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// Copyright 2018-2024 Emil Dotchevski and Reverge Studios, Inc.
// 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)
// This is the program presented in
// https://boostorg.github.io/leaf/#introduction-eh.
// It reads a text file in a buffer and prints it to std::cout, using LEAF to
// handle errors. This version uses exception handling. The version that does
// not use exception handling is in print_file_leaf_result.cpp.
#include <boost/leaf.hpp>
#include <iostream>
#include <memory>
#include <stdio.h>
namespace leaf = boost::leaf;
// First, we need an enum to define our error codes:
enum error_code
{
bad_command_line = 1,
open_error,
read_error,
size_error,
eof_error,
output_error
};
// We will handle all failures in our main function, but first, here are the
// declarations of the functions it calls, each communicating failures by
// throwing exceptions
// Parse the command line, return the file name.
char const * parse_command_line( int argc, char const * argv[] );
// Open a file for reading.
std::shared_ptr<FILE> file_open( char const * file_name );
// Return the size of the file.
std::size_t file_size( FILE & f );
// Read size bytes from f into buf.
void file_read( FILE & f, void * buf, std::size_t size );
// The main function, which handles all errors.
int main( int argc, char const * argv[] )
{
return leaf::try_catch(
[&]
{
char const * file_name = parse_command_line(argc,argv);
auto load = leaf::on_error( leaf::e_file_name{file_name} );
std::shared_ptr<FILE> f = file_open(file_name);
std::size_t s = file_size(*f);
std::string buffer(1 + s, '\0');
file_read(*f, &buffer[0], buffer.size()-1);
std::cout << buffer;
std::cout.flush();
if( std::cout.fail() )
leaf::throw_exception(output_error, leaf::e_errno{errno});
return 0;
},
// Each of the lambdas below is an error handler. LEAF will consider
// them, in order, and call the first one that matches the available
// error objects.
// This handler will be called if the error includes:
// - an object of type error_code equal to open_error, and
// - an object of type leaf::e_errno that has .value equal to ENOENT,
// and
// - an object of type leaf::e_file_name.
[]( leaf::match<error_code, open_error>, leaf::match_value<leaf::e_errno, ENOENT>, leaf::e_file_name const & fn )
{
std::cerr << "File not found: " << fn.value << std::endl;
return 1;
},
// This handler will be called if the error includes:
// - an object of type error_code equal to open_error, and
// - an object of type leaf::e_errno (regardless of its .value), and
// - an object of type leaf::e_file_name.
[]( leaf::match<error_code, open_error>, leaf::e_errno const & errn, leaf::e_file_name const & fn )
{
std::cerr << "Failed to open " << fn.value << ", errno=" << errn << std::endl;
return 2;
},
// This handler will be called if the error includes:
// - an object of type error_code equal to any of size_error,
// read_error, eof_error, and
// - an optional object of type leaf::e_errno (regardless of its
// .value), and
// - an object of type leaf::e_file_name.
[]( leaf::match<error_code, size_error, read_error, eof_error>, leaf::e_errno const * errn, leaf::e_file_name const & fn )
{
std::cerr << "Failed to access " << fn.value;
if( errn )
std::cerr << ", errno=" << *errn;
std::cerr << std::endl;
return 3;
},
// This handler will be called if the error includes:
// - an object of type error_code equal to output_error, and
// - an object of type leaf::e_errno (regardless of its .value),
[]( leaf::match<error_code, output_error>, leaf::e_errno const & errn )
{
std::cerr << "Output error, errno=" << errn << std::endl;
return 4;
},
// This handler will be called if we've got a bad_command_line
[]( leaf::match<error_code, bad_command_line> )
{
std::cout << "Bad command line argument" << std::endl;
return 5;
},
// This last handler matches any error: it prints diagnostic information
// to help debug logic errors in the program, since it failed to match
// an appropriate error handler to the error condition it encountered.
// In this program this handler will never be called.
[]( leaf::error_info const & unmatched )
{
std::cerr <<
"Unknown failure detected" << std::endl <<
"Cryptic diagnostic information follows" << std::endl <<
unmatched;
return 6;
} );
}
// Implementations of the functions called by main:
// Parse the command line, return the file name.
char const * parse_command_line( int argc, char const * argv[] )
{
if( argc == 2 )
return argv[1];
else
leaf::throw_exception(bad_command_line);
}
// Open a file for reading.
std::shared_ptr<FILE> file_open( char const * file_name )
{
if( FILE * f = fopen(file_name, "rb") )
return std::shared_ptr<FILE>(f, &fclose);
else
leaf::throw_exception(open_error, leaf::e_errno{errno});
}
// Return the size of the file.
std::size_t file_size( FILE & f )
{
auto load = leaf::on_error([] { return leaf::e_errno{errno}; });
if( fseek(&f, 0, SEEK_END) )
leaf::throw_exception(size_error);
long s = ftell(&f);
if( s == -1L )
leaf::throw_exception(size_error);
if( fseek(&f,0,SEEK_SET) )
leaf::throw_exception(size_error);
return std::size_t(s);
}
// Read size bytes from f into buf.
void file_read( FILE & f, void * buf, std::size_t size )
{
std::size_t n = fread(buf, 1, size, &f);
if( ferror(&f) )
leaf::throw_exception(read_error, leaf::e_errno{errno});
if( n != size )
leaf::throw_exception(eof_error);
}
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