Re: [Boost-users] Is there an elegant way to initialize an instanceofboost::any with a C-style string constant?

From: Ferdinand Prantl (prantlf_at_[hidden])
Date: 2008-06-30 13:39:03

Well, a little shorter as the static_cast version but still a "superfluous" 
cast.

I could not find anything about it but the thread which I referred in the 
first post to. Do you remember why the constructor has not been added to the 
official version?

Ferda

<peter_foelsche_at_[hidden]> wrote in message 
news:E0675E3EF50A1A48AF78555A6B13296FF9B8E1_at_cos-us-mb03.cos.agilent.com...
> to get this to compile write:
>
>    boost::any foo((const char*)"foo");
>
> I suggested some time ago to add a special constructor:
>
>
> template<class T, size_t SIZE>
> any(T _ap[SIZE]);
>
>
> which should do the same like
> any(T *_p);
>
> Peter
>
> -----Original Message-----
> From: boost-users-bounces_at_[hidden] 
> [mailto:boost-users-bounces_at_[hidden]] On Behalf Of Ferdinand Prantl
> Sent: Monday, June 30, 2008 08:57
> To: boost-users_at_[hidden]
> Subject: [Boost-users] Is there an elegant way to initialize an instance 
> ofboost::any with a C-style string constant?
>
> Hello,
>
> I would like to ask you about the best solution to initialize a
> boost::any instance with a C-string constant. Both the most legible
> (IMHO) possibilities fail:
>
>    boost::any foo("foo");
>
>    boost::any bar;
>    bar = "bar";
>
> Compiler (MSVC 2008 in my case) complains about them:
>
>    ...\boost\any.hpp(117) : error C2536:
> 'boost::any::holder<ValueType>::boost::any::holder<ValueType>::held'
>            : cannot specify explicit initializer for arrays
>      with [ ValueType=const char [4] ]
>    ...\boost\any.hpp(134) : see declaration of
> 'boost::any::holder<ValueType>::held'
>      with [ ValueType=const char [4] ]
>    ...\boost\any.hpp(115) : while compiling class template
>              member function
> 'boost::any::holder<ValueType>::holder(ValueType (&))'
>      with [ ValueType=const char [4] ]
>    ...\boost\any.hpp(41) : see reference to class template instantiation
>              'boost::any::holder<ValueType>' being compiled
>      with [ ValueType=const char [4] ]
>    ...\test.cpp(10) : see reference to function template instantiation
>              'boost::any::any<const char[4]>(ValueType (&))' being 
> compiled
>      with [ ValueType=const char [4] ]
>
> Obviously the string constant is considered of the type chat[4]
> which lacks CopyConstructible and Assignable requirements. Wrapping
> the constant in some expression which satisfies the requiremenst
> spoils the beauty of a simple code or brings unnecessary (for the
> constant handling) object:
>
>    boost::any foo(static_cast<const char *>("foo"));
>    boost::any foo(std::string("foo"));
>
> Such solutions were suggested in the thread
> http://lists.boost.org/Archives/boost/2003/01/43460.php but I was
> hoping there could be something better. I suppose there is no way
> to persuade the compiler to consider the constant to be const char *...
>
> Why has not been the following constructor added to boost::any?
> The array would make it there passing just one conversion:
>
>    template<typename ValueType> any(const ValueType * value)
>        : content(new holder<const ValueType *>(value))
>    {}
>
> Someone could say that that it is more error prone because it enables
> storing a pointer which can be invalid as soon as the boost::any
> instance leaves the scope but the error can be made anyway:
>
>    boost::any foo(std::string("foo").c_str());
>
> Another possibility would be to match the array type exactly but it
> has probably no benefit:
>
>    template<typename ValueType> any(const ValueType value [])
>        : content(new holder<const ValueType *>(value))
>    {}
>
> Do you see any problems with such constructors? How do you use
> string constants with boost::any?
>
> Thank you,
> Ferda Prantl
>
>
>
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