CODEWITHSUNDEEP
c++c++11
VarunPandey
VarunPandey

Reputation: 327

How can I create derived class object from an existing base class object?

I am trying to create a derived class object and populate the data-members (coming from base class) with a existing base class object.

In my process lifecycle, I already had a base class object. For some decision, I have to create a new object (derived from base class). One way to do this is expose the assessors and copy the data. Is there any solution like aggregate initialization or dynamic_cast that I could use instead? 

#include <iostream>
#include <string>

using namespace std;

class Base {
    protected: 
        string name_;

    public:
        Base() : name_ ("foo")
        {
        }
        void ChangeName()
        {
            name_ = std::string {"bar"};
        }
};

class Child final : Base {
    public:
    string GetName()
    {
        return name_;
    }
};

int main()
{
    Base b;
    b.ChangeName();

    Child c = {b};
    cout<<"Hello World. Here is my name: " << c.GetName() << endl;

    return 0;
}

Expected Output: Hello World. Here is my name: bar

Compilation Error

try.cpp:33:17: error: could not convert ‘{b}’ from ‘<brace-enclosed initializer list>’ to ‘Child’
     Child c = {b};

Upvotes: 2

Views: 1774

Answers (2)

SenselessCoder
SenselessCoder

Reputation: 1159

Something like this is possible using the C-style casts:

Child c(*(Child*)&b);

But I wouldn't advise you to use this. It can fail if the memory layouts of the classes aren't similar. The equivalent C++ cast for this situation is this:

Child c(*reinterpret_cast<Child*>(&b));

EDIT: Turns out this is Undefined Behavior. Don't attempt this :)

Upvotes: -1

1201ProgramAlarm
1201ProgramAlarm

Reputation: 32732

If you want to construct a Child from a Base, you need to add a constructor to Child that will do that:

Child(const Base &b): Base(b) { }

You need to be cautious using this, as it will allow constructing a Child from another class that is also derived from Base. And if Child has any other data members, you need to be sure to add appropriate initialization or default values for them.

Upvotes: 5

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