Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. — Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

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Portability Hints: Borland C++ 5.5.1

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.

Preprocessor symbol

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

Core Language

[using-directive] Mixing using-declarations and using-directives

Mixing 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 template] using-declarations for class templates

Identifiers 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 const arg] Deduction of constant arguments to function templates

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

[function address] Resolving addresses of overloaded functions

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.

[string conversion] Converting 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 defaults] Dependent default arguments for template value parameters

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).

[function partial ordering] Partial ordering of function templates

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.

[instantiate memfun ptr] Instantiation with member function pointer

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.)

Library

[cmath.abs] Function double std::abs(double) missing

The 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.

Appendix: Additional issues with Borland C++ version 5.5

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.

[inline friend] Inline friend functions in template classes

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].

2000-09-30 Jens Maurer