Re: [boost] Re: Template argument deduction question

From: Jonathan Wakely (cow_at_[hidden])
Date: 2005-06-03 06:51:35

On Fri, Jun 03, 2005 at 07:38:13AM -0400, David Abrahams wrote:

> Vladimir Prus <ghost_at_[hidden]> writes:
> 
> > Hello,
> > the program_options library fails one tests on EDG compilers (Intel, Como),
> > which a failure to deduce template arguments for a certain call. After reading
> > the standard for some time, I failed to understand the reason.
> >
> > Below is the test program:
> >
> >     
> >     char* cmdline3_[1] = {};
> >     
> >     template<class charT>
> >     void func(const charT* const argv[]) {}
> >     
> >     int main()
> >     {
> >         // P is 'const charT* const argv[]'cmdline3_ 
> >         // A is 'char* cmdline3_[]'
> 
> I think A is   'char*[1]'
> 
> >         // According to 14.8.2.1/2
> >         // If P is not reference type
> >         //    -If A is an array type, the pointer type produced
> >         //     by the array-to-pointer standard convertion (4.2)
> >         //     is used in place of A for type deduction.
> >         //
> >         // So, A should become 'char**'
> >
> >         // The following does not compile.
> >         func(cmdline3_);
> >     
> >         char** p = cmdline3_;
> >         // This does compile, even though the argument type used for
> >         // type deduction should be the same as in example above.
> >         func(p);
> >     
> >     }
> >  
> > Anybody can explain what's going on?
> 
> It's well known that converting a char** into a char const** is a
> type-safety hole, so the language doesn't allow it (see D&E for the
> details, I think).

Or the example in [conv.qual] in the standard:

    int main() {
        const char c = 'c';
        char* pc;
        const char** pcc = &pc;         //1: not allowed
        *pcc = &c;
        *pc = 'C';              //2: modifies a  const  object
    }

>                     'char*[1]' converts (by array-to-pointer) to 'char**',
> but a 'const charT* const[]' parameter is equivalent to char const*
> const*', so a similar logic applies.

That was what I thought the problem was, but why does the second call
succeed?  

jon