My restriction is that f takes a ABC const-ref as an input. Can I construct the variant from the    const ABC& abc Maybe like    variant v(abc);

without deep copying. The Di objects are big.

Then I would apply the visitation.

Maybe storing variant...

Yes, you can store ptrs in the variant.

So f would look like

void f( const ABC& abc ) {   const VariantTypeDefProvidedByLib v(&abc);

   v.apply_visitor( YourVisitor ); }

You cannot construct the variant in f(), because you already lost the compile-time type of the object - it's ABC now. If you'd like to work with ABC's and virtual functions, then why do you need variant? Variant is about preserving the compile-time type.

Hi! The only solution I can come up with is the declaration of the visitor in your ABCBase as virtual function and visitor has a function template to dispatch different types: class ABCVisitor { public: template<class T> void visit(T& t) { // no visit implemented ... } }; class ABCBase { //... dtor, copy ctor, assignment operator etc. public: void accept_visitor(ABCVisitor& v) { do_accept_visitor(v); } private: virtual void accept_visitor(ABCVisitor& v)=0; }; class ABCDerived1 { //... dtor, copy ctor, assignment operator etc. void accept_visitor(ABCVisitor& v) { v.visit(*this); } }; template<> void ABCVisitor::visit<ABCDerived1>(ABCDerived1& derived) { // handle derived here... } Hope that helps, Ovanes On Mon, Aug 3, 2009 at 4:22 PM, Hicham Mouline wrote:

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-----Original Message----- From: boost-users-bounces@lists.boost.org [mailto:boost-users- bounces@lists.boost.org] On Behalf Of Igor R Sent: 03 August 2009 14:50 To: boost-users@lists.boost.org Subject: Re: [Boost-users] variant: bounded types derive from same base class

...

My restriction is that f takes a ABC const-ref as an input. Can I construct the variant from the const ABC& abc Maybe like variant v(abc);

without deep copying. The Di objects are big.

Then I would apply the visitation.

Maybe storing variant...

Yes, you can store ptrs in the variant.

...

So f would look like

void f( const ABC& abc ) { const VariantTypeDefProvidedByLib v(&abc);

v.apply_visitor( YourVisitor ); }

You cannot construct the variant in f(), because you already lost the compile-time type of the object - it's ABC now. If you'd like to work with ABC's and virtual functions, then why do you need variant? Variant is about preserving the compile-time type.

Then that's not what I need. I need a runtime visitor pattern.

Parts of the user code deal with ABCs, and the user also writes functions for specific derived types, that use non common public functions of the derived types.

The f function still needs to dispatch to the appropriate piece of code depending on the ABC type.

void f( const ABC& abc ) {

Switch (abc.getTag()) { case D1tag: // do something ... Case Dntag: } } // I get the warning from g++ if I miss 1 case

Or

void f( const ABC& abc ) { if (dynamic_cast(&abc)) // do something ... if (dynamic_cast(&abc)) // do something } // no warnings here If I miss a case

Maybe what I want is just not possible,

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From: boost-users-bounces@lists.boost.org [mailto:boost-users-bounces@lists.boost.org] On Behalf Of Ovanes Markarian Sent: 03 August 2009 18:32 To: boost-users@lists.boost.org Subject: Re: [Boost-users] variant: bounded types derive from same base class sorry, forgot to derive ABCDerived1 form ABCBase ;) But hope you got the intention. On Mon, Aug 3, 2009 at 5:33 PM, Ovanes Markarian wrote: Hi! The only solution I can come up with is the declaration of the visitor in your ABCBase as virtual function and visitor has a function template to dispatch different types: class ABCVisitor {     public:     template<class T>     void visit(T& t)     {       // no visit implemented ...     } }; class ABCBase {    //... dtor, copy ctor, assignment operator etc.    public:    void accept_visitor(ABCVisitor& v)    {        do_accept_visitor(v);    }      private:    virtual void accept_visitor(ABCVisitor& v)=0; }; class ABCDerived1 : public ABCBase {    //... dtor, copy ctor, assignment operator etc.    void accept_visitor(ABCVisitor& v)    {        v.visit(*this);    } }; template<> void ABCVisitor::visit<ABCDerived1>(ABCDerived1& derived) {    // handle derived here... } Hope that helps, Ovanes ---------------------------------------------------------------------------- ----------------- Unfortunately, that would require changing the ABCBase to add those 2 member functions. I can't touch ABCBase nor Really, I have an existing hierarchy of classes which I cannot touch, but I still would like to have "external" polymorphism behaviour without the need to add base class virtual functions. Rds,