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// Copyright 2018-2022 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 program demonstrates the use of leaf::on_error to print the path an
// error takes as it bubbles up the call stack. The printing code only runs if:
// - An error occurs, and
// - A handler that takes e_error_log argument is present. Otherwise none of the
// error log machinery will be invoked by LEAF.
// This example is similar to error_trace, except the path the error takes is
// not captured, only printed.
#include <boost/leaf.hpp>
#include <iostream>
#include <cstdlib>
#define ENABLE_ERROR_LOG 1
namespace leaf = boost::leaf;
// The error log is activated only if an error handling scope provides a handler
// for e_error_log.
struct e_error_log
{
struct rec
{
char const * file;
int line;
friend std::ostream & operator<<( std::ostream & os, rec const & x )
{
return os << x.file << '(' << x.line << ')';
}
};
e_error_log()
{
std::cerr << "Error! Log:" << std::endl;
}
// Our e_error_log instance is stateless, used only as a target to
// operator<<.
template <class T>
friend std::ostream & operator<<( e_error_log const &, T const & x )
{
return std::cerr << x << std::endl;
}
};
// The ERROR_LOG macro is designed for use in functions that detect or forward
// errors up the call stack. If an error occurs, and if an error handling scope
// provides a handler for e_error_log, the supplied lambda is executed as the
// error bubbles up.
#define ERROR_LOG auto _log = leaf::on_error( []( e_error_log & log ) { log << e_error_log::rec{__FILE__, __LINE__}; } )
// Each function in the sequence below calls the previous function, and each
// function has failure_percent chance of failing. If a failure occurs, the
// ERROR_LOG macro will cause the path the error takes to be printed.
int const failure_percent = 25;
leaf::result<void> f1()
{
ERROR_LOG;
if( (std::rand()%100) > failure_percent )
return { };
else
return leaf::new_error();
}
leaf::result<void> f2()
{
ERROR_LOG;
if( (std::rand()%100) > failure_percent )
return f1();
else
return leaf::new_error();
}
leaf::result<void> f3()
{
ERROR_LOG;
if( (std::rand()%100) > failure_percent )
return f2();
else
return leaf::new_error();
}
leaf::result<void> f4()
{
ERROR_LOG;
if( (std::rand()%100) > failure_percent )
return f3();
else
return leaf::new_error();
}
leaf::result<void> f5()
{
ERROR_LOG;
if( (std::rand()%100) > failure_percent )
return f4();
else
return leaf::new_error();
}
int main()
{
for( int i=0; i!=10; ++i )
leaf::try_handle_all(
[&]() -> leaf::result<void>
{
std::cout << "Run # " << i << ": ";
BOOST_LEAF_CHECK(f5());
std::cout << "Success!" << std::endl;
return { };
},
#if ENABLE_ERROR_LOG // This single #if enables or disables the printing of the error log.
[]( e_error_log const & )
{
},
#endif
[]
{
std::cerr << "Error!" << std::endl;
} );
return 0;
}
////////////////////////////////////////
#ifdef BOOST_LEAF_NO_EXCEPTIONS
namespace boost
{
[[noreturn]] void throw_exception( std::exception const & e )
{
std::cerr << "Terminating due to a C++ exception under BOOST_LEAF_NO_EXCEPTIONS: " << e.what();
std::terminate();
}
struct source_location;
[[noreturn]] void throw_exception( std::exception const & e, boost::source_location const & )
{
throw_exception(e);
}
}
#endif
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