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  <title>Portability Hints: Microsoft Visual C++ 6.0 SP4</title>
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  <h1>Portability Hints: Microsoft Visual C++ 6.0 SP4</h1>

  <p>Similar to the <a href="borland_cpp.html">portability hints for Borland
  C++</a>, this page provides hints on some language features of the
  Microsoft Visual C++ version 6.0 service pack 4 compiler. A list of
  acknowledged deficiencies can be found at the <a href=
  "http://support.microsoft.com/support/kb/articles/q243/4/51.asp">Microsoft
  support site</a>.</p>

  <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.</p>

  <h2>Preprocessor symbol</h2>

  <p>The preprocessor symbol <code>_MSC_VER</code> is defined for all
  Microsoft C++ compilers. Its value is the internal version number of the
  compiler interpreted as a decimal number. Since a few other compilers also
  define this symbol, boost provides the symbol <code>BOOST_MSVC</code>,
  which is defined in <a href="../boost/config.hpp">boost/config.hpp</a> to
  the value of _MSC_VER if and only if the compiler is really Microsoft
  Visual C++. The following table lists some known values.</p>

  <table border="1" summary="">
    <tr>
      <th>Compiler</th>

      <th><code>BOOST_MSVC</code> value</th>
    </tr>

    <tr>
      <td>Microsoft Visual C++ 6.0 (up to SP6)</td>

      <td>1200</td>
    </tr>

    <tr>
      <td>Microsoft embedded Visual C++ 4.0</td>

      <td>1200-1202 (cross compilers)</td>
    </tr>
  </table>

  <h2>Core Language</h2>

  <h3>[chained using] Chaining <code>using</code>-declarations</h3>

  <p>Chaining <code>using</code>-declarations does not work.</p>
  <pre>
void f();

namespace N {
  using ::f;
}

void g()
{
  using N::f;  // C2873: 'f': the symbol cannot be used in a using-declaration
}
</pre>

  <h3>[explicit-instantiation] Explicit function template instantiation</h3>

  <p>Trying to explicitly instantiate a function template leads to the wrong
  function being called silently.</p>
  <pre>
#include &lt;stdio.h&gt;

template&lt;class T&gt;
void f()
{
  printf("%d\n", sizeof(T));
}

int main()
{
  f&lt;double&gt;();      // output: "1"
  f&lt;char&gt;();        // output: "1"
  return 0;
}
</pre>

  <h3>[for-scoping] Scopes of definitions in for-loops</h3>

  <p>The scope of variable definitions in <code>for</code> loops should be
  local to the loop's body, but it is instead local to the enclosing
  block.</p>
  <pre>
int main()
{
  for(int i = 0; i &lt; 5; ++i)
   ;
  for(int i = 0; i &lt; 5; ++i)  // C2374: 'i': Redefinition; multiple initialization
   ;
  return 0;
}
</pre>

  <p><strong>Workaround:</strong> Enclose the offending <code>for</code>
  loops in another pair of curly braces.</p>

  <p>Another possible workaround (brought to my attention by Vesa Karvonen)
  is this:</p>
  <pre>
#ifndef for
#define for if (0) {} else for
#endif
</pre>

  <p>Note that platform-specific inline functions in included headers might
  depend on the old-style <code>for</code> scoping.</p>

  <h3>[inclass-member-init] In-class member initialization</h3>

  <p>In-class member initialization, required to implement a
  Standard-conforming <code>std::numeric_limits</code> template, does not
  work.</p>
  <pre>
struct A
{
  static const int i = 5;      // "invalid syntax for pure virtual method"
};
</pre>

  <p><strong>Workaround:</strong> Either use an enum (which has incorrect
  type, but can be used in compile-time constant expressions), or define the
  value out-of-line (which allows for the correct type, but prohibits using
  the constant in compile-time constant expressions). See <a href=
  "int_const_guidelines.htm">Coding Guidelines for Integral Constant
  Expressions</a> for guidelines how to define member constants portably in
  boost libraries.</p>

  <h3>[koenig-lookup] Argument-dependent lookup</h3>

  <p>Argument-dependent lookup, also called Koenig lookup, works for
  overloaded operators, but not for ordinary functions. No additional
  namespaces induced from the argument types seem to be considered.</p>
  <pre>
namespace N {
  struct A {};
  void f(A);
}

void g()
{
  N::A a;
  f(a);     // 'f': undeclared identifier
}
</pre>

  <h3>[template-friend] Templates as friends</h3>

  <p>A Template cannot be declared a friend of a class.</p>
  <pre>
template&lt;class T&gt;
struct A {};

struct B
{
  template&lt;class T&gt;
  friend struct A;     // "syntax error"
};
</pre>

