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8  <title>Portability Hints: Borland C++ 5.5.1</title>
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33
34  <h1>Portability Hints: Borland C++ 5.5.1</h1>
35
36  <p>It is a general aim for boost libraries to be <a href=
37  "lib_guide.htm#Portability">portable</a>. The primary means for achieving
38  this goal is to adhere to ISO Standard C++. However, ISO C++ is a broad and
39  complex standard and most compilers are not fully conformant to ISO C++
40  yet. In order to achieve portability in the light of this restriction, it
41  seems advisable to get acquainted with those language features that some
42  compilers do not fully implement yet.</p>
43
44  <p>This page gives portability hints on some language features of the
45  Borland C++ version 5.5.1 compiler. Furthermore, the appendix presents
46  additional problems with Borland C++ version 5.5. Borland C++ 5.5.1 is a
47  freely available command-line compiler for Win32 available at <a href=
48  "http://www.borland.com/">http://www.borland.com/</a>.</p>
49
50  <p>Each entry in the following list describes a particular issue, complete
51  with sample source code to demonstrate the effect. Most sample code herein
52  has been verified to compile with gcc 2.95.2 and Comeau C++ 4.2.44.</p>
53
54  <h2>Preprocessor symbol</h2>
55
56  <p>The preprocessor symbol <code>__BORLANDC__</code> is defined for all
57  Borland C++ compilers. Its value is the version number of the compiler
58  interpreted as a hexadecimal number. The following table lists some known
59  values.</p>
60
61  <table border="1" summary="">
62    <tr>
63      <th>Compiler</th>
64
65      <th><code>__BORLANDC__</code> value</th>
66    </tr>
67
68    <tr>
69      <td>Borland C++ Builder 4</td>
70
71      <td>0x0540</td>
72    </tr>
73
74    <tr>
75      <td>Borland C++ Builder 5</td>
76
77      <td>0x0550</td>
78    </tr>
79
80    <tr>
81      <td>Borland C++ 5.5</td>
82
83      <td>0x0550</td>
84    </tr>
85
86    <tr>
87      <td>Borland C++ 5.5.1</td>
88
89      <td>0x0551</td>
90    </tr>
91
92    <tr>
93      <td>Borland C++ Builder 6</td>
94
95      <td>0x0560</td>
96    </tr>
97  </table>
98
99  <h2>Core Language</h2>
100
101  <h3>[using-directive] Mixing <code>using</code>-declarations and
102  <code>using</code>-directives</h3>
103
104  <p>Mixing <code>using</code>-directives (which refer to whole namespaces)
105  and namespace-level <code>using</code>-declarations (which refer to
106  individual identifiers within foreign namespaces) causes ambiguities where
107  there are none. The following code fragment illustrates this:</p>
108  <pre>
109namespace N {
110  int x();
111}
112
113using N::x;
114using namespace N;
115
116int main()
117{
118  &amp;x;     // Ambiguous overload
119}
120</pre>
121
122  <h3>[using template] <code>using</code>-declarations for class
123  templates</h3>
124
125  <p>Identifiers for class templates can be used as arguments to
126  <code>using</code>-declarations as any other identifier. However, the
127  following code fails to compile with Borland C++:</p>
128  <pre>
129template&lt;class T&gt;
130class X { };
131
132namespace N
133{
134  // "cannot use template 'X&lt;T&gt;' without specifying specialization parameters"
135  using ::X;
136};
137</pre>
138
139  <h3>[template const arg] Deduction of constant arguments to function
140  templates</h3>
141
142  <p>Template function type deduction should omit top-level constness.
