//Copyright (c) 2018 Emil Dotchevski
//Copyright (c) 2018 Second Spectrum, 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 example demonstrates how to transport exceptions thrown by a low level function
//through an intermediate context that is not exception-safe, to be handled in a high level
//function which may or may not be exception-safe.

//An real-world example for this use case is when a C API (which may not throw) is implemented
//using a C++ library that throws exceptions. As demonstrated below, these exception objects are
//intercepted and reported by leaf::result<>.

#include <boost/leaf/all.hpp>
#include <iostream>

namespace leaf = boost::leaf;


class error_base: public virtual std::exception { };
class error_a: public virtual error_base { };
class error_b: public virtual error_base { };
class error_c: public virtual error_base { };


//Lower-level library function which throws exceptions.
int compute_answer_throws()
{
	switch( rand()%4 )
	{
		default: return 42;
		case 1: throw error_a();
		case 2: throw error_b();
		case 3: throw error_c();
	}
}


//Calls compute_answer_throws, switches to result<int> for error handling.
leaf::result<int> compute_answer() noexcept
{
	//Convert exceptions of types error_a and error_b to be communicated by leaf::result.
	//Any other exception will be communicated as a std::exception_ptr.
	return leaf::exception_to_result<error_a, error_b>(
		[ ]
		{
			return compute_answer_throws();
		} );
}


//Print the answer if the call to compute_answer is successful.
leaf::result<void> print_answer() noexcept
{
	LEAF_AUTO( r, compute_answer());
	std::cout << "Answer: " << r << std::endl;
	return { };
}


int main()
{
	//Exercise print_answer a few times and handle errors. Note that the exception objects that
	//compute_answer_throws throws are handled as regular LEAF error objects...
	for( int i=0; i!=42; ++i )
	{
		leaf::handle_all(
			[ ]() -> leaf::result<void>
			{
				LEAF_CHECK(print_answer());
				return { };
			},

			[ ]( error_a const & e )
			{
				std::cerr << "Error A!" << std::endl;
			},

			[ ]( error_b const & e )
			{
				std::cerr << "Error B!" << std::endl;
			},

			//...except for error_c errors, which (for demonstration) are captured as exceptions
			//into std::exception_ptr as "unknown" exception. Presumably this should not
			//happen, therefore at this point we treat this situation a logic error: we print
			//diagnostic information and bail out.
			[ ]( std::exception_ptr const * ep )
			{
				std::cerr << "Got unknown error!" << std::endl;

				//Why do we take std::exception_ptr as a pointer? Because handle_all requires
				//that the last handler matches any error -- taken as a pointer, if there isn't a
				//std::exception_ptr associated with the error, the handler will still be matched
				//(with 0 passed for ep). Had we taken it by value or by const &, the program
				//would not compile.
				if( ep )
					leaf::diagnostic_output(std::cerr,*ep);
			} );
	}

	return 0;
}