Re: [boost] Implicit Convert from own Typ to Boost::Variant Type

Subject: Re: [boost] Implicit Convert from own Typ to Boost::Variant Type
From: a.mueller (a.mueller_at_[hidden])
Date: 2011-02-15 01:14:58

Ok thank you  for your response
I shall do so.
----- Original Message ----- 
From: "Jeffrey Lee Hellrung, Jr." <jhellrung_at_[hidden]>
To: <boost_at_[hidden]>
Sent: Friday, February 11, 2011 8:30 PM
Subject: Re: [boost] Implicit Convert from own Typ to Boost::Variant Type

> On 2/10/2011 11:44 PM, a.mueller wrote:
>> Hi,
>> i'm new here and i hope you can help me.
>> My problem i have a class with a Boost Variant type as member and a 
>> implicit convert operator from my class to this Variant type. But when i 
>> want use the conversion in my programm the Compiler give me a error.
>> Compiler Version gcc 4.4.1 on Ubuntu system
>> Boost version 1.45
>> Example Program:
>> #include<iostream>
>>
>> #include<boost/variant.hpp>
>>
>> #include<boost/lexical_cast.hpp>
>>
>> using namespace std;
>>
>>
>> 　
>>
>> typedef boost::variant<float,int>  Testtyp;
>>
>> class Testtypcontainer
>>
>> {
>>
>> public:
>>
>> Testtypcontainer():m_testtyp((float) 5.5){}
>>
>> Testtyp m_testtyp;
>>
>> template<class T>
>>
>> void settesttyp(T testvalue){m_testtyp =testvalue;}
>>
>> Testtyp gettesttyp(){return m_testtyp;}
>>
>> operator Testtyp() const{
>>
>> return m_testtyp;
>>
>> }
>>
>> };
>>
>>
>> int main(){
>>
>> Testtyp help1;
>>
>> Testtypcontainer help3;
>>
>> cout<<help3.gettesttyp()<<endl;
>>
>> help1=(Testtyp)help3; // generate compile error
>>
>>
>> return 0;
>>
>> }
>>
>> P.s. Sorry for my bad english
>
> Looks like the compiler translates your C-style cast
>
> help1=(Testtyp)help3;
>
> into a call to the constructor
>
> template< class T >
> variant< float, int >::variant(T&)
>
> rather than using Testtypcontainer's conversion operator.  This could 
> probably be fixed by SFINAE'ing out certain bindings of T in the variant 
> constructor, i.e., using boost::enable_if to only allow T to be bound such 
> that T& (and const T& for the other constructor overload) is (uniquely?) 
> convertible to one of the variant's types (in this case, float and int).
>
> I would file a trac ticket, maybe simplify your example code:
>
> #include <boost/variant.hpp>
>
> typedef boost::variant<int> variant_t;
>
> struct X
> { operator variant_t() const { return variant_t(); } };
>
> void main()
> {
>     X x;
>     variant_t v = static_cast< variant_t >(x); // COMPILER ERROR
> }
>
> and, if you wish and are able to, submit a patch.
>
> - Jeff
> _______________________________________________
> Unsubscribe & other changes: 
> http://lists.boost.org/mailman/listinfo.cgi/boost