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<title>Portability Hints: Borland C++ 5.5.1</title>
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<p>

<h1>Portability Hints: Borland C++ 5.5.1</h1>

It is a general aim for boost libraries to be
<a href="lib_guide.htm#Portability">portable</a>.  The primary means
for achieving this goal is to adhere to ISO Standard C++.  However,
ISO C++ is a broad and complex standard and most compilers are
not fully conformant to ISO C++ yet.  In order to achieve portability
in the light of this restriction, it seems advisable to get acquainted
with those language features that some compilers do not fully
implement yet.
<p>

This page gives portability hints on some language features of the
Borland C++ version 5.5.1 compiler.  Furthermore, the appendix
presents additional problems with Borland C++ version 5.5.  Borland
C++ 5.5.1 is a freely available command-line compiler for Win32
available at
<a href="http://www.borland.com/">http://www.borland.com/</a>.
<p>

Each entry in the following list describes a particular issue,
complete with sample source code to demonstrate the effect.
Most sample code herein has been verified to compile with gcc 2.95.2
and Comeau C++ 4.2.44.


<h2>Preprocessor symbol</h2>

The preprocessor symbol <code>__BORLANDC__</code> is defined for all
Borland C++ compilers.  Its value is the version number of the
compiler interpreted as a hexadecimal number.  The following table
lists some known values.
<p>

<table border="1">
<tr>
<th>Compiler</th>
<th><code>__BORLANDC__</code> value</th>
</tr>

<tr>
<td>Borland C++ Builder 4</td>
<td>0x0540</td>
</tr>

<tr>
<td>Borland C++ Builder 5</td>
<td>0x0550</td>
</tr>

<tr>
<td>Borland C++ 5.5</td>
<td>0x0550</td>
</tr>

<tr>
<td>Borland C++ 5.5.1</td>
<td>0x0551</td>
</tr>

<tr>
<td>Borland C++ Builder 6</td>
<td>0x0560</td>
</tr>

</table>

<h2>Core Language</h2>

<h3>[using-directive] Mixing <code>using</code>-declarations and
<code>using</code>-directives</h3>

Mixing <code>using</code>-directives (which refer to whole namespaces)
and namespace-level <code>using</code>-declarations (which refer to
individual identifiers within foreign namespaces) causes ambiguities
where there are none.  The following code fragment illustrates this:

<pre>
namespace N {
  int x();
}

using N::x;
using namespace N;

int main()
{
  &amp;x;     // Ambiguous overload
}
</pre>


<h3>[using template] <code>using</code>-declarations for class
templates</h3>

Identifiers for class templates can be used as arguments to
<code>using</code>-declarations as any other identifier.  However, the
following code fails to compile with Borland C++:

<pre>
template&lt;class T&gt;
class X { };

namespace N
{
  // "cannot use template 'X&lt;T&gt;' without specifying specialization parameters"
  using ::X;
};
</pre>


<h3>[template const arg] Deduction of constant arguments to function
templates</h3>

Template function type deduction should omit top-level constness.
However, this code fragment instantiates "f&lt;const int&gt;(int)":

<pre>
template&lt;class T&gt;
void f(T x)
{
        x = 1;  // works
        (void) &amp;x;
        T y = 17;
        y = 20;  // "Cannot modify a const object in function f&lt;const int&gt;(int)"
        (void) &amp;y;
}

int main()
{
        const int i = 17;
        f(i);
}
</pre>

The boost/rational.hpp header exhibits this problem in connection with
the gcd() function.


<h3>[function address] Resolving addresses of overloaded
functions</h3>

Addresses of overloaded functions are not in all contexts properly
resolved (std:13.4 [over.over]); here is a small example:
<pre>
template&lt;class Arg&gt;
void f( void(*g)(Arg) );

void h(int);
void h(double);

template&lt;class T&gt;
void h2(T);

int main()
{
  void (*p)(int) = h;            // this works (std:13.4-1.1)
  void (*p2)(unsigned char) = h2;    // this works as well (std:13.4-1.1)
  f&lt;int&gt;(h2);  // this also works (std:13.4-1.3)

  // "Cannot generate template specialization from h(int)",
  // "Could not find a match for f&lt;Arg&gt;(void (*)(int))"
  f&lt;double&gt;(h);   // should work (std:13.4-1.3)

  f( (void(*)(double))h);  // C-style cast works (std:13.4-1.6 with 5.4)

  // "Overloaded 'h' ambiguous in this context"
  f(static_cast&lt;void(*)(double)&gt;(h)); // should work (std:13.4-1.6 with 5.2.9)
}
</pre>

<strong>Workaround:</strong> Always use C-style casts when determining
addresses of (potentially) overloaded functions.

