lexical_cast & pure virtual functions & VC 8 STL

[sergey_shandar]

VC 8.0 can't compile the valid code:
===== =====
#include <boost/lexical_cast.hpp>

class A 
{
public:
	virtual void out(std::ostream &) const = 0;
};

class B: public A
{
public:
	virtual void out(std::ostream &O) const { O << "B"; }
};

std::ostream &operator<<(std::ostream &O, const A &a)
{
	a.out(O);
	return O;
};

int main()
{
	const A &a = B();
	boost::lexical_cast<std::string>(a);
	return 0;
}
===== =====

The problem occurs in VC STL:
===== =====
c:\program files\microsoft visual studio
8\vc\include\limits(102) : error C2259: 'A' : cannot
instantiate abstract class
===== =====
However, should boost::lexical_cast use
std::numeric_cast in this case?

[redi]

Logged In: YES 
user_id=426241

The code is not valid, because the type of lexical_cast's
Source template argument must be CopyConstructible

http://boost.org/libs/conversion/lexical_cast.htm#lexical_cast

[nobody]

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Perhaps the a being a reference creates the confusion here?  
The type of a is const & A.  And that (as a reference) is 
copy constructible, even though A is not. 

[alnsn]

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user_id=369903

In my opinion, it is an important use-case. I'd change
requirements from

- Both Source and Target are CopyConstructible [20.1.3].

to

- Target is CopyConstructible [20.1.3].
- Source is CopyConstructible or, if not that,
  Source is polymorphic and the source value refers to an
object with dynamic type SourceDyn which is
CopyConstructible and operator<< called for Source is
equivalent to operator<< called for SourceDyn.

[kevlin]

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user_id=32864

I do not believe that there is any longer a reasone that 
Source needs to be restricted to being CopyConstructible. 
There were reasons for this in the past, but those have 
now passed and the requirements can be loosened so that 
the only requirement on Source is that it is 
OutputStreamable.

This would result in the requirements for the two 
parameters being as follows:

  Source:
    OutputStreamable
  Target:
    InputStreamable &&
    CopyConstructible &&
    DefaultConstructible

[alnsn]

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user_id=369903
Originator: NO

Fixed in HEAD.

Affected files:
M ./boost/lexical_cast.hpp
M ./boost/detail/lcast_precision.hpp
M ./libs/conversion/lexical_cast.htm
M ./libs/conversion/test/Jamfile
M ./libs/conversion/test/Jamfile.v2
A ./libs/conversion/test/lexical_cast_abstract_test.cpp

Sergey, please check and close.

[alnsn]

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user_id=369903
Originator: NO

See also thread [boost] numeric_limits<Abstract> or how to fix 1358600?
http://lists.boost.org/Archives/boost/2007/01/index.php
especially John Maddock's proposal
http://lists.boost.org/Archives/boost/2007/01/115765.php
and my reply.