274 lines
10 KiB
C++
274 lines
10 KiB
C++
// Copyright (c) 2001-2010 Hartmut Kaiser
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//
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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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// This example shows how to create a simple lexer recognizing a couple of
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// different tokens aimed at a simple language and how to use this lexer with
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// a grammar. It shows how to associate attributes to tokens and how to access the
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// token attributes from inside the grammar.
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//
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// Additionally, this example demonstrates, how to define a token set usable
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// as the skip parser during parsing, allowing to define several tokens to be
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// ignored.
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//
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// The main purpose of this example is to show how inheritance can be used to
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// overload parts of a base grammar and add token definitions to a base lexer.
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//
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// Further, it shows how you can use the 'omit' attribute type specifier
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// for token definitions to force the token to have no attribute (expose an
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// unused attribute).
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//
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// This example recognizes a very simple programming language having
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// assignment statements and if and while control structures. Look at the file
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// example5.input for an example.
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#include <boost/config/warning_disable.hpp>
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#include <boost/spirit/include/qi.hpp>
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#include <boost/spirit/include/lex_lexertl.hpp>
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#include <boost/spirit/include/phoenix_operator.hpp>
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#include <iostream>
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#include <fstream>
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#include <string>
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#include "example.hpp"
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using namespace boost::spirit;
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using boost::phoenix::val;
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///////////////////////////////////////////////////////////////////////////////
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// Token definition base, defines all tokens for the base grammar below
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///////////////////////////////////////////////////////////////////////////////
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template <typename Lexer>
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struct example5_base_tokens : lex::lexer<Lexer>
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{
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protected:
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// this lexer is supposed to be used as a base type only
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example5_base_tokens() {}
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public:
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void init_token_definitions()
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{
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// define the tokens to match
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identifier = "[a-zA-Z_][a-zA-Z0-9_]*";
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constant = "[0-9]+";
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if_ = "if";
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while_ = "while";
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// associate the tokens and the token set with the lexer
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this->self += lex::token_def<>('(') | ')' | '{' | '}' | '=' | ';' | constant;
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this->self += if_ | while_ | identifier;
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// define the whitespace to ignore (spaces, tabs, newlines and C-style
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// comments)
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this->self("WS")
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= lex::token_def<>("[ \\t\\n]+")
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| "\\/\\*[^*]*\\*+([^/*][^*]*\\*+)*\\/"
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;
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}
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// these tokens have no attribute
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lex::token_def<lex::omit> if_, while_;
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// The following two tokens have an associated attribute type, 'identifier'
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// carries a string (the identifier name) and 'constant' carries the
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// matched integer value.
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//
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// Note: any token attribute type explicitly specified in a token_def<>
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// declaration needs to be listed during token type definition as
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// well (see the typedef for the token_type below).
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//
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// The conversion of the matched input to an instance of this type occurs
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// once (on first access), which makes token attributes as efficient as
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// possible. Moreover, token instances are constructed once by the lexer
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// library. From this point on tokens are passed by reference only,
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// avoiding them being copied around.
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lex::token_def<std::string> identifier;
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lex::token_def<unsigned int> constant;
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};
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///////////////////////////////////////////////////////////////////////////////
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// Grammar definition base, defines a basic language
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///////////////////////////////////////////////////////////////////////////////
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template <typename Iterator, typename Lexer>
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struct example5_base_grammar
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: qi::grammar<Iterator, qi::in_state_skipper<Lexer> >
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{
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template <typename TokenDef>
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example5_base_grammar(TokenDef const& tok)
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: example5_base_grammar::base_type(program)
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{
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using boost::spirit::_val;
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program
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= +block
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;
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block
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= '{' >> *statement >> '}'
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;
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statement
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= assignment
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| if_stmt
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| while_stmt
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;
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assignment
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= (tok.identifier >> '=' >> expression >> ';')
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[
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std::cout << val("assignment statement to: ") << _1 << "\n"
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]
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;
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if_stmt
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= (tok.if_ >> '(' >> expression >> ')' >> block)
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[
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std::cout << val("if expression: ") << _1 << "\n"
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]
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;
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while_stmt
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= (tok.while_ >> '(' >> expression >> ')' >> block)
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[
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std::cout << val("while expression: ") << _1 << "\n"
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]
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;
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// since expression has a variant return type accommodating for
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// std::string and unsigned integer, both possible values may be
