
    checkFormat();
dobValidation();
    return 0;
}
void checkFormat()
{
    //check the length of the string
    int len=input.size();
    if(len!=10)
    {
        cout<<"nPlease enter a valid Date of Birthn";
        cin>>input;
        checkFormat();
        return; 
    }
    char * val;
    val = const_cast<char*>((input.substr(2,1)).c_str());
    //check for the dashes in dob
    if(strcmp(val,"-")!=0)
    {
        cout<<"nPlease enter a valid Date of Birthn";
        cin>>input;
        checkFormat();  
        return; 
    }
        val = const_cast<char*>((input.substr(5,1)).c_str());
    if(strcmp(val,"-")!=0)
    {
        cout<<"nPlease enter a valid Date of Birthn";
        cin>>input;
        checkFormat();      
        return;
    }
    //check for digits
    //extract date from string
    char * date;
    date = const_cast<char*>((input.substr(0,2)).c_str());
    //check char by char for numeric
    char c;
    for(int i=0;i<2;i++)
    {
        c = date[i];
        if(!isdigit(c))
        {
            cout<<"nPlease enter a valid Date of Birthn";
            cin>>input;
            checkFormat();
            return;
        }
    }
    //extract month from string
    char * month;
    month = const_cast<char*>((input.substr(3,2)).c_str());
    //check char by char for numeric    
    for(int i=0;i<2;i++)
    {
        c = month[i];
        if(!isdigit(c))
        {
            cout<<c;
            cout<<"nPlease enter a valid Date of Birthn";
            cin>>input;
            checkFormat();
            return;
        }
    }
    //extract year from string
    char * year;
    year = const_cast<char*>((input.substr(6,4)).c_str());
    //check char by char for numeric    
    for(int i=0;i<4;i++)
    {
        c = year[i];
        if(!isdigit(c))
        {
            cout<<"nPlease enter a valid Date of Birthn";
            cin>>input;
            checkFormat();
            return;
        }
    }   
return;
}

void dobValidation()
{
//        cout<<dob;
    //date
        char * date1;
        date1 = const_cast<char*>((input.substr(0,2)).c_str());
        int dd=atoi(date1);
        //month
        char * month1;
        month1 = const_cast<char*>((input.substr(3,2)).c_str());
        int mm=atoi(month1);
        //year
        char * year1;
        year1 = const_cast<char*>((input.substr(6,4)).c_str());
        int yyyy=atoi(year1);
    //cout<<dd<<mm<<yyyy;
        int days[12]={31,28,31,30,31,30,31,31,30,31,30,31};
        int max_no_of_day = days[mm-1];
        //check for leap year
        if((yyyy%400 ==0 || (yyyy%100 != 0 && yyyy%4 == 0) ) && mm==1)
        {
                        max_no_of_day=29;
        }
        // check date doesnt cross the max limit
        if(dd > max_no_of_day || dd<1)
        {
    //  cout<<"max"<<max_no_of_day<<endl;
    //  cout<<dd<<mm<<yyyy;
                cout<<"nPlease enter a valid Date of Birthn";
                cin>>input;
                dobValidation();
        return;
        }
        // month validation
        if(mm >12 || mm<1)
        {
                cout<<"nPlease enter a valid Date of Birthn";
                cin>>input;
                dobValidation();
        return;
        }
 //year verification
        time_t t = time(0);   // get time now
    struct tm * now = localtime( & t ); //convert to local time
    int current_year = (now->tm_year + 1900);
    int current_month = (now->tm_mon + 1);
    int current_date = (now->tm_mday);

    // date should not exceed current date
   if(yyyy==current_year && mm>current_month)
   {
        cout<<"nPlease enter a valid Date of Birthn";
                cin>>input;
                dobValidation();
        return;
   }
   if(yyyy==current_year && mm==current_month && dd>current_date)
   {
        cout<<"nPlease enter a valid Date of Birthn";
                cin>>input;
                dobValidation();
        return;
   }
   //check whether year crossed current year
    if(yyyy>current_year || yyyy<1900)
    {
        cout<<"nPlease enter a valid Date of Birthn";
                cin>>input;
                dobValidation();
        return;
    }

return;
        }