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| 4 | <!-- Copyright Aleksey Gurtovoy 2006. Distributed under the Boost --> | |
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| 10 | <title>THE BOOST MPL LIBRARY: Exercises</title> | |
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| 16 | </tr></table><div class="header-separator"></div> | |
| 17 | <div class="section" id="exercises"> | |
| 18 | <h1><a class="toc-backref" href="./tutorial-metafunctions.html#id60" name="exercises">Exercises</a></h1> | |
| 19 | <dl> | |
| 20 | <dt>1-0.</dt> | |
| 21 | <dd>Use <tt class="literal"><span class="pre">BOOST_STATIC_ASSERT</span></tt> to add error checking to the <tt class="literal"><span class="pre">binary</span></tt> | |
| 22 | template presented in section <a class="reference" href="./resources.html">1</a>.4.1 so | |
| 23 | that <tt class="literal"><span class="pre">binary<N>::value</span></tt> causes a compilation error if <tt class="literal"><span class="pre">N</span></tt> | |
| 24 | contains digits other than <tt class="literal"><span class="pre">0</span></tt> or <tt class="literal"><span class="pre">1</span></tt>.</dd> | |
| 25 | <dt>1-1.</dt> | |
| 26 | <dd>Turn <tt class="literal"><span class="pre">vector_c<int,1,2,3></span></tt> into a type sequence with elements | |
| 27 | (2,3,4) using <tt class="literal"><span class="pre">transform</span></tt>.</dd> | |
| 28 | <dt>1-2.</dt> | |
| 29 | <dd>Turn <tt class="literal"><span class="pre">vector_c<int,1,2,3></span></tt> into a type sequence with elements | |
| 30 | (1,4,9) using <tt class="literal"><span class="pre">transform</span></tt>.</dd> | |
| 31 | <dt>1-3.</dt> | |
| 32 | <dd>Turn <tt class="literal"><span class="pre">T</span></tt> into <tt class="literal"><span class="pre">T****</span></tt> by using <tt class="literal"><span class="pre">twice</span></tt> twice.</dd> | |
| 33 | <dt>1-4.</dt> | |
| 34 | <dd>Turn <tt class="literal"><span class="pre">T</span></tt> into <tt class="literal"><span class="pre">T****</span></tt> using <tt class="literal"><span class="pre">twice</span></tt> on itself.</dd> | |
| 35 | <dt>1-5.</dt> | |
| 36 | <dd><p class="first">There's still a problem with the dimensional analysis code in | |
| 37 | section 1.1. | |
| 38 | Hint: What happens when you do:</p> | |
| 39 | <pre class="literal-block"> | |
| 40 | f = f + m * a; | |
| 41 | </pre> | |
| 42 | <p>Repair this example using techniques shown in this | |
| 43 | chapter.</p> | |
| 44 | <!-- @ example.wrap('''void will_fail%s | |
| 45 | { quantity<float,force> f(m*a); | |
| 46 | ''' % ma_function_args, '}') | |
| 47 | stack[:0] = dimensional_analysis # stick support code in | |
| 48 | compile('all', expect_error = True) --> | |
| 49 | </dd> | |
| 50 | <dt>1-6.</dt> | |
| 51 | <dd>Build a lambda expression that has functionality equivalent to | |
| 52 | <tt class="literal"><span class="pre">twice</span></tt>. Hint: <tt class="literal"><span class="pre">mpl::apply</span></tt> is a metafunction!</dd> | |
| 53 | <dt>1-7*.</dt> | |
| 54 | <dd><p class="first">What do you think would be the semantics of the following | |
| 55 | constructs:</p> | |
| 56 | <pre class="literal-block"> | |
| 57 | typedef mpl::lambda<mpl::lambda<_1> >::type t1; | |
| 58 | typedef mpl::apply<_1,mpl::plus<_1,_2> >::type t2; | |
| 59 | typedef mpl::apply<_1,std::vector<int> >::type t3; | |
| 60 | typedef mpl::apply<_1,std::vector<_1> >::type t4; | |
| 61 | typedef mpl::apply<mpl::lambda<_1>,std::vector<int> >::type t5; | |
| 62 | typedef mpl::apply<mpl::lambda<_1>,std::vector<_1> >::type t6; | |
| 63 | typedef mpl::apply<mpl::lambda<_1>,mpl::plus<_1,_2> >::type t7; | |
| 64 | typedef mpl::apply<_1,mpl::lambda< mpl::plus<_1,_2> > >::type t8; | |
| 65 | </pre> | |
| 66 | <!-- @example.prepend('#include <vector>') | |
| 67 | compile() --> | |
| 68 | <p class="last">Show the steps used to | |
| 69 | arrive at your answers and write tests verifying your assumptions. | |
| 70 | Did the library behavior match your reasoning? If not, analyze the | |
| 71 | failed tests to discover the actual expression semantics. | |
| 72 | Explain why your assumptions were different, what | |
| 73 | behavior you find more coherent, and why.</p> | |
| 74 | </dd> | |
| 75 | <dt>1-8*.</dt> | |
| 76 | <dd>Our dimensional analysis framework dealt with dimensions, but it | |
| 77 | entirely ignored the issue of <em>units</em>. A length can be | |
| 78 | represented in inches, feet, or meters. A force can be | |
| 79 | represented in newtons or in kg m/sec<sup>2</sup>. Add the | |
| 80 | ability to specify units and test your code. Try to make your | |
| 81 | interface as syntactically friendly as possible for the user.</dd> | |
| 82 | </dl> | |
| 83 | <!-- Along with Alan Turing, Church was one of the founders of computer | |
| 84 | science. --> | |
| 85 | <!-- yes, this is easier to read, but the real beauty of ``apply`` is that | |
| 86 | it not only works on metafunction classes, but on metafunctions passed | |
| 87 | with placeholder arguments. --> | |
| 88 | <!-- Naturally, MPL defines a series of these numbered placeholder | |
| 89 | types, so that we can handle metafunctions with more arguments. --> | |
| 90 | <!-- : in order to pass a template in the ``BinaryOperation`` position, | |
| 91 | ``transform`` would have to be declared:: | |
| 92 | ||
| 93 | template < | |
| 94 | class Sequence1, class Sequence2 | |
| 95 | , template <class,class> class BinaryOperation | |
| 96 | > | |
| 97 | struct transform; // returning a sequence --> | |
| 98 | <!-- "Programming with types" --> | |
| 99 | <!-- Once again, a nested ``value`` member is optional for | |
| 100 | metafunctions, and required for integral type wrappers. --> | |
| 101 | </div> | |
| 102 | ||
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