为C++ Variant Class灵活地在string,int,double之间进行转换

Question

我正在实现一个变体类(不使用boost),我想知道如何处理你存储任何字符串,整数或双精度的情况,并通过ToString()自动将其转换为所需的类型, ToInt()或ToDouble().

例如,

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

ToXXX函数应返回要转换为的类型的引用.现在,我有一个代码,它可以返回与最初分配给(Variant a = Int(7); a.ToInt()工作)相同的类型,并在分配的类型与您要转换的类型不同时引发异常.

抱歉,使用boost不是一种选择.

Answer 1

    #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;
    }    

Answer 2

要实现这样的东西,你需要能够改变存储的类型,因为如果用户通过引用更改变量,他希望更改影响存储的值,所以我写这样的东西:

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;
    }

...

}