143  However, this code fragment instantiates "f&lt;const int&gt;(int)":</p>
144  <pre>
145template&lt;class T&gt;
146void f(T x)
147{
148        x = 1;  // works
149        (void) &amp;x;
150        T y = 17;
151        y = 20;  // "Cannot modify a const object in function f&lt;const int&gt;(int)"
152        (void) &amp;y;
153}
154
155int main()
156{
157        const int i = 17;
158        f(i);
159}
160</pre>
161
162  <h3>[function address] Resolving addresses of overloaded functions</h3>
163
164  <p>Addresses of overloaded functions are not in all contexts properly
165  resolved (std:13.4 [over.over]); here is a small example:</p>
166  <pre>
167template&lt;class Arg&gt;
168void f( void(*g)(Arg) );
169
170void h(int);
171void h(double);
172
173template&lt;class T&gt;
174void h2(T);
175
176int main()
177{
178  void (*p)(int) = h;            // this works (std:13.4-1.1)
179  void (*p2)(unsigned char) = h2;    // this works as well (std:13.4-1.1)
180  f&lt;int&gt;(h2);  // this also works (std:13.4-1.3)
181
182  // "Cannot generate template specialization from h(int)",
183  // "Could not find a match for f&lt;Arg&gt;(void (*)(int))"
184  f&lt;double&gt;(h);   // should work (std:13.4-1.3)
185
186  f( (void(*)(double))h);  // C-style cast works (std:13.4-1.6 with 5.4)
187
188  // "Overloaded 'h' ambiguous in this context"
189  f(static_cast&lt;void(*)(double)&gt;(h)); // should work (std:13.4-1.6 with 5.2.9)
190}
191</pre>
192
193  <p><strong>Workaround:</strong> Always use C-style casts when determining
194  addresses of (potentially) overloaded functions.</p>
195
196  <h3>[string conversion] Converting <code>const char *</code> to
197  <code>std::string</code></h3>
198
199  <p>Implicitly converting <code>const char *</code> parameters to
200  <code>std::string</code> arguments fails if template functions are
201  explicitly instantiated (it works in the usual cases, though):</p>
202  <pre>
203#include &lt;string&gt;
204
205template&lt;class T&gt;
206void f(const std::string &amp; s)
207{}
208
209int main()
210{
211  f&lt;double&gt;("hello");  // "Could not find a match for f&lt;T&gt;(char *)"
212}
213
214</pre>
215
216  <p><strong>Workaround:</strong> Avoid explicit template function
217  instantiations (they have significant problems with Microsoft Visual C++)
218  and pass default-constructed unused dummy arguments with the appropriate
219  type. Alternatively, if you wish to keep to the explicit instantiation, you
220  could use an explicit conversion to <code>std::string</code> or declare the
221  template function as taking a <code>const char *</code> parameter.</p>
222
223  <h3>[template value defaults] Dependent default arguments for template
224  value parameters</h3>
225
226  <p>Template value parameters which default to an expression dependent on
227  previous template parameters don't work:</p>
228  <pre>
229template&lt;class T&gt;
230struct A
231{
232  static const bool value = true;
233};
234
235// "Templates must be classes or functions", "Declaration syntax error"
236template&lt;class T, bool v = A&lt;T&gt;::value&gt;
237struct B {};
238
239int main()
240{
241  B&lt;int&gt; x;
242}
243
244</pre>
245
246  <p><strong>Workaround:</strong> If the relevant non-type template parameter
247  is an implementation detail, use inheritance and a fully qualified
248  identifier (for example, ::N::A&lt;T&gt;::value).</p>
249
250  <h3>[function partial ordering] Partial ordering of function templates</h3>
251
252  <p>Partial ordering of function templates, as described in std:14.5.5.2
253  [temp.func.order], does not work:</p>
254  <pre>
255#include &lt;iostream&gt;
256
257template&lt;class T&gt; struct A {};
258
259template&lt;class T1&gt;
260void f(const A&lt;T1&gt; &amp;)
261{
262  std::cout &lt;&lt; "f(const A&lt;T1&gt;&amp;)\n";
263}
264
265template&lt;class T&gt;
266void f(T)
267{
268  std::cout &lt;&lt; "f(T)\n";
269}
270
271int main()
272{
273  A&lt;double&gt; a;
274  f(a);   // output: f(T)  (wrong)
275  f(1);   // output: f(T)  (correct)
276}
277</pre>
278
279  <p><strong>Workaround:</strong> Declare all such functions uniformly as
280  either taking a value or a reference parameter.</p>
