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arangodb/3rdParty/boost/1.62.0/libs/context/example/v2/parser.cpp

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C++

// Copyright Oliver Kowalke 2014.
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE_1_0.txt or copy at
// http://www.boost.org/LICENSE_1_0.txt)
#include <cstdlib>
#include <exception>
#include <functional>
#include <iostream>
#include <memory>
#include <sstream>
#include <boost/context/all.hpp>
namespace ctx = boost::context;
/*
* grammar:
* P ---> E '\0'
* E ---> T {('+'|'-') T}
* T ---> S {('*'|'/') S}
* S ---> digit | '(' E ')'
*/
class Parser{
char next;
std::istream& is;
std::function<void(char)> cb;
char pull(){
return std::char_traits<char>::to_char_type(is.get());
}
void scan(){
do{
next=pull();
}
while(isspace(next));
}
public:
Parser(std::istream& is_,std::function<void(char)> cb_) :
next(), is(is_), cb(cb_)
{}
void run() {
scan();
E();
}
private:
void E(){
T();
while (next=='+'||next=='-'){
cb(next);
scan();
T();
}
}
void T(){
S();
while (next=='*'||next=='/'){
cb(next);
scan();
S();
}
}
void S(){
if (isdigit(next)){
cb(next);
scan();
}
else if(next=='('){
cb(next);
scan();
E();
if (next==')'){
cb(next);
scan();
}else{
throw std::runtime_error("parsing failed");
}
}
else{
throw std::runtime_error("parsing failed");
}
}
};
int main() {
try {
std::istringstream is("1+1");
bool done=false;
std::exception_ptr except;
// execute parser in new execution context
boost::context::execution_context<char> source(
[&is,&done,&except](ctx::execution_context<char> sink,char){
// create parser with callback function
Parser p( is,
[&sink](char ch){
// resume main execution context
auto result = sink(ch);
sink = std::move(std::get<0>(result));
});
try {
// start recursive parsing
p.run();
} catch (...) {
// store other exceptions in exception-pointer
except = std::current_exception();
}
// set termination flag
done=true;
// resume main execution context
return sink;
});
// user-code pulls parsed data from parser
// invert control flow
auto result = source('\0');
source = std::move(std::get<0>(result));
char c = std::get<1>(result);
if ( except) {
std::rethrow_exception(except);
}
while( ! done) {
printf("Parsed: %c\n",c);
std::tie(source,c) = source('\0');
if (except) {
std::rethrow_exception(except);
}
}
std::cout << "main: done" << std::endl;
return EXIT_SUCCESS;
} catch ( std::exception const& e) {
std::cerr << "exception: " << e.what() << std::endl;
}
return EXIT_FAILURE;
}