c++ How to create an instance of derived class template

Question

I have base class and a bunch of derived classes (only one here for simplicity). I also have holder class with one of derived classes as a template argument. I want holder object to create an instance of derived class. Here is the code:

class base {
protected:
    int value;
public:
    base() : value (0) { }
    base(int value) : value(value) { }
};

class derived : public base { };

template <class T>
class holder {
public:
    holder(T) {}
    T create(int value) {
        return T(value);
    }
};

int _tmain(int argc, _TCHAR* argv[])
{
    holder<base*> h(&derived());
    derived* d = h.create(1); // error here
}

I get an error error C2440: 'initializing' : cannot convert from 'base *' to 'derived *'. I guess that's because type of variable is holder<base*>, so create method is called with base as template argument. But how do I cast it properly if I have a lot of derived classes?

UPD. I changed holder::create method so it uses std::remove_pointer but I still get the same compile error.

T create(int value) {
    return new (std::remove_pointer<T>::type)(value);
}

Are you aware that you are creating a base* with an int ? Does your code compile ? — perencia
– perencia, Commented May 22, 2016 at 10:56
What are you trying to do? Didn't the compiler also issue an error about taking an address of a temporary? — LogicStuff
– LogicStuff, Commented May 22, 2016 at 10:57
@SlardarZhang I also need to store a list of holder's, each may hold different subclasses of base (I should have mentioned it in post). So (as far as I understand c++) , no - it can only be holder<base*> — Vlad Shevchenko
– Vlad Shevchenko, Commented May 22, 2016 at 11:03
@perencia I see now, but don't understand what can I do with it. — Vlad Shevchenko
– Vlad Shevchenko, Commented May 22, 2016 at 11:11

AlphaRL · Accepted Answer · 2016-05-22 11:35:01Z

1

You can let holder holds derived type rather than base type, and use boost::any or std::any (c++ 17) to store all the holders.

#include "iostream"
#include "boost/any.hpp"
#include "vector"

class base {
protected:
    int value;
public:
    base() : value (0) { }
    base(int value) : value(value) { }
};

class derived1 : public base {
public:
    derived1(int value) : base(value) {};
};
class derived2 : public base {
public:
    derived2(int value) : base(value) {};
};

template <class T>
class holder {
public:
    holder() {}
    T* create(int value) {
        return new T(value);
    }
};

int main()
{
    std::vector<boost::any> list;
    holder<derived1> h1;
    holder<derived2> h2;
    list.push_back(h1);
    list.push_back(h2);
    derived1* pd1 = boost::any_cast<holder<derived1>>(list[0]).create(1);
    derived2* pd2 = boost::any_cast<holder<derived2>>(list[1]).create(2);
}

answered May 22, 2016 at 11:35

AlphaRL

1,6291 gold badge12 silver badges20 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Vlad Shevchenko Over a year ago

That seams to do what I need. Do you by any chance know is there any class for Qt?

AlphaRL Over a year ago

I didn't heard that.@VladShevchenko

Collectives™ on Stack Overflow

c++ How to create an instance of derived class template

1 Answer 1

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related