"Eric Niebler" <[EMAIL PROTECTED]> writes:
> David Abrahams wrote:
>> Boost CVS is offline at the moment so I can't update, but with a
>> recent CVS HEAD, I am getting:
>> quickbook:
>> /home/dave/boost/tools/quickbook/detail/post_process.cpp:374: void
>> quickbook::post_process(const std::string&, std::ostream&, int,
>> int): Assertion `r.full' failed.
>> Any clues?
>
> Can you send along the .qbk file? Also, you might temporarily try
> using the new --no-pretty-print switch to disable the post-processor,
> since that's what seems to be going awry.
Voila.
[part implicit_cast
[version 1]
[copyright 2005-2006 ]
[id implicit_cast]
[category implicit_cast]
[authors [Abrahams,David],[Little,Andy]]
[license
Distributed under the Boost Software License, Version 1.0.
(See accompanying file LICENSE_1_0.txt or copy at
<ulink url="http://www.boost.org/LICENSE_1_0.txt";>
http://www.boost.org/LICENSE_1_0.txt
</ulink>)
]
]
[section:index Boost.Implicit_cast]
* [link synopsis Synopsis]
* [link summary Summary]
* [link example Example]
[#synopsis]
[h3 Synopsis]
header [EMAIL PROTECTED]/../../boost/implicit_cast.hpp
"boost/implicit_cast.hpp"]
template <typename T> T implicit_cast( T1 x);
Requires: T1 is implicitly convertible to T.
Returns: An object of type T whose value is the result of implicit
conversion of x from type T1 to T.
[#summary]
[h3 Summary]
implicit_cast is used when the source type is implicitly
convertible to the target type, to force the implicit conversion.
It's less liberal than static_cast, which will convert in the opposite
direction.
[/ I don't see any point in saying this -- DWA
implicit_cast<t>(S) will usually [link note1
'''<superscript>1</superscript>'''] succeed if [EMAIL
PROTECTED]/../../doc/html/boost_typetraits/reference.html#boost_typetraits.is_convertible
boost::is_convertible<S,T>]::value is true, but will always cause a
compile_time error if
[EMAIL
PROTECTED]/../../doc/html/boost_typetraits/reference.html#boost_typetraits.is_convertible
boost::is_convertible<S,T>]::value is false.
]
[#example]
[h3 Example]
udtA below can only be initialized using the explicit or 'value
initialisation' syntax,
'''
<code>udtA( <replaceable>some-expression</replaceable> )</>
'''
.
#include <boost/implicit_cast.hpp>
template <typename T>
struct udtA{
typedef T value_type;
T val;
template <typename T1>
explicit udtA( T1 const & t) : val(t){}
};
The problem is that udtA can be value initialized by an unexpectedly
wide range of types as shown in the first example below. The
initialisation of a udtA<double> object is safe because it can only be
value initialized by builtin types. The nested udtA<udtA<double> >
allows initialisation by its value_type, a udtA<double>, as shown in
the initialisation of vA1, but it also allows initialisation by an int
as shown in vA2. This may be, but is probably not, what the author of
udtA intended.
udtA<double> vA0(1);
udtA<udtA<double> >vA1(vA0);
udtA<udtA<double> >vA2(1);
This problem can be prevented by adding `boost::implicit_cast` in the
initialiser for val as shown in udtB.
template <typename T>
struct udtB{
typedef T value_type;
T val;
template <typename T1>
explicit udtB( T1 const & t) : val(boost::implicit_cast<T>(t)){}
};
Attempting to initialise a udtB by a type not implicitly convertible to its
value_type will be prevented, as in the attempted initialisation of vB2 by an
int shown below.
udtB<double> vB0(1);
udtB<udtB<double> >vB1(vB0);
udtB<udtB<double> >vB2(1); // Error!
[#note1]
Note1\n
The following is one situation where is_convertible<foo,int> will return true
but where implicit_cast won't succeed.
struct foo{
operator int() { throw "fooled ya"; }
};
[endsect]
--
Dave Abrahams
Boost Consulting
www.boost-consulting.com
