How to write a clone method easily?

Question

I have a base class with a virtual clone new method

class A
{
    virtual A* cloneNew() const { return new A; }
};

and its derivatives

class A1 : public A
{
    virtual A1* cloneNew() const { return new A1; }
};

class A2 : public A
{
    virtual A2* cloneNew() const { return new A2; }
};

Now I want to use macro or other way to make its re-implementation more easily like

class A1: public A
{
    CLONE_NEW; // no type A1 here
};

Is it possible to do it? Does decltype(this) help?

Having a clone method is usually an indication of something wrong in the design (code smell). What are you really trying to do? — Martin York, Oct 26 '13 at 14:53
@Loki Astari Can you elaborate on why a clone method is code smell? — Max, Oct 26 '13 at 15:32
@LokiAstari I have dynamically-typed items in a tree of a GUI. I want to create a new similar item when an item is clicked. — user1899020, Oct 26 '13 at 15:50
It's not very C++ like (hence the code smell). Looks like something I would do in Java. Also see: http://stackoverflow.com/q/6626201/14065 — Martin York, Oct 26 '13 at 18:35

Kerrek SB · Accepted Answer · 2013-10-26T14:12:25.600

3

The following works fine for me, and can easily be turned into a macro:

struct Foo
{
    virtual auto clone() -> decltype(this)
    {
        return new auto(*this);
    }
};

If you want the clone() function to be const, you cannot use new auto and you have to work a bit harder with the return type:

#include <type_traits>

virtual auto clone() const -> std::decay<decltype(*this)>::type *
{
    return new std::decay<decltype(*this)>::type(*this);
}

edited Oct 26 '13 at 14:12

answered Oct 26 '13 at 14:06

Kerrek SB

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@user1899020 MSVC11 (The C++ compiler of VS2012) [does not support C++11 completely](http://msdn.microsoft.com/en-us/library/vstudio/hh567368.aspx). On the other hand, as far I know MSVC11 supports trailing return type and the `decltype` specifier, so the problem should be at the implementation of `std::decay`. Whats exactly your compiler error? – Manu343726 Oct 26 '13 at 14:35
MSVC11 shows `this` can only be used in the internal body of non-static methods of a class. The problem is on `-> decltype(this)` – user1899020 Oct 26 '13 at 14:37
1

@user1899020 I have tried that in my VS2012 installation and fails too. Seems like a MSVC11 bug. – Manu343726 Oct 26 '13 at 14:48

How to write a clone method easily?

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