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