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105 lines
3.0 KiB
C++
105 lines
3.0 KiB
C++
//Copyright (c) 2018 Emil Dotchevski
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//Copyright (c) 2018 Second Spectrum, Inc.
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//Distributed under the Boost Software License, Version 1.0. (See accompanying
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//file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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//This example demonstrates how to transport exceptions thrown by a low level function
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//through an intermediate context that is not exception-safe, to be handled in a high level
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//function which may or may not be exception-safe.
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//An real-world example for this use case is when a C API (which may not throw) is implemented
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//using a C++ library that throws exceptions. As demonstrated below, these exception objects are
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//intercepted and reported by leaf::result<>.
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#include <boost/leaf/all.hpp>
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#include <iostream>
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namespace leaf = boost::leaf;
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class error_base: public virtual std::exception { };
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class error_a: public virtual error_base { };
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class error_b: public virtual error_base { };
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class error_c: public virtual error_base { };
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//Lower-level library function which throws exceptions.
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int compute_answer_throws()
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{
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switch( rand()%4 )
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{
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default: return 42;
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case 1: throw error_a();
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case 2: throw error_b();
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case 3: throw error_c();
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}
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}
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//Calls compute_answer_throws, switches to result<int> for error handling.
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leaf::result<int> compute_answer() noexcept
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{
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//Convert exceptions of types error_a and error_b to be communicated by leaf::result.
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//Any other exception will be communicated as a std::exception_ptr.
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return leaf::exception_to_result<error_a, error_b>(
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[ ]
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{
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return compute_answer_throws();
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} );
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}
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//Print the answer if the call to compute_answer is successful.
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leaf::result<void> print_answer() noexcept
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{
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LEAF_AUTO( r, compute_answer());
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std::cout << "Answer: " << r << std::endl;
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return { };
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}
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int main()
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{
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//Exercise print_answer a few times and handle errors. Note that the exception objects that
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//compute_answer_throws throws are handled as regular LEAF error objects...
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for( int i=0; i!=42; ++i )
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{
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leaf::handle_all(
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[ ]() -> leaf::result<void>
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{
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LEAF_CHECK(print_answer());
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return { };
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},
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[ ]( error_a const & e )
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{
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std::cerr << "Error A!" << std::endl;
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},
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[ ]( error_b const & e )
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{
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std::cerr << "Error B!" << std::endl;
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},
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//...except for error_c errors, which (for demonstration) are captured as exceptions
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//into std::exception_ptr as "unknown" exception. Presumably this should not
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//happen, therefore at this point we treat this situation a logic error: we print
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//diagnostic information and bail out.
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[ ]( std::exception_ptr const * ep )
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{
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std::cerr << "Got unknown error!" << std::endl;
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//Why do we take std::exception_ptr as a pointer? Because handle_all requires
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//that the last handler matches any error -- taken as a pointer, if there isn't a
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//std::exception_ptr associated with the error, the handler will still be matched
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//(with 0 passed for ep). Had we taken it by value or by const &, the program
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//would not compile.
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if( ep )
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leaf::diagnostic_output(std::cerr,*ep);
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} );
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}
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return 0;
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}
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