C++ Convert from defined superclass to subclass using =

Question

I'm trying to make something to define a storage container for any subclass of Tag ; ie TAG_String , in the code in the following files:

main.cpp

#include <iostream>
#include "Tag.h"

using namespace std;

int main() {
    nbt::TAG_Compound d("Data");
    d["str"] = new nbt::TAG_String("str");
    d["str"].value("hi");
    cout << d["str"].value() << endl;
    //cout << val() << endl;
    return 0;
}

Tag.cpp

#include "Tag.h"

using namespace std;

nbt::Tag::Tag(string nam){
    name = nam;
}

/*nbt::Tag& nbt::Tag::operator=(nbt::Tag& nw){
    nbt::Tag *tis = this;
    tis = nw;
    return tis&;
}*/

string nbt::Tag::Name(){
    return name;
}

void nbt::Tag::Name(string nam){
    name = nam;
}

void nbt::TAG_Compound::load(string file){

}

void nbt::TAG_Compound::save(string file){

}

nbt::Tag& nbt::TAG_Compound::operator[](string lookup){
    for(int i=0;i<val.size();i++){
        if(val[i].Name() == lookup){
            return val[i];
        }
    }
    nbt::Tag tag("");
    val.push_back(tag);
    return val[val.size()-1];
}

void nbt::TAG_String::load(string file){

}

void nbt::TAG_String::save(string file){

}

string nbt::TAG_String::value(){
    return val;
}

void nbt::TAG_String::value(string v){
    val = v;
}

(In that i know there are empty functions; they will be used later)

Tag.h

#ifndef NBTTAG_H_INCLUDED
#define NBTTAG_H_INCLUDED

#include <string>
#include <vector>

namespace nbt{

    class Tag {

        std::string name;

    public:

        //void save(std::string);
        //void load(std::string);
        std::string Name();
        void Name(std::string);
        Tag(std::string);
        //Tag& operator=(Tag&);

    };

    class TAG_Compound : public Tag {

        std::vector<Tag> val;

    public:

        void save(std::string);
        void load(std::string);
        Tag& operator[](std::string);
        using Tag::Tag;

    };
    class TAG_String : public Tag {

        std::string val;

    public:

        void save(std::string);
        void load(std::string);
        std::string value();
        void value(std::string);
        using Tag::Tag;

    };
}
#endif // NBTTAG_H_INCLUDED

Is there even a way? (I am conviced there is, so if anyone can prove me wrog, that would be a good thing.)

Answer 1

Make a class which contains a reference to the Tag class, eg as follows:

class Container {
public:
    Container( Tag *tag ) {
        containedTag = tag;
    }

    Tag *containedTag;
};

Then instantiate it as follows:

TAG_String tag = TAG_String( "String!" );
Container container = Container( &tag );

I've noticed you have attempted to do something similar with the TAG_Compound class, however the vector is only accepting Tag values, and would need to accept a pointer or a reference instead. Remember, in C++, you can only pass a child class instance off as an instance of its parent class if you're passing a reference or pointer to it, you cannot actually make it an instance of the parent class.