1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Macro constraints</title> 5<link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../../index.html" title="Chapter 1. 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Now I will discuss what this means in terms of preprocessor 33 metaprogramming and input to macros in general. 34 </p> 35<h5> 36<a name="variadic_macro_data.vmd_specific.vmd_constraints.h0"></a> 37 <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 38 input</a> 39 </h5> 40<p> 41 When a programmer designs any kinds of callables in C++ ( functions, member 42 functions etc. ), he specifies what the types of input and the return value 43 are. The C++ compiler enforces this specification at compile time. Similarly 44 at run-time a callable may check that its input falls within certain documented 45 and defined boundaries and react accordingly if it does not. This is all 46 part of the constraints for any callable in C++ and should be documented 47 by any good programmer. 48 </p> 49<p> 50 The C++ preprocessor is much "dumber" than the C++ compiler and 51 even with the preprocessor metaprogramming constructs which Paul Mensonides 52 has created in Boost PP there is far less the preprocessor metaprogrammer 53 can do at preprocessing time to constrain argument input to a macro than 54 a programmer can do at compile-time and/or at run-time to constrain argument 55 input to a C++ callable. Nevertheless it is perfectly valid to document what 56 a macro expects as its argument input and, if a programmer does not follow 57 the constraint, the macro will fail to work properly. In the ideal case in 58 preprocessor metaprogramming the macro could tell whether or not the constraint 59 was met and could issue some sort of intelligible preprocessing error when 60 this occurred, but even within the reality of preprocessor metaprogramming 61 with Boost PP this is not always possible to do. Nevertheless if the user 62 of a macro does not follow the constraints for a macro parameter, as specified 63 in the documentation of a particular macro being invoked, any error which 64 occurs is the fault of that user. I realize that this may go against the 65 strongly held concept that programming errors must always be met with some 66 sort of compile-time or run-time occurrence which allows the programmer to 67 correct the error, rather than a silent failure which masks the error. Because 68 the preprocessor is "dumber" and cannot provide this occurrence 69 in all cases the error could unfortunately be masked, despite the fact that 70 the documentation specifies the correct input constraint(s). In the case 71 of the already discussed macro BOOST_VMD_IS_EMPTY, this masking of the error 72 could only occur with a preprocessor ( Visual C++ ) which is not C++ standard 73 conformant. 74 </p> 75<p> 76 The Boost PP library does have a way of generating a preprocessing error, 77 without generating preprocessor output, but once again this way does not 78 work with the non-conformant preprocessor of Visual C++. The means to do 79 so using Boost PP is through the BOOST_PP_ASSERT macro. As will be seen and 80 discussed later VMD has an equivalent macro which will work with Visual C++ 81 by producing incorrect C++ output rather than a preprocessing error, but 82 even this is not a complete solution since the incorrect C++ output produced 83 could be hidden. 84 </p> 85<p> 86 Even the effort to produce a preprocessing error, or incorrect output inducing 87 a compile-time error, does not solve the problem of constrained input for 88 preprocessor metaprogramming. Often it is impossible to determine if the 89 input meets the constraints which the preprocessor metaprogrammer places 90 on it and documents. Certain preprocessing tokens cannot be checked reliably 91 for particular values, or a range of values, without the checking mechanism 92 itself creating a preprocessing error or undefined behavior. 93 </p> 94<p> 95 This does not mean that one should give up attempting to check macro input 96 constraints. If it can be done I see the value of such checks and a number 97 of VMD macros, discussed later, are designed as preprocessing input constraint 98 checking macros. But the most important thing when dealing with macro input 99 constraints is that they should be carefully documented, and that the programmer 100 should know that if the constraints are not met either preprocessing errors 101 or incorrect macro results could be the results. 102 </p> 103<p> 104 The VMD library, in order to present more preprocessor programming functionality 105 and flexibility, allows that erroneous results could occur if certain input 106 constraints are not met, whether the erroneous results are preprocessing 107 errors or incorrect output from a VMD macro. At the same time the VMD does 108 everything that the preprocessor is capable of doing to check the input constraints, 109 and carefully documents for each macro in the library what the input for 110 each could be in order to avoid erroneous output. 111 </p> 112<p> 113 Documented macro input constraints are just as valid in the preprocessor 114 as compile-time/run-time constraints are valid in C++, even if the detection 115 of such constraints and/or the handling of constraints that are not met are 116 far more difficult, if not impossible, in the preprocessor than in the compile-time/run-time 117 processing of C++. 118 </p> 119<p> 120 The VMD library uses constraints for most of it macros and the documentation 121 for those macros mentions the constraints that apply in order to use the 122 macro. 123 </p> 124</div> 125<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 126<td align="left"></td> 127<td align="right"><div class="copyright-footer">Copyright © 2010-2017 Tropic Software 128 East Inc</div></td> 129</tr></table> 130<hr> 131<div class="spirit-nav"> 132<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> 133</div> 134</body> 135</html> 136
