Code to convert Roman Numerals to Decimals. (RN2DEC created by ButchDean72 - https://repl.it/@ButchDean72/RN2DEC)

RN2DEC.cpp

#include <cstdio>
#include <unordered_map>
#include <vector>
#include <cstring>
#include <memory>

const std::string sample = "MCMLXXII";

struct ROMAN_NUMERALS
{
    char letter;
    int value;
};

class CRNBase
{
    public:
    
        CRNBase(){}
        virtual ~CRNBase(){}
        
    protected:
        
        unsigned int FetchRNValue(const char c) const;
        
    private:
    
        std::unordered_map<char, unsigned int> rnValsHash = {{'M', 1000}, {'D', 500}, {'C', 100}, {'L', 50}, {'X', 10}, {'V', 5}, {'I', 1}};
};

unsigned int CRNBase::FetchRNValue(const char c) const
{
    std::unordered_map<char, unsigned int>::const_iterator itr = rnValsHash.find(c);
    
    if(itr == rnValsHash.end())
        return 0;
    
    return itr->second;
}

class CRN2Dec : public CRNBase
{
    public:
    
        CRN2Dec(){}
        ~CRN2Dec(){}
        
        unsigned int DecValue(std::string rnStr);
        
    private:
    
        std::vector<int> values;
    
};

unsigned int CRN2Dec::DecValue(std::string rnStr)
{
    int total = 0, current = 0;
    
    for(std::string::iterator i = rnStr.begin(); i != rnStr.end(); ++i)
    {
        current = FetchRNValue(*i);
        
        if(values.empty())
        {
            values.push_back(current);
            
            continue;
        }
        
        if(current > values[values.size() - 1])
        {
            values[values.size() - 1] = current - values[values.size() - 1];
        }
        else
        {
            values.push_back(current);
        }
    }
    
    for(int j = 0; j != values.size(); ++j)
    {
        total += values[j];
    }
    
    return total;
}

int main() 
{
    std::unique_ptr<CRN2Dec> rn2d(new CRN2Dec());
    
    std::printf("Decimal for %s is %d\n", sample.c_str(), rn2d->DecValue(sample));
    
    return 0;
}