ceph/src/boost/libs/context/example/v1/parser.cpp

   1
   2 //          Copyright Oliver Kowalke 2014.
   3 // Distributed under the Boost Software License, Version 1.0.
   4 //    (See accompanying file LICENSE_1_0.txt or copy at
   5 //          http://www.boost.org/LICENSE_1_0.txt)
   6
   7 #include <cstdlib>
   8 #include <exception>
   9 #include <functional>
  10 #include <iostream>
  11 #include <memory>
  12 #include <sstream>
  13
  14 #include <boost/context/all.hpp>
  15
  16 /*
  17  * grammar:
  18  *   P ---> E '\0'
  19  *   E ---> T {('+'|'-') T}
  20  *   T ---> S {('*'|'/') S}
  21  *   S ---> digit | '(' E ')'
  22  */
  23 class Parser{
  24    char next;
  25    std::istream& is;
  26    std::function<void(char)> cb;
  27
  28    char pull(){
  29         return std::char_traits<char>::to_char_type(is.get());
  30    }
  31
  32    void scan(){
  33        do{
  34            next=pull();
  35        }
  36        while(isspace(next));
  37    }
  38
  39 public:
  40    Parser(std::istream& is_,std::function<void(char)> cb_) :
  41       next(), is(is_), cb(cb_)
  42     {}
  43
  44    void run() {
  45       scan();
  46       E();
  47    }
  48
  49 private:
  50    void E(){
  51       T();
  52       while (next=='+'||next=='-'){
  53          cb(next);
  54          scan();
  55          T();
  56       }
  57    }
  58
  59    void T(){
  60       S();
  61       while (next=='*'||next=='/'){
  62          cb(next);
  63          scan();
  64          S();
  65       }
  66    }
  67
  68    void S(){
  69       if (isdigit(next)){
  70          cb(next);
  71          scan();
  72       }
  73       else if(next=='('){
  74          cb(next);
  75          scan();
  76          E();
  77          if (next==')'){
  78              cb(next);
  79              scan();
  80          }else{
  81              throw std::runtime_error("parsing failed");
  82          }
  83       }
  84       else{
  85          throw std::runtime_error("parsing failed");
  86       }
  87    }
  88 };
  89
  90 int main() {
  91     try {
  92         std::istringstream is("1+1");
  93         bool done=false;
  94         std::exception_ptr except;
  95
  96         // create handle to main execution context
  97         auto sink(boost::context::execution_context::current());
  98         // execute parser in new execution context
  99         boost::context::execution_context source(
 100                 [&sink,&is,&done,&except](void*){
 101                 // create parser with callback function
 102                 Parser p(is,
 103                          [&sink](char ch){
 104                                 // resume main execution context
 105                                 sink(&ch);
 106                         });
 107                     try {
 108                         // start recursive parsing
 109                         p.run();
 110                     } catch (...) {
 111                         // store other exceptions in exception-pointer
 112                         except = std::current_exception();
 113                     }
 114                     // set termination flag
 115                     done=true;
 116                     // resume main execution context
 117                     sink();
 118                 });
 119
 120         // user-code pulls parsed data from parser
 121         // invert control flow
 122         void* vp = source();
 123         if (except) {
 124             std::rethrow_exception(except);
 125         }
 126         while( ! done) {
 127             printf("Parsed: %c\n",* static_cast<char*>(vp));
 128             vp = source();
 129             if (except) {
 130                 std::rethrow_exception(except);
 131             }
 132         }
 133         std::cout << "main: done" << std::endl;
 134         return EXIT_SUCCESS;
 135     } catch ( std::exception const& e) {
 136         std::cerr << "exception: " << e.what() << std::endl;
 137     }
 138     return EXIT_FAILURE;
 139 }