<h3>[string conversion] Converting <code>const char *</code> to
<code>std::string</code></h3>

Implicitly converting <code>const char *</code> parameters to
<code>std::string</code> arguments fails if template functions are
explicitly instantiated (it works in the usual cases, though):

<pre>
#include &lt;string&gt;

template&lt;class T&gt;
void f(const std::string &amp; s)
{}

int main()
{
  f&lt;double&gt;("hello");  // "Could not find a match for f&lt;T&gt;(char *)"
}

</pre>

<strong>Workaround:</strong> Avoid explicit template function
instantiations (they have significant problems with Microsoft Visual
C++) and pass default-constructed unused dummy arguments with the
appropriate type.  Alternatively, if you wish to keep to the explicit
instantiation, you could use an explicit conversion to
<code>std::string</code> or declare the template function as taking a
<code>const char *</code> parameter.


<h3>[template value defaults] Dependent default arguments for template
value parameters</h3>

Template value parameters which default to an expression dependent on
previous template parameters don't work:

<pre>
template&lt;class T&gt;
struct A
{
  static const bool value = true;
};

// "Templates must be classes or functions", "Declaration syntax error"
template&lt;class T, bool v = A&lt;T&gt;::value&gt;
struct B {};

int main()
{
  B&lt;int&gt; x;
}

</pre>


<strong>Workaround:</strong> If the relevant non-type template
parameter is an implementation detail, use inheritance and a fully
qualified identifier (for example, ::N::A&lt;T&gt;::value).


<h3>[function partial ordering] Partial ordering of function
templates</h3>

Partial ordering of function templates, as described in std:14.5.5.2
[temp.func.order], does not work:

<pre>
#include &lt;iostream&gt;

template&lt;class T&gt; struct A {};

template&lt;class T1&gt;
void f(const A&lt;T1&gt; &)
{
  std::cout << "f(const A&lt;T1&gt;&)\n";
}

template&lt;class T&gt;
void f(T)
{
  std::cout << "f(T)\n";
}

int main()
{
  A&lt;double&gt; a;
  f(a);   // output: f(T)  (wrong)
  f(1);   // output: f(T)  (correct)
}
</pre>

<strong>Workaround:</strong> Declare all such functions uniformly as
either taking a value or a reference parameter.


<h3>[instantiate memfun ptr] Instantiation with member function pointer</h3>

When directly instantiating a template with some member function
pointer, which is itself dependent on some template parameter, the
compiler cannot cope:
<pre>
template&lt;class U&gt; class C { };
template&lt;class T&gt;
class A
{
  static const int v = C&lt;void (T::*)()&gt;::value;
};
</pre>

<strong>Workaround:</strong> Use an intermediate <code>typedef</code>:

<pre>
template&lt;class U&gt; class C { };
template&lt;class T&gt;
class A
{
  typedef void (T::*my_type)();
  static const int v = C&lt;my_type&gt;::value;
};
</pre>

(Extracted from e-mail exchange of David Abrahams, Fernando Cacciola,
and Peter Dimov; not actually tested.)


<h2>Library</h2>


<h3>[cmath.abs] Function <code>double std::abs(double)</code>
missing</h3>

The function <code>double std::abs(double)</code> should be defined
(std:26.5-5 [lib.c.math]), but it is not:

<pre>
#include &lt;cmath&gt;

int main()
{
  double (*p)(double) = std::abs;  // error
}
</pre>

Note that <code>int std::abs(int)</code> will be used without warning
if you write <code>std::abs(5.1)</code>.
<p>
Similar remarks apply to seemingly all of the other standard math
functions, where Borland C++ fails to provide <code>float</code> and
<code>long double</code> overloads.
<p>
<strong>Workaround:</strong> Use <code>std::fabs</code> instead if
type genericity is not required.

<h2>Appendix: Additional issues with Borland C++ version 5.5</h2>

These issues are documented mainly for historic reasons.  If you are
still using Borland C++ version 5.5, you are strongly encouraged to
obtain an upgrade to version 5.5.1, which fixes the issues described
in this section.

<h3>[inline friend] Inline friend functions in template classes</h3>

If a friend function of some class has not been declared before the
friend function declaration, the function is declared at the namespace
scope surrounding the class definition.  Together with class templates
and inline definitions of friend functions, the code in the following
fragment should declare (and define) a non-template function "bool
N::f(int,int)", which is a friend of class N::A&lt;int&gt;.  However,
Borland C++ v5.5 expects the function f to be declared beforehand:

<pre>
namespace N {
template&lt;class T&gt;
class A
{
  // "f is not a member of 'N' in function main()"
  friend bool f(T x, T y) { return x < y; }
};
}

int main()
{
  N::A&lt;int&gt; a;
}
</pre>

This technique is extensively used in boost/operators.hpp.  Giving in
to the wish of the compiler doesn't work in this case, because then
the "instantiate one template, get lots of helper functions at
namespace scope" approach doesn't work anymore.  Defining
BOOST_NO_OPERATORS_IN_NAMESPACE (a define
BOOST_NO_INLINE_FRIENDS_IN_CLASS_TEMPLATES would match this case
better) works around this problem and leads to another one, see
[using-template].

<p>

<hr>

2000-09-30 <a href="../people/jens_maurer.htm">Jens Maurer</a>
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