ceph/src/boost/libs/leaf/examples/print_file/print_file_eh.cpp

   1 // Copyright (c) 2018-2020 Emil Dotchevski and Reverge Studios, Inc.
   2
   3 // Distributed under the Boost Software License, Version 1.0. (See accompanying
   4 // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
   5
   6 // This is the program presented in https://boostorg.github.io/leaf/#introduction-eh.
   7
   8 // It reads a text file in a buffer and prints it to std::cout, using LEAF to handle errors.
   9 // This version uses exception handling. The version that does not use exception
  10 // handling is in print_file_result.cpp.
  11
  12 #include <boost/leaf/exception.hpp>
  13 #include <boost/leaf/handle_errors.hpp>
  14 #include <boost/leaf/pred.hpp>
  15 #include <boost/leaf/on_error.hpp>
  16 #include <boost/leaf/common.hpp>
  17 #include <iostream>
  18 #include <memory>
  19 #include <stdio.h>
  20
  21 namespace leaf = boost::leaf;
  22
  23
  24 // Exception type hierarchy.
  25 struct bad_command_line: std::exception { };
  26 struct input_error: std::exception { };
  27 struct open_error: input_error { };
  28 struct read_error: input_error { };
  29 struct size_error: input_error { };
  30 struct eof_error: input_error { };
  31 struct output_error: std::exception { };
  32
  33
  34 // We will handle all failures in our main function, but first, here are the declarations of the functions it calls, each
  35 // communicating failures by throwing exceptions
  36
  37 // Parse the command line, return the file name.
  38 char const * parse_command_line( int argc, char const * argv[] );
  39
  40 // Open a file for reading.
  41 std::shared_ptr<FILE> file_open( char const * file_name );
  42
  43 // Return the size of the file.
  44 int file_size( FILE & f );
  45
  46 // Read size bytes from f into buf.
  47 void file_read( FILE & f, void * buf, int size );
  48
  49
  50 // The main function, which handles all errors.
  51 int main( int argc, char const * argv[] )
  52 {
  53     return leaf::try_catch(
  54
  55         [&]
  56         {
  57             char const * file_name = parse_command_line(argc,argv);
  58
  59             auto load = leaf::on_error( leaf::e_file_name{file_name} );
  60
  61             std::shared_ptr<FILE> f = file_open(file_name);
  62
  63             int s = file_size(*f);
  64
  65             std::string buffer(1 + s, '\0');
  66             file_read(*f, &buffer[0], buffer.size()-1);
  67
  68             std::cout << buffer;
  69             std::cout.flush();
  70             if( std::cout.fail() )
  71                 throw leaf::exception(output_error{}, leaf::e_errno{errno});
  72
  73             return 0;
  74         },
  75
  76         // Each of the lambdas below is an error handler. LEAF will consider them, in order, and call the first one that matches
  77         // the available error objects.
  78
  79         // This handler will be called if the error includes:
  80         // - a caught exception of type open_error, and
  81         // - an object of type leaf::e_errno that has .value equal to ENOENT, and
  82         // - an object of type leaf::e_file_name.
  83         []( open_error &, leaf::match_value<leaf::e_errno, ENOENT>, leaf::e_file_name const & fn )
  84         {
  85             std::cerr << "File not found: " << fn.value << std::endl;
  86             return 1;
  87         },
  88
  89         // This handler will be called if the error includes:
  90         // - a caught exception of type open_error, and
  91         // - an object of type leaf::e_errno (regardless of its .value), and
  92         // - an object of type leaf::e_file_name.
  93         []( open_error &, leaf::e_errno const & errn, leaf::e_file_name const & fn )
  94         {
  95             std::cerr << "Failed to open " << fn.value << ", errno=" << errn << std::endl;
  96             return 2;
  97         },
  98
  99         // This handler will be called if the error includes:
 100         // - a caught exception of type input_error, and
 101         // - an optional object of type leaf::e_errno (regardless of its .value), and
 102         // - an object of type leaf::e_file_name.
 103         []( input_error &, leaf::e_errno const * errn, leaf::e_file_name const & fn )
 104         {
 105             std::cerr << "Failed to access " << fn.value;
 106             if( errn )
 107                 std::cerr << ", errno=" << *errn;
 108             std::cerr << std::endl;
 109             return 3;
 110         },
 111
 112         // This handler will be called if the error includes:
 113         // - a caught exception of type std::ostream::failure, and
 114         // - an object of type leaf::e_errno (regardless of its .value),
 115         []( output_error &, leaf::e_errno const & errn )
 116         {
 117             std::cerr << "Output error, errno=" << errn << std::endl;
 118             return 4;
 119         },
 120
 121         // This handler will be called if we've got a bad_command_line
 122         []( bad_command_line & )
 123         {
 124             std::cout << "Bad command line argument" << std::endl;
 125             return 5;
 126         },
 127
 128         // This last handler matches any error: it prints diagnostic information to help debug logic errors in the program, since it
 129         // failed to match  an appropriate error handler to the error condition it encountered. In this program this handler will
 130         // never be called.
 131         []( leaf::error_info const & unmatched )
 132         {
 133             std::cerr <<
 134                 "Unknown failure detected" << std::endl <<
 135                 "Cryptic diagnostic information follows" << std::endl <<
 136                 unmatched;
 137             return 6;
 138         } );
 139 }
 140
 141
 142 // Implementations of the functions called by main:
 143
 144
 145 // Parse the command line, return the file name.
 146 char const * parse_command_line( int argc, char const * argv[] )
 147 {
 148     if( argc==2 )
 149         return argv[1];
 150     else
 151         throw leaf::exception(bad_command_line{});
 152 }
 153
 154
 155 // Open a file for reading.
 156 std::shared_ptr<FILE> file_open( char const * file_name )
 157 {
 158     if( FILE * f = fopen(file_name, "rb") )
 159         return std::shared_ptr<FILE>(f, &fclose);
 160     else
 161         throw leaf::exception(open_error{}, leaf::e_errno{errno});
 162 }
 163
 164
 165 // Return the size of the file.
 166 int file_size( FILE & f )
 167 {
 168     auto load = leaf::on_error([] { return leaf::e_errno{errno}; });
 169
 170     if( fseek(&f, 0, SEEK_END) )
 171         throw leaf::exception(size_error{});
 172
 173     int s = ftell(&f);
 174     if( s==-1L )
 175         throw leaf::exception(size_error{});
 176
 177     if( fseek(&f,0,SEEK_SET) )
 178         throw leaf::exception(size_error{});
 179
 180     return s;
 181 }
 182
 183
 184 // Read size bytes from f into buf.
 185 void file_read( FILE & f, void * buf, int size )
 186 {
 187     int n = fread(buf, 1, size, &f);
 188
 189     if( ferror(&f) )
 190         throw leaf::exception(read_error{}, leaf::e_errno{errno});
 191
 192     if( n!=size )
 193         throw leaf::exception(eof_error{});
 194 }