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   8   <title>Portability Hints: Microsoft Visual C++ 6.0 SP4</title>
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  34   <h1>Portability Hints: Microsoft Visual C++ 6.0 SP4</h1>
  35
  36   <p>Similar to the <a href="borland_cpp.html">portability hints for Borland
  37   C++</a>, this page provides hints on some language features of the
  38   Microsoft Visual C++ version 6.0 service pack 4 compiler. A list of
  39   acknowledged deficiencies can be found at the <a href=
  40   "http://support.microsoft.com/support/kb/articles/q243/4/51.asp">Microsoft
  41   support site</a>.</p>
  42
  43   <p>Each entry in the following list describes a particular issue, complete
  44   with sample source code to demonstrate the effect. Most sample code herein
  45   has been verified to compile with gcc 2.95.2 and Comeau C++ 4.2.44.</p>
  46
  47   <h2>Preprocessor symbol</h2>
  48
  49   <p>The preprocessor symbol <code>_MSC_VER</code> is defined for all
  50   Microsoft C++ compilers. Its value is the internal version number of the
  51   compiler interpreted as a decimal number. Since a few other compilers also
  52   define this symbol, boost provides the symbol <code>BOOST_MSVC</code>,
  53   which is defined in <a href="../boost/config.hpp">boost/config.hpp</a> to
  54   the value of _MSC_VER if and only if the compiler is really Microsoft
  55   Visual C++. The following table lists some known values.</p>
  56
  57   <table border="1" summary="">
  58     <tr>
  59       <th>Compiler</th>
  60
  61       <th><code>BOOST_MSVC</code> value</th>
  62     </tr>
  63
  64     <tr>
  65       <td>Microsoft Visual C++ 6.0 (up to SP6)</td>
  66
  67       <td>1200</td>
  68     </tr>
  69
  70     <tr>
  71       <td>Microsoft embedded Visual C++ 4.0</td>
  72
  73       <td>1200-1202 (cross compilers)</td>
  74     </tr>
  75   </table>
  76
  77   <h2>Core Language</h2>
  78
  79   <h3>[chained using] Chaining <code>using</code>-declarations</h3>
  80
  81   <p>Chaining <code>using</code>-declarations does not work.</p>
  82   <pre>
  83 void f();
  84
  85 namespace N {
  86   using ::f;
  87 }
  88
  89 void g()
  90 {
  91   using N::f;  // C2873: 'f': the symbol cannot be used in a using-declaration
  92 }
  93 </pre>
  94
  95   <h3>[explicit-instantiation] Explicit function template instantiation</h3>
  96
  97   <p>Trying to explicitly instantiate a function template leads to the wrong
  98   function being called silently.</p>
  99   <pre>
 100 #include &lt;stdio.h&gt;
 101
 102 template&lt;class T&gt;
 103 void f()
 104 {
 105   printf("%d\n", sizeof(T));
 106 }
 107
 108 int main()
 109 {
 110   f&lt;double&gt;();      // output: "1"
 111   f&lt;char&gt;();        // output: "1"
 112   return 0;
 113 }
 114 </pre>
 115
 116   <h3>[for-scoping] Scopes of definitions in for-loops</h3>
 117
 118   <p>The scope of variable definitions in <code>for</code> loops should be
 119   local to the loop's body, but it is instead local to the enclosing
 120   block.</p>
 121   <pre>
 122 int main()
 123 {
 124   for(int i = 0; i &lt; 5; ++i)
 125    ;
 126   for(int i = 0; i &lt; 5; ++i)  // C2374: 'i': Redefinition; multiple initialization
 127    ;
 128   return 0;
 129 }
 130 </pre>
 131
 132   <p><strong>Workaround:</strong> Enclose the offending <code>for</code>
 133   loops in another pair of curly braces.</p>
 134
 135   <p>Another possible workaround (brought to my attention by Vesa Karvonen)
 136   is this:</p>
 137   <pre>
 138 #ifndef for
 139 #define for if (0) {} else for
 140 #endif
 141 </pre>
 142
 143   <p>Note that platform-specific inline functions in included headers might
 144   depend on the old-style <code>for</code> scoping.</p>
 145
 146   <h3>[inclass-member-init] In-class member initialization</h3>
 147
 148   <p>In-class member initialization, required to implement a
 149   Standard-conforming <code>std::numeric_limits</code> template, does not
 150   work.</p>
 151   <pre>
 152 struct A
 153 {
 154   static const int i = 5;      // "invalid syntax for pure virtual method"
 155 };
 156 </pre>
 157
 158   <p><strong>Workaround:</strong> Either use an enum (which has incorrect
 159   type, but can be used in compile-time constant expressions), or define the
 160   value out-of-line (which allows for the correct type, but prohibits using
 161   the constant in compile-time constant expressions). See <a href=
 162   "int_const_guidelines.htm">Coding Guidelines for Integral Constant
 163   Expressions</a> for guidelines how to define member constants portably in
 164   boost libraries.</p>
 165
 166   <h3>[koenig-lookup] Argument-dependent lookup</h3>
 167
 168   <p>Argument-dependent lookup, also called Koenig lookup, works for
 169   overloaded operators, but not for ordinary functions. No additional
 170   namespaces induced from the argument types seem to be considered.</p>
 171   <pre>
 172 namespace N {
 173   struct A {};
 174   void f(A);
 175 }
 176
 177 void g()
 178 {
 179   N::A a;
 180   f(a);     // 'f': undeclared identifier
