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Rearrangement of links and better explanation of variadic macro support.

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Edward Diener
2020-09-19 09:14:26 -04:00
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<meta content="text/html; charset=windows-1252" http-equiv="content-type">
<meta content="text/html; charset=windows-1252"
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<title>contents.html</title>
<link rel="stylesheet" type="text/css" href="styles.css">
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<body>
<h4><a href="AppendixA-AnIntroductiontoPreprocessorMetaprogramming.html"
target="_top">Introduction</a></h4>
<h4><a href="index.html" target="_top">Introduction</a></h4>
<h4><a
href="AppendixA-AnIntroductiontoPreprocessorMetaprogramming.html"
target="_top">Basic Intro</a></h4>
<a href="topics.html"> </a>
<h4><a href="topics.html">Topics</a></h4>
<a href="topics.html">
</a>
<h4><a href="topics.html"></a><a href="terms.html">Terminology</a></h4>
<a href="terms.html">
</a>
<h4><a href="terms.html"></a><a href="data.html">Data Types</a></h4>
<a href="data.html">
</a>
<h4><a href="data.html"></a><a href="ref.html">Reference</a></h4>
<a href="ref.html">
</a>
<h4><a href="ref.html"></a><a href="headers.html">Headers</a></h4>
<a href="headers.html">
</a>
<h4><a href="headers.html"></a><a href="examples.html">Examples</a></h4>
<a href="examples.html">
</a>
<h4><a href="examples.html"></a><a href="miscellanea.html">Miscellanea</a></h4>
<a href="topics.html"> </a>
<h4><a href="terms.html">Terminology</a></h4>
<a href="terms.html"> </a>
<h4><a href="data.html">Data Types</a></h4>
<a href="data.html"> </a>
<h4><a href="ref.html">Reference</a></h4>
<a href="ref.html"> </a>
<h4><a href="headers.html">Headers</a></h4>
<a href="headers.html"> </a>
<h4><a href="examples.html">Examples</a></h4>
<a href="examples.html"> </a>
<h4><a href="miscellanea.html">Miscellanea</a></h4>
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<!--
<EFBFBD> Copyright Housemarque Oy 2002<30> Copyright Paul Mensonides 2002

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both C++ and C compilation. It does not depend on any other Boost
libraries and therefore may be used as a standalone library.<br>
</div>
<div>The library assumes that the compiler supports variadic macros.
Since variadic macros are a C99 or C++11 feature the library
ostensibly requires at least compilation at those C or C++
standard levels. A number of compilers support variadic macros at
a lower level, so it still may be possible using the library at
the C++03 compilation level, as long as strict C++03 compliance is
not enforced.<br>
<div>The library requires that the compiler supports <a
href="topics/variadic_macros.html">variadic macros</a>. Since
variadic macros are a C99 or C++11 feature the library ostensibly
requires at least compilation at those C or C++ standard levels. A
number of compilers support variadic macros at a lower level, so
it still may be possible using the library at that compilation
level, as long as strict C++98/C++03 compliance is not enforced.<br>
</div>
<div> An excerpt from <i>C++ Template Metaprogramming: Concepts,
Tools, and Techniques from Boost and Beyond</i> by Dave Abrahams

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</head>
<body>
<h4>Variadic Macros</h4>
<div> Variadic macros are supported by nearly all compilers. This library assumes
the compiler supports variadic macros. Variadic macros are macros of the form: </div>
<div> Variadic macros are supported by nearly all compilers.
Variadic macros are macros of the form: </div>
<div class="code">
<pre>#define SOME_MACRO(ZeroOrMoreParameters,...) macro expansion possible specifying __VA_ARGS__</pre>
</div>
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</div>
<u> <span style="font-weight: bold;">Preprocessor Library Support<br>
</span></u>
<div>The library assumes support for variadic macros for all
compilers using the library.<br>
<div>The library requires support for variadic macros for all
compilers using the library. This usually means that compilation
at the C level is C99 or higher and that compilation at the C++
level is C++11 or higher. Many C++ compilers support variadic
macros at the C++98/C++03 level as long as strict compliance is
not turned on for those compilers at that level. In particular
both the gcc and clang compilers will support variadic macros at
the C++98/C++03 levels as long as strict ANSI compliance is not
turned on at that level. For those compilers this largely means
that the <code>-pedantic</code> or <code>-pedantic-errors</code>
option is not used at the C++98/C++03 level of compilation. Boost
C++ is deprecating compiling Boost libraries at the C++98/C++03
level, so if you must still use this library at that level be
aware of these aforementioned caveats.<br>
<br>
<a name="vmvcquirk"></a>Visual C++'s default preprocessor has a few
quirks related to variadic macros which require the end-user to code
slightly differently. When Visual C++'s default preprocessor is being
used BOOST_PP_VARIADICS_MSVC is 1, otherwise it is 0. In this way the
end-user can test for the presence of Visual C++'s default preprocessor
and code accordingly.<br>
<a name="vmvcquirk"></a>Visual C++'s default preprocessor has a
few quirks related to variadic macros which require the end-user
to code slightly differently. When Visual C++'s default
preprocessor is being used BOOST_PP_VARIADICS_MSVC is 1, otherwise
it is 0. In this way the end-user can test for the presence of
Visual C++'s default preprocessor and code accordingly.<br>
<br>
Support for working with variadic data is largely centered on
being able to convert variadic data to other library data types,
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of the ability of the variadic parameters to encompass a variable
number of arguments. The library has functionality which can know
the number of variadic arguments passed when invoking a variadic
macro. This allows the same variadic macro to work
with different numbers of parameters, therefore providing more
than one syntactical equivalent for the same macro name.<br>
macro. This allows the same variadic macro to work with different
numbers of parameters, therefore providing more than one
syntactical equivalent for the same macro name.<br>
<br>
The macros in the library which offer this enhanced functionality
are all centered on <i>tuple</i> manipulation. With variadic
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<div>An array as a preprocessor data type is a two-element tuple
where the first element is the array size and the second element
is a tuple which constitutes the array data. Because a tuple knows
its own size because of compiler support for variadic macros, there is
no reason to use the array preprocessor data type as opposed to
the tuple preprocessor data type; the tuple data type now has all
of the functionality which the array data type has and is
syntactically easier to use. The preprocessor array data type is
essentially obsolete for modern C++ compilers.</div>
its own size because of compiler support for variadic macros,
there is no reason to use the array preprocessor data type as
opposed to the tuple preprocessor data type; the tuple data type
now has all of the functionality which the array data type has and
is syntactically easier to use. The preprocessor array data type
is essentially obsolete for modern C++ compilers.</div>
<u style="font-weight: bold;">Using Variadic Data</u>
<div>Variadic data exists in the form of comma-separated
preprocessor tokens. This is the case whether the variadic data
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to work and may fail.<br>
</span><br>
This occurs because of a preprocessor weakness in a number of
compilers, currently most notably Visual C++'s default preprocessor.
Even passing variadic data as arguments to a non-variadic macro, when
it is not represented in the form of&nbsp; __VA_ARGS__, may fail with
certain compilers.<br>
compilers, currently most notably Visual C++'s default
preprocessor. Even passing variadic data as arguments to a
non-variadic macro, when it is not represented in the form
of&nbsp; __VA_ARGS__, may fail with certain compilers.<br>
<br>
What follows are very simple examples, showing how variadic data
can be passed to a non-variadic macro.<br>