devn00b
/
EQ2EMu


			
				
					
						
						
							123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135
							<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
<title>Macro constraints</title>
<link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css">
<meta name="generator" content="DocBook XSL Stylesheets V1.79.1">
<link rel="home" href="../../index.html" title="Chapter&#160;1.&#160;The Variadic Macro Data Library 1.9">
<link rel="up" href="../vmd_specific.html" title="Specific macros for working with data types">
<link rel="prev" href="../vmd_specific.html" title="Specific macros for working with data types">
<link rel="next" href="vmd_identifier.html" title="Identifiers">
</head>
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
<table cellpadding="2" width="100%"><tr>
<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td>
<td align="center"><a href="../../../../../../index.html">Home</a></td>
<td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td>
<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
<td align="center"><a href="../../../../../../more/index.htm">More</a></td>
</tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="../vmd_specific.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../vmd_specific.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="vmd_identifier.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="variadic_macro_data.vmd_specific.vmd_constraints"></a><a class="link" href="vmd_constraints.html" title="Macro constraints">Macro
      constraints</a>
</h3></div></div></div>
<p>
        When discussing the BOOST_VMD_IS_EMPTY macro I mentioned constraining input
        to the macro. Now I will discuss what this means in terms of preprocessor
        metaprogramming and input to macros in general.
      </p>
<h5>
<a name="variadic_macro_data.vmd_specific.vmd_constraints.h0"></a>
        <span class="phrase"><a name="variadic_macro_data.vmd_specific.vmd_constraints.constrained_input"></a></span><a class="link" href="vmd_constraints.html#variadic_macro_data.vmd_specific.vmd_constraints.constrained_input">Constrained
        input</a>
      </h5>
<p>
        When a programmer designs any kinds of callables in C++ ( functions, member
        functions etc. ), he specifies what the types of input and the return value
        are. The C++ compiler enforces this specification at compile time. Similarly
        at run-time a callable may check that its input falls within certain documented
        and defined boundaries and react accordingly if it does not. This is all
        part of the constraints for any callable in C++ and should be documented
        by any good programmer.
      </p>
<p>
        The C++ preprocessor is much "dumber" than the C++ compiler and
        even with the preprocessor metaprogramming constructs which Paul Mensonides
        has created in Boost PP there is far less the preprocessor metaprogrammer
        can do at preprocessing time to constrain argument input to a macro than
        a programmer can do at compile-time and/or at run-time to constrain argument
        input to a C++ callable. Nevertheless it is perfectly valid to document what
        a macro expects as its argument input and, if a programmer does not follow
        the constraint, the macro will fail to work properly. In the ideal case in
        preprocessor metaprogramming the macro could tell whether or not the constraint
        was met and could issue some sort of intelligible preprocessing error when
        this occurred, but even within the reality of preprocessor metaprogramming
        with Boost PP this is not always possible to do. Nevertheless if the user
        of a macro does not follow the constraints for a macro parameter, as specified
        in the documentation of a particular macro being invoked, any error which
        occurs is the fault of that user. I realize that this may go against the
        strongly held concept that programming errors must always be met with some
        sort of compile-time or run-time occurrence which allows the programmer to
        correct the error, rather than a silent failure which masks the error. Because
        the preprocessor is "dumber" and cannot provide this occurrence
        in all cases the error could unfortunately be masked, despite the fact that
        the documentation specifies the correct input constraint(s). In the case
        of the already discussed macro BOOST_VMD_IS_EMPTY, this masking of the error
        could only occur with a preprocessor ( Visual C++ ) which is not C++ standard
        conformant.
      </p>
<p>
        The Boost PP library does have a way of generating a preprocessing error,
        without generating preprocessor output, but once again this way does not
        work with the non-conformant preprocessor of Visual C++. The means to do
        so using Boost PP is through the BOOST_PP_ASSERT macro. As will be seen and
        discussed later VMD has an equivalent macro which will work with Visual C++
        by producing incorrect C++ output rather than a preprocessing error, but
        even this is not a complete solution since the incorrect C++ output produced
        could be hidden.
      </p>
<p>
        Even the effort to produce a preprocessing error, or incorrect output inducing
        a compile-time error, does not solve the problem of constrained input for
        preprocessor metaprogramming. Often it is impossible to determine if the
        input meets the constraints which the preprocessor metaprogrammer places
        on it and documents. Certain preprocessing tokens cannot be checked reliably
        for particular values, or a range of values, without the checking mechanism
        itself creating a preprocessing error or undefined behavior.
      </p>
<p>
        This does not mean that one should give up attempting to check macro input
        constraints. If it can be done I see the value of such checks and a number
        of VMD macros, discussed later, are designed as preprocessing input constraint
        checking macros. But the most important thing when dealing with macro input
        constraints is that they should be carefully documented, and that the programmer
        should know that if the constraints are not met either preprocessing errors
        or incorrect macro results could be the results.
      </p>
<p>
        The VMD library, in order to present more preprocessor programming functionality
        and flexibility, allows that erroneous results could occur if certain input
        constraints are not met, whether the erroneous results are preprocessing
        errors or incorrect output from a VMD macro. At the same time the VMD does
        everything that the preprocessor is capable of doing to check the input constraints,
        and carefully documents for each macro in the library what the input for
        each could be in order to avoid erroneous output.
      </p>
<p>
        Documented macro input constraints are just as valid in the preprocessor
        as compile-time/run-time constraints are valid in C++, even if the detection
        of such constraints and/or the handling of constraints that are not met are
        far more difficult, if not impossible, in the preprocessor than in the compile-time/run-time
        processing of C++.
      </p>
<p>
        The VMD library uses constraints for most of it macros and the documentation
        for those macros mentions the constraints that apply in order to use the
        macro.
      </p>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright &#169; 2010-2017 Tropic Software
      East Inc</div></td>
</tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="../vmd_specific.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../vmd_specific.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="vmd_identifier.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
</div>
</body>
</html>