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<title>Portability Hints: Borland C++ 5.5.1</title>
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<table border="1" bgcolor="#007F7F" cellpadding="2" summary="">
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<td bgcolor="#FFFFFF"><img src="../boost.png" alt=
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<h1>Portability Hints: Borland C++ 5.5.1</h1>
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<p>It is a general aim for boost libraries to be <a href=
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"lib_guide.htm#Portability">portable</a>. The primary means for achieving
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this goal is to adhere to ISO Standard C++. However, ISO C++ is a broad and
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complex standard and most compilers are not fully conformant to ISO C++
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yet. In order to achieve portability in the light of this restriction, it
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seems advisable to get acquainted with those language features that some
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compilers do not fully implement yet.</p>
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<p>This page gives portability hints on some language features of the
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Borland C++ version 5.5.1 compiler. Furthermore, the appendix presents
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additional problems with Borland C++ version 5.5. Borland C++ 5.5.1 is a
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freely available command-line compiler for Win32 available at <a href=
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"http://www.borland.com/">http://www.borland.com/</a>.</p>
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<p>Each entry in the following list describes a particular issue, complete
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with sample source code to demonstrate the effect. Most sample code herein
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has been verified to compile with gcc 2.95.2 and Comeau C++ 4.2.44.</p>
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<h2>Preprocessor symbol</h2>
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<p>The preprocessor symbol <code>__BORLANDC__</code> is defined for all
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Borland C++ compilers. Its value is the version number of the compiler
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interpreted as a hexadecimal number. The following table lists some known
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values.</p>
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<table border="1" summary="">
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<tr>
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<th>Compiler</th>
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<th><code>__BORLANDC__</code> value</th>
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</tr>
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<tr>
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<td>Borland C++ Builder 4</td>
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<td>0x0540</td>
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</tr>
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<tr>
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<td>Borland C++ Builder 5</td>
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<td>0x0550</td>
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</tr>
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<tr>
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<td>Borland C++ 5.5</td>
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<td>0x0550</td>
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</tr>
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<tr>
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<td>Borland C++ 5.5.1</td>
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<td>0x0551</td>
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</tr>
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<tr>
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<td>Borland C++ Builder 6</td>
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<td>0x0560</td>
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</tr>
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</table>
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<h2>Core Language</h2>
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<h3>[using-directive] Mixing <code>using</code>-declarations and
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<code>using</code>-directives</h3>
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<p>Mixing <code>using</code>-directives (which refer to whole namespaces)
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and namespace-level <code>using</code>-declarations (which refer to
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individual identifiers within foreign namespaces) causes ambiguities where
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there are none. The following code fragment illustrates this:</p>
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<pre>
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namespace N {
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int x();
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}
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using N::x;
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using namespace N;
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int main()
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{
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&x; // Ambiguous overload
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}
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</pre>
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<h3>[using template] <code>using</code>-declarations for class
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templates</h3>
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<p>Identifiers for class templates can be used as arguments to
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<code>using</code>-declarations as any other identifier. However, the
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following code fails to compile with Borland C++:</p>
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<pre>
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template<class T>
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class X { };
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namespace N
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{
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// "cannot use template 'X<T>' without specifying specialization parameters"
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using ::X;
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};
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</pre>
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<h3>[template const arg] Deduction of constant arguments to function
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templates</h3>
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<p>Template function type deduction should omit top-level constness.
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However, this code fragment instantiates "f<const int>(int)":</p>
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<pre>
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template<class T>
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void f(T x)
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{
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x = 1; // works
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(void) &x;
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T y = 17;
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y = 20; // "Cannot modify a const object in function f<const int>(int)"
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(void) &y;
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}
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int main()
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{
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const int i = 17;
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f(i);
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}
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</pre>
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<h3>[function address] Resolving addresses of overloaded functions</h3>
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<p>Addresses of overloaded functions are not in all contexts properly
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resolved (std:13.4 [over.over]); here is a small example:</p>
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<pre>
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template<class Arg>
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void f( void(*g)(Arg) );
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void h(int);
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void h(double);
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template<class T>
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void h2(T);
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int main()
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{
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void (*p)(int) = h; // this works (std:13.4-1.1)
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void (*p2)(unsigned char) = h2; // this works as well (std:13.4-1.1)
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f<int>(h2); // this also works (std:13.4-1.3)
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// "Cannot generate template specialization from h(int)",
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// "Could not find a match for f<Arg>(void (*)(int))"
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f<double>(h); // should work (std:13.4-1.3)
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f( (void(*)(double))h); // C-style cast works (std:13.4-1.6 with 5.4)
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// "Overloaded 'h' ambiguous in this context"
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f(static_cast<void(*)(double)>(h)); // should work (std:13.4-1.6 with 5.2.9)
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}
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</pre>
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<p><strong>Workaround:</strong> Always use C-style casts when determining
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addresses of (potentially) overloaded functions.</p>
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<h3>[string conversion] Converting <code>const char *</code> to
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<code>std::string</code></h3>
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<p>Implicitly converting <code>const char *</code> parameters to
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<code>std::string</code> arguments fails if template functions are
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explicitly instantiated (it works in the usual cases, though):</p>
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<pre>
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#include <string>
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template<class T>
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void f(const std::string & s)
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{}
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int main()
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{
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f<double>("hello"); // "Could not find a match for f<T>(char *)"
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}
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</pre>
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<p><strong>Workaround:</strong> Avoid explicit template function
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instantiations (they have significant problems with Microsoft Visual C++)
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and pass default-constructed unused dummy arguments with the appropriate
