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Removed bool_testable, added note about portability of separate, explicit instantiation, changed license of documentation

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[SVN r21110]
This commit is contained in:
Daniel Frey
2003-12-03 07:01:28 +00:00
parent 929517d6d7
commit 2f5945d0cd
6 changed files with 17 additions and 229 deletions
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operators.htm
+17 -151
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@@ -72,8 +72,6 @@
<li><a href="#ordering">Ordering Note</a></li>
<li><a href="#symmetry">Symmetry Note</a></li>
<li><a href="#safe_bool_note">Safe-Bool Note</a></li>
</ul>
</li>
@@ -342,12 +340,19 @@ class MyInt
<li><code><a href="#indexable">indexable&lt;&gt;</a></code></li>
<li><code><a href="#bool_testable">bool_testable&lt;&gt;</a></code></li>
<li>Any composite operator template that includes at least one of the
above</li>
</ul>
<p>As Daniel Kr&uuml;gler pointed out, this technique violates 14.6.5/2
and is thus non-portable. The reasoning is, that the operators injected
by the instantiation of e.g.
<code>less_than_comparable&lt;myclass&gt;</code> can not be found
by ADL according to the rules given by 3.4.2/2, since myclass is
not an associated class of
<code>less_than_comparable&lt;myclass&gt;</code>.
Thus only use this technique if all else fails.</p>
<h3>Requirement <a name="portability">Portability</a></h3>
<p>Many compilers (<i>e.g.</i> MSVC 6.3, GCC 2.95.2) will not enforce the
@@ -531,17 +536,6 @@ const point&lt;float&gt; pi_over_4_normalized = pi_over_4 / length(pi_over_4);
Return convertible to <code>bool</code>.</td>
</tr>
<tr>
<td><code><a name="bool_testable">bool_testable&lt;T&gt;</a></code></
td>
<td><code>bool operator!(const T&)</code></td>
<td><code>static_cast&lt;bool&gt;(t)</code>.<br>
<code>T</code> convertible to <code>bool</code>. See the <a href=
"#safe_bool_note">Safe-Bool Note</a>.</td>
</tr>
<tr>
<td><code><a name="addable1">addable&lt;T&gt;</a></code><br>
<code>addable1&lt;T&gt;</code></td>
@@ -989,132 +983,6 @@ T operator+( T lhs, const T&amp; rhs )
that don't implement the NRVO. <br>
<br>
<h4><a name="safe_bool_note">Safe-Bool</a> Note</h4>
<p><code><a href="#bool_testable">bool_testable</a></code> provides the
antithesis of operator bool, such that the expression <code>if (!p)</code>
is valid, whilst also making <code>operator bool</code> safer by preventing
accidental conversions to integer types. If <code>operator bool</code>
were declared for a class without using
<code><a href="#bool_testable">bool_testable</code></a> then expressions
such as :</p>
<pre>
void f(int);
void g(object o)
{
f(o);
}
</pre>
<p>would compile silently, not warning the user of the (probably)
unintended behaviour of passing <code>0</code> or <code>1</code> to
<code>f()</code>.</p> <code>bool_testable&lt;&gt;</code> prevents these
accidental conversions by declaring a private conversion operator to
<code>signed char</code>, and not defining the body.</p>
<h5>Example :</h5>
<pre>
class Stream : boost::bool_testable&lt;Stream&gt;
{
public:
explicit Stream(const char * source);
operator bool() const;
// non-member bool operator!(const T&) const auto-generated and
// operator bool made safe by bool_testable&lt;&gt;
bool can_print();
};
void g(int);
void f()
{
if (Stream s1("source.txt"))
{
// use s1 ...
}
// or..
Stream s2("source.txt");
if (!s2)
{
// handle problem
}
// or ..
if (s2 && s2.can_print()) //* see <a href="#bool_testable_msvc">note</a>
{
// print something...
}
g(stream); // Will not compile, but would compile
// fine without bool_testable&lt;&gt;
}
</pre>
<h5><a name="bool_testable_msvc">Note for MSVC version 6 users</a> :</h5>
<p>Due to a bug in MSVC6, whilst compiling the above code, the compiler
will incorrectly diagnose an '<code>ambiguous operator &&</code>' at
the asterisked line. The workaround for this is to either:</p>
<pre>
if (s2)
if (s2.can_print())
</pre>
<p>or</p>
<pre>
if (!!s2 && s2.can_print())
</pre>
<p>This problem also affects the other logical operators.</p>
<h5>Rationale for Implementation</h5>
<p>Another possible implementation for <code>bool_testable</code> was the
safe-bool idiom as found in <a href=
"../smart_ptr/shared_ptr.htm#conversions"><code>shared_ptr&lt;&gt;</code>
</a> written by <a href="../../people/peter_dimov.htm">Peter Dimov</a>.
This implementation required the user to provide <code>operator!</code>,
and provided a conversion operator to a pointer to member function. This
method had the advantage of more descriptive diagnostic messages on most
platforms, when the bool conversion was used incorrectly. Unfortunately
that implementation had issues when used with a class with other user
defined integer conversion operators. This is because the integer
conversion operator provided a better match than the conversion to pointer
to member function. For example:</p>
<pre>
class MyInt : alternative_bool_testable&lt;MyInt&gt;
{
public:
bool operator!() const;
operator int() const;
// alternative_bool_testable&lt;MyInt&gt; provides :
//
// typedef void (MyInt::*member_fn)();
// operator member_fn() { return &MyInt::f; }
};
void g(int);
void f(const MyInt& i)
{
if (!i) // fine: calls operator!
...
g(i); // fine: calls operator int
if (i) // error: calls operator int NOT operator member_fn()
...
}
</pre>
<h3><a name="grpd_oprs">Grouped Arithmetic Operators</a></h3>
<p>The following templates provide common groups of related operations.
@@ -2203,10 +2071,6 @@ public:
<dd>Contributed the NRVO-friendly and symmetric implementation of
arithmetic operators.</dd>
<dt>Sam Partington</dt>
<dd>Contributed the bool_testable class.</dd>
</dl>
<h2>Note for Users of <a name="old_lib_note">Older Versions</a></h2>
@@ -2255,13 +2119,15 @@ public:
backward-compatible.</p>
<hr>
<p>Revised: 30 Oct 2001</p>
<p>Revised: 03 Dec 2003</p>
<p>Copyright &copy; David Abrahams and Beman Dawes 1999-2001. Permission
to copy, use, modify, sell and distribute this document is granted
provided this copyright notice appears in all copies. This document is
provided "as is" without express or implied warranty, and with no claim
as to its suitability for any purpose.</p>
<p>Copyright &copy; Beman Dawes, David Abrahams, 1999-2001.</p>
<p>Copyright &copy; Daniel Frey, 2002-2003.</p>
<p>Use, modification, and distribution is subject to the Boost Software
License, Version 1.0. (See accompanying file
<a href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or copy at
<a href="http://www.boost.org/LICENSE_1_0.txt">
www.boost.org/LICENSE_1_0.txt</a>)</p>
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