[GiNaC-list] Python wrapper for a GiNaC derived library

Ondrej Certik ondrej at certik.cz
Thu Aug 3 23:12:59 CEST 2006

> Yes, as long as you are using references you are fine (... well not
> really, see below ...). You can call virtual methods through references

that's what I mean - use references, I think I am fine, see below.

> and you can cast them back to references to the appropriate child-class of
> basic. However, at some point you will need to create real objects. This
> is because writing
> basic &e = power(a+b, 2)
> is not allowed. The compiler will reject it. It would create a reference
> out of a temporary which you cannot do in C++. Oh well, I said that you
> will need to create real objects, so let us write:
> basic e = power(a+b, 2)
> No! Also not good! It will create a new basic object with only the
> basic-parts of the power. Not the base and exponent.
> The problem that he compiler rejects references to temporaries is going to
> hunt you. The signature of the power constructor cannot be
> power::power(basic &b, basic &e) because you cannot pass a+b to the first
> (reference!) argument of the constructor. What shall we do then? Leave out
> the &'s. No! this will discard all the add-specific data of a+b and only
> copy the, ehhm well, bare basics.
> The point is that your Python data types are already doing for you
> automatically what GiNaCs exes are doing in C++: being a reference to a
> garbage-collected data type. I don't think it is possible to do without
> that.

I am not sure I understand. Why cannot we use it like in the following
example? I compiled it and it works as expected:

#include <iostream>

class basic
        basic() {};

class power:public basic
        power(basic *a, basic *b)
        void print()
            std::cout<<this->a << "  ^  " << this->b << std::endl;
        //a ... base, b... exponent
        basic *a,*b;

basic* pow(basic*a, basic*b)
    return new power(a,b);

int main()
    basic *e1=new basic(); //some complicated expression
    basic *e2=new basic(); //some complicated expression

    basic *p=new power(e1,e2);

    //p->print(); //this line won't compile in C++, in python its ok
    ((power *)p)->print(); //OK, prints: 0x804a008  ^  0x804a018

    //because there is no operator for power in C++ to be overloaded,
we can create a
    //function to do that
    basic *q=pow(e2,e1);
    ((power *)q)->print(); //OK, prints: 0x804a018  ^  0x804a008
    return 0;


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