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// returned to the calling rule
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expression
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= tok.identifier [ _val = _1 ]
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| tok.constant [ _val = _1 ]
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;
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}
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typedef qi::in_state_skipper<Lexer> skipper_type;
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qi::rule<Iterator, skipper_type> program, block, statement;
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qi::rule<Iterator, skipper_type> assignment, if_stmt;
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qi::rule<Iterator, skipper_type> while_stmt;
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// the expression is the only rule having a return value
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typedef boost::variant<unsigned int, std::string> expression_type;
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qi::rule<Iterator, expression_type(), skipper_type> expression;
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};
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///////////////////////////////////////////////////////////////////////////////
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// Token definition for derived lexer, defines additional tokens
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///////////////////////////////////////////////////////////////////////////////
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template <typename Lexer>
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struct example5_tokens : example5_base_tokens<Lexer>
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{
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typedef example5_base_tokens<Lexer> base_type;
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example5_tokens()
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{
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// define the additional token to match
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else_ = "else";
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// associate the new token with the lexer, note we add 'else' before
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// anything else to add it to the token set before the identifier
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// token, otherwise "else" would be matched as an identifier
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this->self = else_;
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// now add the token definitions from the base class
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this->base_type::init_token_definitions();
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}
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// this token has no attribute
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lex::token_def<lex::omit> else_;
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};
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///////////////////////////////////////////////////////////////////////////////
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// Derived grammar definition, defines a language extension
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///////////////////////////////////////////////////////////////////////////////
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template <typename Iterator, typename Lexer>
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struct example5_grammar : example5_base_grammar<Iterator, Lexer>
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{
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template <typename TokenDef>
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example5_grammar(TokenDef const& tok)
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: example5_base_grammar<Iterator, Lexer>(tok)
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{
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// we alter the if_stmt only
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this->if_stmt
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= this->if_stmt.copy() >> -(tok.else_ >> this->block)
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;
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}
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};
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///////////////////////////////////////////////////////////////////////////////
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int main()
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{
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// iterator type used to expose the underlying input stream
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typedef std::string::iterator base_iterator_type;
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// This is the lexer token type to use. The second template parameter lists
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// all attribute types used for token_def's during token definition (see
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// example5_base_tokens<> above). Here we use the predefined lexertl token
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// type, but any compatible token type may be used instead.
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//
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// If you don't list any token attribute types in the following declaration
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// (or just use the default token type: lexertl_token<base_iterator_type>)
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// it will compile and work just fine, just a bit less efficient. This is
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// because the token attribute will be generated from the matched input
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// sequence every time it is requested. But as soon as you specify at
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// least one token attribute type you'll have to list all attribute types
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// used for token_def<> declarations in the token definition class above,
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// otherwise compilation errors will occur.
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typedef lex::lexertl::token<
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base_iterator_type, boost::mpl::vector<unsigned int, std::string>
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> token_type;
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// Here we use the lexertl based lexer engine.
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typedef lex::lexertl::lexer<token_type> lexer_type;
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// This is the token definition type (derived from the given lexer type).
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typedef example5_tokens<lexer_type> example5_tokens;
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// this is the iterator type exposed by the lexer
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typedef example5_tokens::iterator_type iterator_type;
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// this is the type of the grammar to parse
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typedef example5_grammar<iterator_type, example5_tokens::lexer_def> example5_grammar;
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// now we use the types defined above to create the lexer and grammar
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// object instances needed to invoke the parsing process
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example5_tokens tokens; // Our lexer
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example5_grammar calc(tokens); // Our parser
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std::string str (read_from_file("example5.input"));
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// At this point we generate the iterator pair used to expose the
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// tokenized input stream.
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std::string::iterator it = str.begin();
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iterator_type iter = tokens.begin(it, str.end());
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iterator_type end = tokens.end();
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// Parsing is done based on the token stream, not the character
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// stream read from the input.
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// Note how we use the lexer defined above as the skip parser. It must
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// be explicitly wrapped inside a state directive, switching the lexer
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// state for the duration of skipping whitespace.
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std::string ws("WS");
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bool r = qi::phrase_parse(iter, end, calc, qi::in_state(ws)[tokens.self]);
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if (r && iter == end)
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{
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std::cout << "-------------------------\n";
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std::cout << "Parsing succeeded\n";
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std::cout << "-------------------------\n";
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}
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else
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{
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std::cout << "-------------------------\n";
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std::cout << "Parsing failed\n";
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std::cout << "-------------------------\n";
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}
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std::cout << "Bye... :-) \n\n";
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return 0;
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}
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