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type. Alternatively, if you wish to keep to the explicit instantiation, you
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could use an explicit conversion to <code>std::string</code> or declare the
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template function as taking a <code>const char *</code> parameter.</p>
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<h3>[template value defaults] Dependent default arguments for template
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value parameters</h3>
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<p>Template value parameters which default to an expression dependent on
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previous template parameters don't work:</p>
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<pre>
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template<class T>
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struct A
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{
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static const bool value = true;
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};
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// "Templates must be classes or functions", "Declaration syntax error"
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template<class T, bool v = A<T>::value>
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struct B {};
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int main()
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{
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B<int> x;
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}
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</pre>
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<p><strong>Workaround:</strong> If the relevant non-type template parameter
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is an implementation detail, use inheritance and a fully qualified
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identifier (for example, ::N::A<T>::value).</p>
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<h3>[function partial ordering] Partial ordering of function templates</h3>
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<p>Partial ordering of function templates, as described in std:14.5.5.2
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[temp.func.order], does not work:</p>
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<pre>
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#include <iostream>
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template<class T> struct A {};
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template<class T1>
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void f(const A<T1> &)
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{
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std::cout << "f(const A<T1>&)\n";
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}
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template<class T>
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void f(T)
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{
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std::cout << "f(T)\n";
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}
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int main()
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{
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A<double> a;
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f(a); // output: f(T) (wrong)
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f(1); // output: f(T) (correct)
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}
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</pre>
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<p><strong>Workaround:</strong> Declare all such functions uniformly as
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either taking a value or a reference parameter.</p>
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<h3>[instantiate memfun ptr] Instantiation with member function
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pointer</h3>
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<p>When directly instantiating a template with some member function
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pointer, which is itself dependent on some template parameter, the compiler
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cannot cope:</p>
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<pre>
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template<class U> class C { };
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template<class T>
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class A
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{
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static const int v = C<void (T::*)()>::value;
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};
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</pre>
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<p><strong>Workaround:</strong> Use an intermediate
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<code>typedef</code>:</p>
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<pre>
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template<class U> class C { };
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template<class T>
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class A
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{
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typedef void (T::*my_type)();
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static const int v = C<my_type>::value;
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};
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</pre>
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<p>(Extracted from e-mail exchange of David Abrahams, Fernando Cacciola,
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and Peter Dimov; not actually tested.)</p>
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<h2>Library</h2>
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<h3>[cmath.abs] Function <code>double std::abs(double)</code> missing</h3>
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<p>The function <code>double std::abs(double)</code> should be defined
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(std:26.5-5 [lib.c.math]), but it is not:</p>
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<pre>
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#include <cmath>
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int main()
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{
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double (*p)(double) = std::abs; // error
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}
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</pre>
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<p>Note that <code>int std::abs(int)</code> will be used without warning if
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you write <code>std::abs(5.1)</code>.</p>
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<p>Similar remarks apply to seemingly all of the other standard math
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functions, where Borland C++ fails to provide <code>float</code> and
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<code>long double</code> overloads.</p>
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<p><strong>Workaround:</strong> Use <code>std::fabs</code> instead if type
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genericity is not required.</p>
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<h2>Appendix: Additional issues with Borland C++ version 5.5</h2>
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<p>These issues are documented mainly for historic reasons. If you are
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still using Borland C++ version 5.5, you are strongly encouraged to obtain
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an upgrade to version 5.5.1, which fixes the issues described in this
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section.</p>
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<h3>[inline friend] Inline friend functions in template classes</h3>
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<p>If a friend function of some class has not been declared before the
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friend function declaration, the function is declared at the namespace
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scope surrounding the class definition. Together with class templates and
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inline definitions of friend functions, the code in the following fragment
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should declare (and define) a non-template function "bool N::f(int,int)",
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which is a friend of class N::A<int>. However, Borland C++ v5.5
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expects the function f to be declared beforehand:</p>
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<pre>
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namespace N {
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template<class T>
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class A
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{
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// "f is not a member of 'N' in function main()"
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friend bool f(T x, T y) { return x < y; }
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};
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}
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int main()
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{
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N::A<int> a;
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}
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</pre>
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<p>This technique is extensively used in boost/operators.hpp. Giving in to
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the wish of the compiler doesn't work in this case, because then the
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"instantiate one template, get lots of helper functions at namespace scope"
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approach doesn't work anymore. Defining BOOST_NO_OPERATORS_IN_NAMESPACE (a
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define BOOST_NO_INLINE_FRIENDS_IN_CLASS_TEMPLATES would match this case
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better) works around this problem and leads to another one, see
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[using-template].</p>
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<hr>
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<p><a href="http://validator.w3.org/check?uri=referer"><img border="0" src=
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"http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01 Transitional"
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height="31" width="88"></a></p>
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<p>Revised
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<!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->03
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December, 2006<!--webbot bot="Timestamp" endspan i-checksum="38512" --></p>
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<p><i>Copyright © 2000-2002 <a href="../people/jens_maurer.htm">Jens
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Maurer</a></i></p>
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<p><i>Distributed under the Boost Software License, Version 1.0. (See
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accompanying file <a href="../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or copy
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at <a href=
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"http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p>
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</body>
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</html>
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