 181 }
 182 </pre>
 183
 184   <h3>[template-friend] Templates as friends</h3>
 185
 186   <p>A Template cannot be declared a friend of a class.</p>
 187   <pre>
 188 template&lt;class T&gt;
 189 struct A {};
 190
 191 struct B
 192 {
 193   template&lt;class T&gt;
 194   friend struct A;     // "syntax error"
 195 };
 196 </pre>
 197
 198   <h3>[member-template-outofline] Out-of-line definitions of member
 199   templates</h3>
 200
 201   <p>Defining member templates outside their enclosing class does not
 202   work.</p>
 203   <pre>
 204 template&lt;class T&gt;
 205 struct A
 206 {
 207   template&lt;class U&gt;
 208   void f();
 209 };
 210
 211 template&lt;class T&gt;
 212 template&lt;class U&gt;   // "syntax error"
 213 void A&lt;T&gt;::f()      // "T: undeclared identifier"
 214 {
 215 }
 216 </pre>
 217
 218   <p><strong>Workaround:</strong> Define member templates in-line within
 219   their enclosing class.</p>
 220
 221   <h3>[partial-spec] Partial specialization</h3>
 222
 223   <p>Partial specialization of class templates does not work.</p>
 224   <pre>
 225 template&lt;class T&gt;
 226 struct A {};
 227
 228 template&lt;class T&gt;
 229 struct B {};
 230
 231 template&lt;class T&gt;
 232 struct A&lt;B&lt;T&gt; &gt; {};  // template class was already defined as a non-template
 233 </pre>
 234
 235   <p><strong>Workaround:</strong> In some situations where interface does not
 236   matter, class member templates can simulate partial specialization.</p>
 237
 238   <h3>[template-value] Dependent template value parameters</h3>
 239
 240   <p>Template value parameters whose type depends on a previous template
 241   parameter provoke an internal compiler error if the correct syntax (with
 242   "typename") is used.</p>
 243   <pre>
 244 template&lt;class T, typename T::result_type&gt; // C1001: INTERNAL COMPILER ERROR: msc1.cpp, line 1794
 245 struct B {};
 246  // (omit "typename" and it compiles)
 247
 248 </pre>
 249
 250   <p><strong>Workaround:</strong> Leave off the "typename" keyword. That
 251   makes the program non-conforming, though.</p>
 252
 253   <h3>[wchar_t] <code>wchar_t</code> is not built-in</h3>
 254
 255   <p>The type <code>wchar_t</code> is not a built-in type.</p>
 256   <pre>
 257 wchar_t x;  // "missing storage class or type identifier"
 258 </pre>
 259
 260   <p><strong>Workaround:</strong> When using Microsoft Visual C++, the header
 261   <a href="../boost/config.hpp">boost/config.hpp</a> includes
 262   <code>&lt;cstddef&gt;</code>, which defines <code>wchar_t</code> as a
 263   typedef for <code>unsigned short</code>. Note that this means that the
 264   compiler does not regard <code>wchar_t</code> and <code>unsigned
 265   short</code> as distinct types, as is required by the standard, and so
 266   ambiguities may emanate when overloading on <code>wchar_t</code>. The macro
 267   <code>BOOST_NO_INTRINSIC_WCHAR_T</code> is defined in this situation.</p>
 268
 269   <h3>[delete-const-pointer] Deleting <code>const X *</code> does not
 270   work</h3>
 271
 272   <p>Trying to delete a pointer to a cv-qualified type gives an error:</p>
 273   <pre>
 274 void f()
 275 {
 276   const int *p = new int(5);
 277   delete p;        // C2664: cannot convert from "const int *" to "void *"
 278 }
 279 </pre>
 280
 281   <p><strong>Workaround:</strong> Define the function</p>
 282   <pre>
 283 inline void operator delete(const void *p) throw()
 284 { operator delete(const_cast&lt;void*&gt;(p)); }
 285 </pre>
 286
 287   <p>and similar functions for the other cv-qualifier combinations, for
 288   operator delete[], and for the <code>std::nothrow</code> variants.</p>
 289
 290   <h2>Standard Library</h2>
 291
 292   <h3>[clib-namespace] C library names in global namespace instead of
 293   std</h3>
 294
 295   <p>Library names from the &lt;c...&gt; headers are in the global namespace
 296   instead of namespace std.</p>
 297
 298   <p><b>Workaround:</b>&nbsp; The header <a href=
 299   "../libs/config/config.htm">boost/config.hpp</a> will define
 300   BOOST_NO_STDC_NAMESPACE. It can be used as follows:</p>
 301   <pre>
 302 # ifdef BOOST_NO_STDC_NAMESPACE
 303     namespace std { using ::abs; using ::fabs; }
 304 # endif
 305 </pre>
 306
 307   <p>Because std::size_t and std::ptrdiff_t are so commonly used, the
 308   workaround for these is already provided in boost/config.hpp.</p>
 309   <hr>
 310
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 314
 315   <p>Revised
 316   <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->04 December, 2006<!--webbot bot="Timestamp" endspan i-checksum="38514" --></p>
 317
 318   <p><i>Copyright &copy; 2001-2002 <a href="../people/jens_maurer.htm">Jens
 319   Maurer</a></i></p>
 320
 321   <p><i>Distributed under the Boost Software License, Version 1.0. (See
 322   accompanying file <a href="../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or copy
 323   at <a href=
 324   "http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p>
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