Flexibly convert between string, int, double for C   Variant Class
Variant a = 7;
cout << "The answer is" + a.ToString() << endl; // should print "The answer is 7"
a = "7.4";
double &b = a.ToDouble();
b += 1;
cout << a.ToString() << endl; // should print 8.4

#include <string>
    #include <iostream>
    class Variant{
    public:
        Variant(){
            data.type = UNKOWN;
            data.intVal = 0;
        }
        Variant(int v){
            data.type = INT;
            data.intVal = v;
        }
        Variant(double v){
            data.type = DOUBLE;
            data.realVal = v;
        }
        Variant(std::string v){
            data.type = STRING;
            data.strVal = new std::string(v);
        }
            //the missing copy constructor
             Variant(Variant const& other)
             {
                *this = other;// redirect to the copy assignment
              }

        ~Variant(){
            if(STRING == data.type){
                delete data.strVal;
            }
        }

        Variant& operator = (const Variant& other){
            if(this != &other)
            {
                if(STRING == data.type){
                    delete data.strVal;
                }

                switch(other.data.type){
                case STRING:{
                        data.strVal = new std::string(*(other.data.strVal));
                        data.type = STRING;
                        break;
                    }
                default:{
                        memcpy(this, &other, sizeof(Variant));
                        break;
                    }
                }
            }
            return *this;
        }
        Variant& operator = (int newVal){
            if(STRING == data.type){
                delete data.strVal;
            }
            data.type = INT;
            data.intVal= newVal;
            return *this;
        }

        Variant& operator = (double newVal){
            if(STRING == data.type){
                delete data.strVal;
            }
            data.type = DOUBLE;
            data.realVal= newVal;
            return *this;
        }

        Variant& operator = (std::string& newVal){
            if(STRING == data.type){
                delete data.strVal;
            }
            data.type = STRING;
            data.strVal= new std::string(newVal);
            return *this;
        }
        operator int&() {
            if(INT == data.type)
            {
                return data.intVal;
            }
            //do type conversion if you like
            throw std::runtime_error("bad cast");
        }

        operator double&()  {
            if(DOUBLE == data.type){
                return data.realVal;
            }
            //do type conversion if you like
            throw std::runtime_error("bad cast");
        }
        operator std::string&() {
            if(STRING == data.type){
                return *data.strVal;
            }
            throw std::runtime_error("bad cast");
        }
    private:
        enum Type{
            UNKOWN=0,
            INT,
            DOUBLE,
            STRING
        };
        struct{
            Type type;
            union
            {
                int intVal;
                double realVal;
                std::string* strVal;
            };
        } data;
    };


    int main(){
        Variant v("this is string");//string
        v=1;//int
        v=1.0;//double
        v=std::string("another string");//
        Variant v2; //unkown type
        v2=v;//string
        std::cout << (std::string&)v2 << std::endl;
        return 0;
    }

class Variant
{
private:
    enum StoreType
    {
        Integer,
        Float,
        String,
    }
    store_type;

    union
    {
       int * as_integer;
       double * as_double;
       std::string * as_string;
    }
    store_pointer;

    // convert to type
    void integer_to_double();
    void integer_to_string();
    void double_to_integer();
    void double_to_string();
    void string_to_integer();
    void string_to_double();

public:

...

    int & ToInt()
    {
        switch (store_type)
        {
            case Integer: break;
            case Double: double_to_integer(); break;
            case String: string_to_integer(); break;
        }
        return * as_integer;
    }

...

}

class Variant {
    enum internaltype {stringtype, inttype, doubletype} curtype;
    std::string strdata; //can't be in union
    union {
        int intdata;
        double doubledata;
    };
public:
    Variant() :curtype(inttype) {}
    Variant(const std::string& s) : curtype(stringtype), strdata(s) {}
    Variant(int i) : curtype(inttype) {intdata = i;}
    Variant(double d) : curtype(doubletype) {doubledata = d;}

    std::string& ToString() {
        std::stringstream ss;
        switch(curtype) {
        case inttype:
                      ss << intdata;
                      ss >> stringdata;
                      break;
        case doubletype:
                      ss << doubledata;
                      ss >> stringdata;
                      break;
        }
        curtype = stringtype;
        return &strdata;
    }
    int& ToInt() {/*much the same*/}
    double& ToDouble() {/*much the same*/}
};

#include <boost/any.hpp>
#include <boost/lexical_cast.hpp>
#include <string>
#include <typeinfo>

class Variant
{
    boost::any value;
public:
    Variant(int n): value(n) {}
    Variant(double d): value(d) {}
    Variant(const std::string& s): value(s) {}
    Variant(const char* s): value(std::string(s)) {} //without this, string literals create ambiguities

    int& ToInt() { return convert<int>();}
    double& ToDouble() { return convert<double>(); }
    std::string& ToString() { return convert<std::string>(); }

private:
    template <class T>
    T& convert()
    {
        if (typeid(T) != value.type()) { //otherwise no conversion required
            if (typeid(int) == value.type()) {
                value = boost::lexical_cast<T>(boost::any_cast<int>(value));
            }
            else if (typeid(double) == value.type()) {
                value = boost::lexical_cast<T>(boost::any_cast<double>(value));
            }
            else if (typeid(std::string) == value.type()) {
                value = boost::lexical_cast<T>(boost::any_cast<std::string>(value));
            }
        }
        return *boost::any_cast<T>(&value);
    }
};

#include <iostream>
using namespace std;

int main()
{
    Variant a = 7;
    cout << "The answer is" + a.ToString() << endl; // should print "The answer is 7"
    a = "7.4";
    double &b = a.ToDouble();
    b += 1;
    cout << a.ToString() << endl; // should print 8.4
}

class Variant
{
private:
    enum data_type {
     ...
    };

    data_type variant_type;

    union Data {
       char *string;
       int integer;
       double dbl;
    } data;

public:
    Variant(const int data);
    Variant(const double data);
    Variant(const char *data);

    //  I think that implementation of the constructors should be pretty straight forward.

    int ToInt() const;
    double ToDouble() const;
    std::string ToString() const;
};

class Variant
{
private:
    char data_string[16384];

public:
    Variant(const int data);
    Variant(const double data);
    Variant(const char *data);

    //  I think that implementation of the constructors should be pretty straight forward.

    int ToInt() const;
    double ToDouble() const;
    std::string ToString() const;
};