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31 When discussing the BOOST_VMD_IS_EMPTY macro I mentioned constraining input
32 to the macro. Now I will discuss what this means in terms of preprocessor
33 metaprogramming and input to macros in general.
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
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.
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
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
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.
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.
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.
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
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
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