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

30 Jun 2008

      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

 wrote in message 
news:E0675E3EF50A1A48AF78555A6B13296FF9B8E1@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
any(T _ap[SIZE]);
which should do the same like
any(T *_p);
Peter
-----Original Message-----
From: boost-users-bounces@lists.boost.org 
[mailto:boost-users-bounces@lists.boost.org] On Behalf Of Ferdinand Prantl
Sent: Monday, June 30, 2008 08:57
To: boost-users@lists.boost.org
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(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("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(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(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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