template, cast, and operator
 

Hello,

the following does not compile due to problems with the + operator. The same thing works without templates. How can I achive this?

Thanks a lot,

Thomas


template <typename T = double> class CVariable {
};

template <typename T = double> class CSymbolic {
public:
CSymbolic(CVariable<T>){}
CSymbolic(){}
};

template <typename T> CSymbolic<T> operator+(CSymbolic<T> a1, CSymbolic<T> a2) {
return CSymbolic();
}

int main() {
CVariable<> x, y;
CSymbolic<> f1 = x+y; // problem here
}

Re: template, cast, and operator
 

On Monday, January 7, 2013 12:29:58 PM UTC-6, thomas...@mail.com wrote:
> template <typename T> CSymbolic<T> operator+(CSymbolic<T> a1, CSymbolic<T> a2) {
> return CSymbolic();
> }

Since CVariable<> is not convertible to CSymbolic, this overload is
not selected. Make it convertible (you may need a forward
declaration), or have one more overload.

--
Zhihao Yuan, ID lichray
The best way to predict the future is to invent it.
__________________________________________________ _
4BSD -- http://4bsd.biz/

Re: template, cast, and operator
 

On 1/7/2013 1:29 PM, thomas.grund@mail.com wrote:
> the following does not compile due to problems with the + operator. The same thing works without templates. How can I achive this?
>
> Thanks a lot,
>
> Thomas
>
>
> template <typename T = double> class CVariable {
> };
>
> template <typename T = double> class CSymbolic {
> public:
> CSymbolic(CVariable<T>){}
> CSymbolic(){}
> };
>
> template <typename T> CSymbolic<T> operator+(CSymbolic<T> a1, CSymbolic<T> a2) {
> return CSymbolic();
> }
>
> int main() {
> CVariable<> x, y;
> CSymbolic<> f1 = x+y; // problem here
> }

What are you trying to achieve? The problem is simple: there is no
operator+ defined for operands of type CVariable<double>. The compiler
is unable to use the operator+ that you defined for CSymbolic<> because
when trying to figure out the type T to instantiate your operator+
*template*, it cannot (shall not) consider conversions.

V
--
I do not respond to top-posted replies, please don't ask

Re: template, cast, and operator
 

On 1/7/2013 1:47 PM, Zhihao Yuan wrote:
> On Monday, January 7, 2013 12:29:58 PM UTC-6, thomas...@mail.com wrote:
>> template <typename T> CSymbolic<T> operator+(CSymbolic<T> a1, CSymbolic<T> a2) {
>> return CSymbolic();
>> }
>
> Since CVariable<> is not convertible to CSymbolic,

Uh... But it *is* convertible. Please look again, CSymbolic has a
c-tor that takes CVariable as an argument.

> this overload is
> not selected. Make it convertible (you may need a forward
> declaration), or have one more overload.
>

V
--
I do not respond to top-posted replies, please don't ask

Re: template, cast, and operator
 

On Monday, January 7, 2013 7:49:23 PM UTC+1, Victor Bazarov wrote:
> On 1/7/2013 1:29 PM, Thomas Grund wrote:
>
> > the following does not compile due to problems with the + operator. Thesame thing works without templates. How can I achive this?
>
> >
>
> > Thanks a lot,
>
> >
>
> > Thomas
>
> >
>
> >
>
> > template <typename T = double> class CVariable {
>
> > };
>
> >
>
> > template <typename T = double> class CSymbolic {
>
> > public:
>
> > CSymbolic(CVariable<T>){}
>
> > CSymbolic(){}
>
> > };
>
> >
>
> > template <typename T> CSymbolic<T> operator+(CSymbolic<T> a1, CSymbolic<T> a2) {
>
> > return CSymbolic();
>
> > }
>
> >
>
> > int main() {
>
> > CVariable<> x, y;
>
> > CSymbolic<> f1 = x+y; // problem here
>
> > }
>
>
>
> What are you trying to achieve? The problem is simple: there is no
>
> operator+ defined for operands of type CVariable<double>. The compiler
>
> is unable to use the operator+ that you defined for CSymbolic<> because
>
> when trying to figure out the type T to instantiate your operator+
>
> *template*, it cannot (shall not) consider conversions.
>
>
>
> V
>
> --
>
> I do not respond to top-posted replies, please don't ask

I am playing around with symbolic calculus (see ginac-library). There is a typical structure of a base class from which several things are derived as sum, product, variable and so on. Then there is a wrapper (CSymbolic) whichwrapps all different types (contains pointer to base class). Everything works if the underlying numerical type is fixed (e.g. double). I want to makethis numerical type to be defined by the user to allow for instance interval arithmetic (interval type instead of double). Hope I made things clear.

Thanks for solutions!

