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It isn't uncommon to need token-wise arithmetic operations forpurposes other than invoking Preprocessor Library repetitionmacros. For example, let's write a metafunction to generate

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function types from "signature" type sequences that specify thefunction's return and parameter types:: template <unsigned Size, class Signature>

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struct to_function_impl; template <class Signature> struct to_function

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: to_function_impl<mpl::size<Signature>::type, Signature> {};The challenge now is to implement ``to_function_impl``. For

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``Size == 3``, an appropriate specialization might look like this:: template <class Signature> struct to_function_impl<3,Signature>

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{ typedef mpl::begin<Signature>::type i0; typedef mpl::deref<i0>::type t0;

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typedef mpl::next<i0>::type i1; typedef mpl::deref<i1>::type t1; typedef mpl::next<i1>::type i2;

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typedef mpl::deref<i2>::type t2; typedef t0 type(t1,t2); };

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A local macro to generate a single ``to_function_impl``specialization would look something like this:

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.. parsed-literal:: #define to_function_impl_spec(size) \\ template <class Signature> \\

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struct to_function_impl<3,Signature> \\ { \\ typedef mpl::begin<Signature>::type i0; \\ typedef mpl::deref<i0>::type t0; \\

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\\ BOOST_PP_REPEAT_FROM_TO(1, size, to_function_t, ~) \\ \\ typedef t0 type(BOOST_PP_ENUM_SHIFTED_PARAMS(size,t)); \\

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}; #define to_function_t(z, n, unused) \\ typedef mpl::next<BOOST_PP_CAT(i,\ **BOOST_PP_DEC(n)**)>::type \\

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BOOST_PP_CAT(i,n); \\ \\ typedef mpl::deref<BOOST_PP_CAT(i,n)>::type BOOST_PP_CAT(t,n);

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We've used some new library macros above; here is a brief rundown:* ``BOOST_PP_REPEAT_FROM_TO`` is just like ``BOOST_PP_REPEAT``, except that it accepts an initial repetition index. Since every

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function has a return type, we don't need to worry about the case where ``Size == 0``.* ``BOOST_PP_ENUM_SHIFTED_PARAMS`` is just like

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``BOOST_PP_ENUM_PARAMS``, except that repetition indices start at ``1`` instead of ``0``.* ``BOOST_PP_CAT`` implements token pasting; its two arguments are

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"glued" together into a single token. Since this is a general-purpose macro, it sits in ``cat.hpp`` at the top level of the library's directory tree. [#paste]_

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.. [#paste] The preprocessor's built-in token-pasting operator, ``##``, often yields surprising results by taking effect before its arguments have been expanded. By contrast, ``BOOST_PP_CAT`` delays concatenation until after its arguments have been fully

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evaluated.* Finally, though it only performs trivial arithmetic, ``BOOST_PP_DEC`` plays a crucial role in generating an

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appropriate prior iterator identifier for our own code in ``to_function_t``.If we didn't have ``BOOST_PP_REPEAT_FROM_TO`` at our disposal in

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the previous example, we might've had to use ``BOOST_PP_REPEAT``,which always starts iterating at ``0``. Consequently``to_function_t`` would've been responsible for producing thedeclarations of ``i0`` and ``t0`` as well as those of the other

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nested types. To manage that, it would need a way to selectdifferent expansions depending on the value of ``n``.In its ``control/`` directory, the Preprocessor Library supplies a

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macro ``BOOST_PP_IF(c,t,f)`` that fulfills a similar role to theone filled by ``mpl::if_``. Rewriting the example accordingly, weget:

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.. parsed-literal:: #define to_function_impl_spec(size) \\ template <class Signature> \\

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struct to_function_impl<3,Signature> \\ { \\ BOOST_PP_REPEAT_FROM_TO(1, size, to_function_t, ~) \\ \\

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typedef t0 type(BOOST_PP_ENUM_SHIFTED_PARAMS(size,t)); \\ }; #define to_function_t(z, n, unused) \\

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typedef BOOST_PP_IF( \\ n, \\ mpl::next<BOOST_PP_CAT(i,BOOST_PP_DEC(n))>::type, \\ typedef mpl::begin<Signature>::type i0; \\

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) \\ BOOST_PP_CAT(i,n); \\ \\ typedef mpl::deref<BOOST_PP_CAT(i,n)>::type BOOST_PP_CAT(t,n);

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Although the formulation above will work, it does unnecessary workwhen ``n == 0``, evaluating the "true" branch of the conditionalonly to discard it. -->

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