[boost] Re: Template argument deduction question

From: David Abrahams (dave_at_[hidden])
Date: 2005-06-03 06:38:13

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).  '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.

    

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com