76 lines
2.1 KiB
Plaintext
76 lines
2.1 KiB
Plaintext
[/==============================================================================
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Copyright (C) 2001-2015 Joel de Guzman
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Copyright (C) 2001-2011 Hartmut Kaiser
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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===============================================================================/]
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[section Sum - adding numbers]
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Here's a parser that sums a comma-separated list of numbers.
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Ok we've glossed over some details in our previous examples. First, our
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includes:
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#include <boost/spirit/home/x3.hpp>
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#include <iostream>
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#include <string>
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Then some using directives:
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namespace x3 = boost::spirit::x3;
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namespace ascii = boost::spirit::x3::ascii;
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using x3::double_;
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using ascii::space;
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using x3::_attr;
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Now the actual parser:
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template <typename Iterator>
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bool adder(Iterator first, Iterator last, double& n)
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{
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auto assign = [&](auto& ctx){ n = _attr(ctx); };
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auto add = [&](auto& ctx){ n += _attr(ctx); };
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bool r = x3::phrase_parse(first, last,
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// Begin grammar
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(
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double_[assign] >> *(',' >> double_[add])
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)
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,
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// End grammar
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space);
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if (first != last) // fail if we did not get a full match
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return false;
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return r;
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}
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The full cpp file for this example can be found here:
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[@../../../example/x3/sum.cpp sum.cpp]
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This is almost like our original numbers list example. We're incrementally
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building on top of our examples. This time though, like in the complex number
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example, we'll be adding the smarts. There's an accumulator (`double& n`) that
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adds the numbers parsed. On a successful parse, this number is the sum of all
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the parsed numbers.
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The first `double_` parser attaches this action:
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[&](auto& ctx){ n = _attr(ctx); }
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This assigns the parsed result (actually, the attribute of `double_`) to `n`.
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The second `double_` parser attaches this action:
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[&](auto& ctx){ n += _attr(ctx); }
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So, subsequent numbers add into `n`.
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That wasn't too bad, was it :-) ?
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[endsect]
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