No matching function for call / Candidate expects 1 argument, 0 provided

First off, I am a beginner at C++ coding, so if possible try to explain things in the simplest possible terms so that I can make heads or tails of it.

I have done a lot of research, but most of the examples just don't make any sense to me. I have tried implementing various solutions, but I either get the same bug or make things worse.

The program is supposed to be able to take in a string of Roman numerals, and convert them to an integer value. Since this is a homework assignment, I must all of this within a Roman_int class and perform the operation through member functions within the class.

The program I have created contains a member function called as_int() which does the work of converts a string of roman numeral into a numeric integer value. As far as I can tell, that part works fine.

Another thing to note, I am using a special header provided by my university, which handles things like using the std library, so things might look a bit weird.

Here's what I have:

#include "std_lib_facilities_4.h"
/* 
M=1000
D=500
C=100
L=50
X=10
V=5
I=1
*/

class Roman_int
{
    string r;

 public:
    int as_int()
    {
        int val;
        for(int i=r.size()-1; i>0; --i)
        {
            if(i==r.size()-1)
            {
                switch(r[i])
                {
                    case 'M':
                        val+=1000;
                        break;
                    case 'D':
                        val+=500;
                        break;
                    case 'C':
                        val+=100;
                        break;
                    case 'L':
                        val+=50;
                        break;
                    case 'X':
                        val+=10;
                        break;
                    case 'V':
                        val+=5;
                        break;
                    case 'I':
                        val+=1;
                        break;
                    default:
                        error("invalid input");
                        break;
                }
            }
            else
            {
                switch(r[i])
                {
                    case 'M':
                        val+=1000;
                        break;
                    case 'D':
                        if(r[i+1]== 'M')
                            val-=500;
                        else
                            val+=500;
                            break;
                    case 'C':
                        if(r[i+1]== 'M' || r[i+1]== 'D')
                            val-=100;
                        else
                            val+=100;
                            break;
                    case 'L':
                        if(r[i+1]== 'M' || r[i+1]== 'D' || r[i+1]== 'C')
                            val-=50;
                        else
                            val+=50;
                            break;
                    case 'X':
                        if(r[i+1]== 'M' || r[i+1]== 'D' || r[i+1]== 'C' || r[i+1]== 'L')
                            val-=10;
                        else
                            val+=10;
                            break;
                    case 'V':
                        if(r[i+1]== 'M' || r[i+1]== 'D' || r[i+1]== 'C' || r[i+1]== 'L'  || r[i+1]== 'X')
                            val-=5;
                        else
                            val+=5;
                            break;
                    case 'I':
                        if(r[i+1]== 'I')
                            val+=1;
                        else
                            val-=1;
                        break;
                    default:
                        error("invalid input");
                        break;
                }
            }
        }
        return val;
    }

    friend istream& operator>>(istream& is, Roman_int& roman)
    {
        string rom_num;
        is >> rom_num;
        if(!is) return is;
        roman = Roman_int(rom_num);
        return is;
    }

    friend ostream& operator<<(ostream& os, Roman_int& roman)
    {
        cout << roman.r << endl;
        return os;
    }

};



int main()
{
    Roman_int r;
    cout << "Enter a Roman numeral in all capital letters: " << endl;
    cin >> r;
    cout << "Roman " << r << " equals " << r.as_int() << endl;
}

The problem I am having is with declaring my Roman_int object in my main function. I get this error:

hw6_pr3.cpp: In function ‘std::istream& operator>>(std::istream&, Roman_int&)’:
hw6_pr3.cpp:114:28: error: no matching function for call to ‘Roman_int::Roman_int(String&)’
   roman = Roman_int(rom_num);
                            ^
hw6_pr3.cpp:114:28: note: candidates are:
hw6_pr3.cpp:17:7: note: Roman_int::Roman_int()
 class Roman_int
       ^
hw6_pr3.cpp:17:7: note:   candidate expects 0 arguments, 1 provided
hw6_pr3.cpp:17:7: note: Roman_int::Roman_int(const Roman_int&)
hw6_pr3.cpp:17:7: note:   no known conversion for argument 1 from ‘String’ to ‘const Roman_int&’
hw6_pr3.cpp:17:7: note: Roman_int::Roman_int(Roman_int&&)
hw6_pr3.cpp:17:7: note:   no known conversion for argument 1 from ‘String’ to ‘Roman_int&&’

From what I have read, I'm pretty sure it has something to do with either the default constructor or how my class is being initialized, but I have no idea where I am supposed to go from there, as I have never encountered this sort of problem with any of the classes I have created. If anyone could explain this in simple terms, that would extremely helpful.