  <h3>[member-template-outofline] Out-of-line definitions of member
  templates</h3>

  <p>Defining member templates outside their enclosing class does not
  work.</p>
  <pre>
template&lt;class T&gt;
struct A
{
  template&lt;class U&gt;
  void f();
};

template&lt;class T&gt;
template&lt;class U&gt;   // "syntax error"
void A&lt;T&gt;::f()      // "T: undeclared identifier"
{
}
</pre>

  <p><strong>Workaround:</strong> Define member templates in-line within
  their enclosing class.</p>

  <h3>[partial-spec] Partial specialization</h3>

  <p>Partial specialization of class templates does not work.</p>
  <pre>
template&lt;class T&gt;
struct A {};

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

template&lt;class T&gt;
struct A&lt;B&lt;T&gt; &gt; {};  // template class was already defined as a non-template
</pre>

  <p><strong>Workaround:</strong> In some situations where interface does not
  matter, class member templates can simulate partial specialization.</p>

  <h3>[template-value] Dependent template value parameters</h3>

  <p>Template value parameters whose type depends on a previous template
  parameter provoke an internal compiler error if the correct syntax (with
  "typename") is used.</p>
  <pre>
template&lt;class T, typename T::result_type&gt; // C1001: INTERNAL COMPILER ERROR: msc1.cpp, line 1794
struct B {};
 // (omit "typename" and it compiles)

</pre>

  <p><strong>Workaround:</strong> Leave off the "typename" keyword. That
  makes the program non-conforming, though.</p>

  <h3>[wchar_t] <code>wchar_t</code> is not built-in</h3>

  <p>The type <code>wchar_t</code> is not a built-in type.</p>
  <pre>
wchar_t x;  // "missing storage class or type identifier"
</pre>

  <p><strong>Workaround:</strong> When using Microsoft Visual C++, the header
  <a href="../boost/config.hpp">boost/config.hpp</a> includes
  <code>&lt;cstddef&gt;</code>, which defines <code>wchar_t</code> as a
  typedef for <code>unsigned short</code>. Note that this means that the
  compiler does not regard <code>wchar_t</code> and <code>unsigned
  short</code> as distinct types, as is required by the standard, and so
  ambiguities may emanate when overloading on <code>wchar_t</code>. The macro
  <code>BOOST_NO_INTRINSIC_WCHAR_T</code> is defined in this situation.</p>

  <h3>[delete-const-pointer] Deleting <code>const X *</code> does not
  work</h3>

  <p>Trying to delete a pointer to a cv-qualified type gives an error:</p>
  <pre>
void f()
{
  const int *p = new int(5);
  delete p;        // C2664: cannot convert from "const int *" to "void *"
}
</pre>

  <p><strong>Workaround:</strong> Define the function</p>
  <pre>
inline void operator delete(const void *p) throw()
{ operator delete(const_cast&lt;void*&gt;(p)); }
</pre>

  <p>and similar functions for the other cv-qualifier combinations, for
  operator delete[], and for the <code>std::nothrow</code> variants.</p>

  <h2>Standard Library</h2>

  <h3>[clib-namespace] C library names in global namespace instead of
  std</h3>

  <p>Library names from the &lt;c...&gt; headers are in the global namespace
  instead of namespace std.</p>

  <p><b>Workaround:</b>&nbsp; The header <a href=
  "../libs/config/config.htm">boost/config.hpp</a> will define
  BOOST_NO_STDC_NAMESPACE. It can be used as follows:</p>
  <pre>
# ifdef BOOST_NO_STDC_NAMESPACE
    namespace std { using ::abs; using ::fabs; }
# endif
</pre>

  <p>Because std::size_t and std::ptrdiff_t are so commonly used, the
  workaround for these is already provided in boost/config.hpp.</p>
  <hr>

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  <p>Revised 
  <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->04 December, 2006<!--webbot bot="Timestamp" endspan i-checksum="38514" --></p>

  <p><i>Copyright &copy; 2001-2002 <a href="../people/jens_maurer.htm">Jens
  Maurer</a></i></p>

  <p><i>Distributed under the Boost Software License, Version 1.0. (See
  accompanying file <a href="../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or copy
  at <a href=
  "http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p>
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