CODEWITHSUNDEEP
c++new-operator
Wentinn Liao
Wentinn Liao

Reputation: 11

Is there a __new__ equivalent for C++?

The __new__ override in Python allows you to construct an object of a completely different class using the constructor of one class. Was wondering if there was any way to achieve this in C++, or any other language for that matter? Writing a certain solving software, originally in Python, then moved to Java for speed reasons and also clarity of structure, and had a pseudo way of achieving this for Java, but since C++ is more flexible and versatile maybe the real thing might be possible. Thanks the consideration

Upvotes: 0

Views: 214

Answers (2)

nada
nada

Reputation: 2117

A literal translation of what you want to do would probably go like this:

class A {};
class B {};
class C {};

void* create_object(char class_letter) {
    switch (class_letter) {
        case 'A': return new A;
        case 'B': return new B;
        case 'C': return new C;
        default: return new A;
    }
}

Everyone who had his first C++ lessons including the usual recommendations knows this is bound to explode. Why? Look: You could call this function like this:

void* ptr = create_object('B');

You get a void* which is not a nice thing to have, because now it's your duty to cast it to the correct type for example by doing:

B* b_ptr = (B*)ptr; // eww!

and equally important - now it's also your duty to delete it (yuck!), which is a popular thing us coders tend to forget.

Bottom line is, it's doable - as always, but ugly. So use a different design pattern if you plan on implementing this in C++.

Upvotes: 0

Robert Andrzejuk
Robert Andrzejuk

Reputation: 5222

Memory for an object is allocated before the constructor is called.

And since C++ is not a dynamically typed system, you cannot change the type of the object at runtime.

Use a factory function instead.

Upvotes: 5

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