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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>
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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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