66 lines
1.5 KiB
Plaintext
66 lines
1.5 KiB
Plaintext
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[#if_]
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[section if_]
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[h1 Synopsis]
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template <class P, class T, class F>
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struct if_;
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This is a [link parser_combinator parser combinator].
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[table Arguments
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[[Name] [Type]]
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[[`P`] [[link parser parser]]]
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[[`T`] [[link metaprogramming_value template metaprogramming value]]]
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[[`F`] [[link metaprogramming_value template metaprogramming value]]]
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]
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[h1 Description]
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`if_` always accepts the input string. The result of parsing is `T`, when `P`
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accepts the input and `F` otherwise.
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[h1 Header]
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#include <boost/metaparse/if_.hpp>
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[h1 Expression semantics]
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For any `p` parser, `t` and `f` classes the following are equivalent:
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if_<p, t, f>
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one_of<last_of<p, return_<t>>, return_<f>>
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[h1 Example]
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#include <boost/metaparse/if_.hpp>
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#include <boost/metaparse/int_.hpp>
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#include <boost/metaparse/string.hpp>
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#include <boost/metaparse/start.hpp>
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#include <boost/metaparse/get_result.hpp>
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#include <type_traits>
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using namespace boost::metaparse;
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using int11 = std::integral_constant<int, 11>;
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using int13 = std::integral_constant<int, 13>;
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static_assert(
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get_result<
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if_<int_, int11, int13>::apply<BOOST_METAPARSE_STRING("1234"), start>
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>::type::value == 11,
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"When the int_ parser succeeds, the result of parsing should be 11"
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);
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static_assert(
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get_result<
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if_<int_, int11, int13>::apply<BOOST_METAPARSE_STRING("foo"), start>
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>::type::value == 13,
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"When the int_ parser fails, the result of parsing should be 13"
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);
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[endsect]
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