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

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

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>

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.


<h2>Preprocessor symbol</h2>

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

<tr>
<td>Microsoft Visual C++ 6.0 (up to SP4)</td>
<td>1200</td>
</tr>

</table>


<h2>Core Language</h2>

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

Chaining <code>using</code>-declarations does not work.
<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>

Trying to explicitly instantiate a function template leads to the
wrong function being called silently.

<pre>
#include &lt;stdio.h&gt;

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

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


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

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.


<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>

<strong>Workaround:</strong> Enclose the offending <code>for</code>
loops in another pair of curly braces.
<p>
Another possible workaround (brought to my attention by Vesa Karvonen)
is this:
<pre>
#ifndef for
#define for if (0) {} else for
#endif
</pre>

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


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

In-class member initialization, required to implement a
Standard-conforming <code>std::numeric_limits</code> template, does
not work.

<pre>
struct A
{
  static const int i = 5;      // &quot;invalid syntax for pure virtual method&quot;
};
</pre>

<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.


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

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.  

<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>

A Template cannot be declared a friend of a class.

<pre>
template&lt;class T&gt;
struct A {};

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


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

Defining member templates outside their enclosing class does not work.

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

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


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

Partial specialization of class templates does not work.

<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>

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


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

Template value parameters whose type depends on a previous template
parameter provoke an internal compiler error if the correct syntax
(with "typename") is used.

<pre>
template&lt;class T, typename T::result_type&gt; // C1001: INTERNAL COMPILER ERROR: msc1.cpp, line 1794
struct B {};
 // (omit &quot;typename&quot; and it compiles)

</pre>

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


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

The type <code>wchar_t</code> is not a built-in type.

<pre>
wchar_t x;  // &quot;missing storage class or type identifier&quot;
</pre>

<strong>Workaround:</strong> When using Microsoft Visual C++, the
header
<a href="../boost/config.hpp">boost/config.hpp</a>
includes <code>&lt;cstddef></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.


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

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

<strong>Workaround:</strong> Define the function
<pre>
inline void operator delete(const void *p) throw()
{ operator delete(const_cast&lt;void*>(p)); }
</pre>
and similar functions for the other cv-qualifier combinations, for
operator delete[], and for the <code>std::nothrow</code> variants.


<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><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:
<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>&nbsp;
<hr>

2001-05-04 <a href="../people/jens_maurer.htm">Jens Maurer</a>
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