Thomas

Re: template, cast, and operator
 

On Monday, January 7, 2013 12:51:34 PM UTC-6, Victor Bazarov wrote:
> Uh... But it *is* convertible. Please look again, CSymbolic has a
> c-tor that takes CVariable as an argument.

Oops, sorry.

The problem is caused by function template deducution, conversion is
not considered here.

Enable it, in a quick and dirty way:

template <template <typename> class C, typename T,
typename = typename std::enable_if<
std::is_convertible<C<T>, CSymbolic<T>>::value>::type>
CSymbolic<T> operator+(C<T> a1, C<T> a2) {
return {};
}

--
Zhihao Yuan, ID lichray
The best way to predict the future is to invent it.
__________________________________________________ _
4BSD -- http://4bsd.biz/

Re: template, cast, and operator
 

On Monday, January 7, 2013 8:46:19 PM UTC+1, Zhihao Yuan wrote:
> On Monday, January 7, 2013 12:51:34 PM UTC-6, Victor Bazarov wrote: > Uh.... But it *is* convertible. Please look again, CSymbolic has a > c-tor thattakes CVariable as an argument. Oops, sorry. The problem is caused by function template deducution, conversion is not considered here. Enable it, in a quick and dirty way: template <template <typename> class C, typename T, typename = typename std::enable_if< std::is_convertible<C<T>, CSymbolic<T>>::value>::type> CSymbolic<T> operator+(C<T> a1, C<T> a2) { return {}; } --Zhihao Yuan, ID lichray The best way to predict the future is to invent it.. __________________________________________________ _ 4BSD -- http://4bsd.biz/

I have to learn this technique and will give it a try.

Thank you very much!

Thomas

Re: template, cast, and operator
 

On Tuesday, January 8, 2013 2:29:58 AM UTC+8, thomas...@mail.com wrote:
> Hello,
>
>
>
> the following does not compile due to problems with the + operator. The same thing works without templates. How can I achive this?
>
>
>
> Thanks a lot,
>
>
>
> Thomas
>
>
>
>
>
> template <typename T = double> class CVariable {
>
> };
>
>
>
> template <typename T = double> class CSymbolic {
>
> public:
>
> CSymbolic(CVariable<T>){}
>
> CSymbolic(){}
>
> };
>
>
>
> template <typename T> CSymbolic<T> operator+(CSymbolic<T> a1, CSymbolic<T> a2) {
>
> return CSymbolic();
>
> }
>
>
>
> int main() {
>
> CVariable<> x, y;
>

OBJECT a= OBJECT b by the copy constructor first.

Then it is trivial that
a+=c for the object modifier +=.


If the object size of b can't copy to a,
pray for the old code can be morphed to use
objects in the derived classes.




> CSymbolic<> f1 = x+y; // problem here
>
> }

Re: template, cast, and operator
 

在 2013年1月9日星期三UTC+8上午11时25分14秒 ，Dan Stewart写道：
> On Mon, 07 Jan 2013 10:29:58 -0800, thomas.grund wrote:
>
>
>
> > Hello,
>
> >
>
> > the following does not compile due to problems with the + operator. The
>
> > same thing works without templates. How can I achive this?
>
> >
>
> > Thanks a lot,
>
> >
>
> > Thomas
>
> >
>
> >
>
> > template <typename T = double> class CVariable {
>
> > };
>
> >
>
> > template <typename T = double> class CSymbolic {
>
> > public:
>
> > CSymbolic(CVariable<T>){} CSymbolic(){}
>
> > };
>
> >
>
> > template <typename T> CSymbolic<T> operator+(CSymbolic<T> a1,
>
> > CSymbolic<T> a2) {
>
> > return CSymbolic();
>
> > }
>
> >
>
> > int main() {
>
> > CVariable<> x, y;
>
> > CSymbolic<> f1 = x+y; // problem here
>
> > }
>
>
>
> In this case when you call x + y you are trying to instantiate the
>
> function
>
>
>
> CSymbolic<double> operator+(CSymbolic<double> a1, CSymbolic<double> b1)
>
>
>
> with template argument CVariable<double>
>
>
>
> However, there are only 2 cases where conversion from a template argument
>
> to a template parameter is allowed:
>
>
>
> a) Template argument is a reference/pointer to T and template parameter
>
> is a reference/pointer to const T
>
>
>
> or
>
>
>
> b) Template argument is an array or function, in which case the template
>
> parameter is deduced to a pointer to the array or function type
>
>
>
> In this case you are trying instantiate the above function using a user-
>
> defined conversion from CVariable<double> to CSymbolic<double>, which
>
> templates won't allow, since the intended conversion doesn't fall under
>
> a) or b). Hope this helps.
I am thinking there is no closure concept in C++
operator reloading.

Anyway, for fixed types languages, I don't think
it is too difficult for various types.