...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
It is a general aim for boost libraries to be portable. 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.
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 http://www.borland.com/.
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.
The preprocessor symbol __BORLANDC__
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.
Compiler | __BORLANDC__ value |
---|---|
Borland C++ Builder 4 | 0x0540 |
Borland C++ Builder 5 | 0x0550 |
Borland C++ 5.5 | 0x0550 |
Borland C++ 5.5.1 | 0x0551 |
Borland C++ Builder 6 | 0x0560 |
using
-declarations and
using
-directivesMixing using
-directives (which refer to whole namespaces)
and namespace-level using
-declarations (which refer to
individual identifiers within foreign namespaces) causes ambiguities where
there are none. The following code fragment illustrates this:
namespace N { int x(); } using N::x; using namespace N; int main() { &x; // Ambiguous overload }
using
-declarations for class
templatesIdentifiers for class templates can be used as arguments to
using
-declarations as any other identifier. However, the
following code fails to compile with Borland C++:
template<class T> class X { }; namespace N { // "cannot use template 'X<T>' without specifying specialization parameters" using ::X; };
Template function type deduction should omit top-level constness. However, this code fragment instantiates "f<const int>(int)":
template<class T> void f(T x) { x = 1; // works (void) &x; T y = 17; y = 20; // "Cannot modify a const object in function f<const int>(int)" (void) &y; } int main() { const int i = 17; f(i); }
Addresses of overloaded functions are not in all contexts properly resolved (std:13.4 [over.over]); here is a small example:
template<class Arg> void f( void(*g)(Arg) ); void h(int); void h(double); template<class T> 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<int>(h2); // this also works (std:13.4-1.3) // "Cannot generate template specialization from h(int)", // "Could not find a match for f<Arg>(void (*)(int))" f<double>(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<void(*)(double)>(h)); // should work (std:13.4-1.6 with 5.2.9) }
Workaround: Always use C-style casts when determining addresses of (potentially) overloaded functions.
const char *
to
std::string
Implicitly converting const char *
parameters to
std::string
arguments fails if template functions are
explicitly instantiated (it works in the usual cases, though):
#include <string> template<class T> void f(const std::string & s) {} int main() { f<double>("hello"); // "Could not find a match for f<T>(char *)" }
Workaround: 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 std::string
or declare the
template function as taking a const char *
parameter.
Template value parameters which default to an expression dependent on previous template parameters don't work:
template<class T> struct A { static const bool value = true; }; // "Templates must be classes or functions", "Declaration syntax error" template<class T, bool v = A<T>::value> struct B {}; int main() { B<int> x; }
Workaround: If the relevant non-type template parameter is an implementation detail, use inheritance and a fully qualified identifier (for example, ::N::A<T>::value).
Partial ordering of function templates, as described in std:14.5.5.2 [temp.func.order], does not work:
#include <iostream> template<class T> struct A {}; template<class T1> void f(const A<T1> &) { std::cout << "f(const A<T1>&)\n"; } template<class T> void f(T) { std::cout << "f(T)\n"; } int main() { A<double> a; f(a); // output: f(T) (wrong) f(1); // output: f(T) (correct) }
Workaround: Declare all such functions uniformly as either taking a value or a reference parameter.
When directly instantiating a template with some member function pointer, which is itself dependent on some template parameter, the compiler cannot cope:
template<class U> class C { }; template<class T> class A { static const int v = C<void (T::*)()>::value; };
Workaround: Use an intermediate
typedef
:
template<class U> class C { }; template<class T> class A { typedef void (T::*my_type)(); static const int v = C<my_type>::value; };
(Extracted from e-mail exchange of David Abrahams, Fernando Cacciola, and Peter Dimov; not actually tested.)
double std::abs(double)
missingThe function double std::abs(double)
should be defined
(std:26.5-5 [lib.c.math]), but it is not:
#include <cmath> int main() { double (*p)(double) = std::abs; // error }
Note that int std::abs(int)
will be used without warning if
you write std::abs(5.1)
.
Similar remarks apply to seemingly all of the other standard math
functions, where Borland C++ fails to provide float
and
long double
overloads.
Workaround: Use std::fabs
instead if type
genericity is not required.
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.
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<int>. However, Borland C++ v5.5 expects the function f to be declared beforehand:
namespace N { template<class T> 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<int> a; }
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].
Revised 03 December, 2006
Copyright © 2000-2002 Jens Maurer
Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)