281
282  <h3>[instantiate memfun ptr] Instantiation with member function
283  pointer</h3>
284
285  <p>When directly instantiating a template with some member function
286  pointer, which is itself dependent on some template parameter, the compiler
287  cannot cope:</p>
288  <pre>
289template&lt;class U&gt; class C { };
290template&lt;class T&gt;
291class A
292{
293  static const int v = C&lt;void (T::*)()&gt;::value;
294};
295</pre>
296
297  <p><strong>Workaround:</strong> Use an intermediate
298  <code>typedef</code>:</p>
299  <pre>
300template&lt;class U&gt; class C { };
301template&lt;class T&gt;
302class A
303{
304  typedef void (T::*my_type)();
305  static const int v = C&lt;my_type&gt;::value;
306};
307</pre>
308
309  <p>(Extracted from e-mail exchange of David Abrahams, Fernando Cacciola,
310  and Peter Dimov; not actually tested.)</p>
311
312  <h2>Library</h2>
313
314  <h3>[cmath.abs] Function <code>double std::abs(double)</code> missing</h3>
315
316  <p>The function <code>double std::abs(double)</code> should be defined
317  (std:26.5-5 [lib.c.math]), but it is not:</p>
318  <pre>
319#include &lt;cmath&gt;
320
321int main()
322{
323  double (*p)(double) = std::abs;  // error
324}
325</pre>
326
327  <p>Note that <code>int std::abs(int)</code> will be used without warning if
328  you write <code>std::abs(5.1)</code>.</p>
329
330  <p>Similar remarks apply to seemingly all of the other standard math
331  functions, where Borland C++ fails to provide <code>float</code> and
332  <code>long double</code> overloads.</p>
333
334  <p><strong>Workaround:</strong> Use <code>std::fabs</code> instead if type
335  genericity is not required.</p>
336
337  <h2>Appendix: Additional issues with Borland C++ version 5.5</h2>
338
339  <p>These issues are documented mainly for historic reasons. If you are
340  still using Borland C++ version 5.5, you are strongly encouraged to obtain
341  an upgrade to version 5.5.1, which fixes the issues described in this
342  section.</p>
343
344  <h3>[inline friend] Inline friend functions in template classes</h3>
345
346  <p>If a friend function of some class has not been declared before the
347  friend function declaration, the function is declared at the namespace
348  scope surrounding the class definition. Together with class templates and
349  inline definitions of friend functions, the code in the following fragment
350  should declare (and define) a non-template function "bool N::f(int,int)",
351  which is a friend of class N::A&lt;int&gt;. However, Borland C++ v5.5
352  expects the function f to be declared beforehand:</p>
353  <pre>
354namespace N {
355template&lt;class T&gt;
356class A
357{
358  // "f is not a member of 'N' in function main()"
359  friend bool f(T x, T y) { return x &lt; y; }
360};
361}
362
363int main()
364{
365  N::A&lt;int&gt; a;
366}
367</pre>
368
369  <p>This technique is extensively used in boost/operators.hpp. Giving in to
370  the wish of the compiler doesn't work in this case, because then the
371  "instantiate one template, get lots of helper functions at namespace scope"
372  approach doesn't work anymore. Defining BOOST_NO_OPERATORS_IN_NAMESPACE (a
373  define BOOST_NO_INLINE_FRIENDS_IN_CLASS_TEMPLATES would match this case
374  better) works around this problem and leads to another one, see
375  [using-template].</p>
376  <hr>
377
378  <p><a href="http://validator.w3.org/check?uri=referer"><img border="0" src=
379  "http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01 Transitional"
380  height="31" width="88"></a></p>
381
382  <p>Revised
383  <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->03
384  December, 2006<!--webbot bot="Timestamp" endspan i-checksum="38512" --></p>
385
386  <p><i>Copyright &copy; 2000-2002 <a href="../people/jens_maurer.htm">Jens
387  Maurer</a></i></p>
388
389  <p><i>Distributed under the Boost Software License, Version 1.0. (See
390  accompanying file <a href="../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or copy
391  at <a href=
392  "http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p>
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