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### Macro constraints

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.

##### Constrained input

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.

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.

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.

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.

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.

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.

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++.

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.