C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  pointer to pointer conversion          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message pw4getter Guest Posted: Fri Mar 05, 2004 3:06 pm    Post subject: pointer to pointer conversion Is there any rationale to prohibit implicit conversion from derived** to base** ? To illustrate my question I submit couple lines below: class base { public: base(){} virtual ~base(){} }; class derived: public base { derived(){} virtual ~derived(){} }; void do_something(base**ppb) {} void test() { derived my_d; derived * pd = &my_d; do_something(&pd); // Error in VC6,VC.NET And in BC 5; bse * pb = &my_d; do_sometning(&pb); // ok, as it should. } Back to top Leor Zolman Guest Posted: Fri Mar 05, 2004 3:40 pm    Post subject: Re: pointer to pointer conversion On 5 Mar 2004 07:06:10 -0800, [email]pw4getter (AT) yahoo (DOT) com[/email] (pw4getter) wrote: Quote: Is there any rationale to prohibit implicit conversion from derived** to base** ? To illustrate my question I submit couple lines below: class base { public: base(){} virtual ~base(){} }; class derived: public base { derived(){} virtual ~derived(){} }; void do_something(base**ppb) {} void test() { derived my_d; derived * pd = &my_d; do_something(&pd); // Error in VC6,VC.NET And in BC 5; bse * pb = &my_d; do_sometning(&pb); // ok, as it should. } I wasn't clear on why myself, so I did a little research and found this wonderful article from the CUJ expert's forum: http://www.cuj.com/documents/s=7995/cujcexp1905hyslop/ I think in there is justification for why that's an error, but I'm going to have to re-read it myself to remember exactly what the rationale is... -leor Leor Zolman BD Software [email]leor (AT) bdsoft (DOT) com[/email] www.bdsoft.com -- On-Site Training in C/C++, Java, Perl & Unix C++ users: Download BD Software's free STL Error Message Decryptor at www.bdsoft.com/tools/stlfilt.html Back to top Victor Bazarov Guest Posted: Fri Mar 05, 2004 3:42 pm    Post subject: Re: pointer to pointer conversion "pw4getter" <pw4getter (AT) yahoo (DOT) com> wrote... Quote: Is there any rationale to prohibit implicit conversion from derived** to base** ? Yes. Simple, really. While 'derived' and 'base' are related (implied from your question and the use of those words as type names) and the conversion between pointers to them is allowed, derived* and base* are NOT related, therefore there is no implicit conversion between pointers to them. Victor Back to top Nick Hounsome Guest Posted: Fri Mar 05, 2004 7:09 pm    Post subject: Re: pointer to pointer conversion "pw4getter" <pw4getter (AT) yahoo (DOT) com> wrote Quote: Is there any rationale to prohibit implicit conversion from derived** to base** ? To illustrate my question I submit couple lines below: Try the following program: struct base {int ib}; struct derived : base {int d;} derived d[2]; derived* dp = d; derived** dpp = &dp; base** bpp = reinterpret_cast<base**>(dpp); // force it base*bp = *bpp; assert(bp+1 == dp+1); // this will fail Back to top Andrey Tarasevich Guest Posted: Fri Mar 05, 2004 9:49 pm    Post subject: Re: pointer to pointer conversion Leor Zolman wrote: Quote: ... I think in there is justification for why that's an error, but I'm going to have to re-read it myself to remember exactly what the rationale is... ... At some level of abstraction, the rationale behind this limitation has the same roots as the well-known limitation that prohibits the following conversion char** p = 0; const char** pc = p; // ERROR If the original conversion was allowed it would provide a way to circumvent type-safety rules, just like 'const char**' -> 'char**' conversion would allow violations of const-correctness rules. -- Best regards, Andrey Tarasevich Back to top Casey Carter Guest Posted: Fri Mar 05, 2004 11:41 pm    Post subject: Re: pointer to pointer conversion [email]pw4getter (AT) yahoo (DOT) com[/email] (pw4getter) wrote in message news:<7914aaeb.0403050706.5c91f6e (AT) posting (DOT) google.com>... Quote: Is there any rationale to prohibit implicit conversion from derived** to base** ? Yes: if you convert a derived** into a base**, then someone could assign a base* into it that does not point to a derived. e.g.: derived* d = new derived(); derived** dp = &d; base** bp = dp; // Implicit conversion *bp = new base(); After this snippet, **dp is a base, but not a derived. 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