[boost] Re: Template argument deduction question

From: David Abrahams (dave_at_[hidden])
Date: 2005-06-03 08:47:38

Vladimir Prus <ghost_at_[hidden]> writes:

> David Abrahams wrote:
>
>>> 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]'
>
> Yes, but it still should be converted to char** for template argument
> deduction.
>
>>>         // 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).  
>
> I know that. What's why the function argument is not (const charT* argv[])
> but (const charT* const argv[])
>
>> '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.
>
> I don't think so. In the second call, passing "char**" to the same function
> works without any problems. It looks like in the first call, compiler
> "forgets" to promote array to pointer.

On second thought, I think you've found an EDG bug.

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