fiber/examples/adapt_method_calls.cpp

//          Copyright Nat Goodspeed 2015.
// 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)

#include <boost/fiber/all.hpp>
#include <memory>                   // std::shared_ptr
#include <thread>
#include <chrono>
#include <iostream>
#include <sstream>
#include <exception>
#include <cassert>

/*****************************************************************************
*   example async API
*****************************************************************************/
// introduce class-scope typedef
struct AsyncAPIBase
{
    // error callback accepts an int error code; 0 == success
    typedef int errorcode;
};

//[Response
// every async operation receives a subclass instance of this abstract base
// class through which to communicate its result
struct Response
{
    typedef std::shared_ptr<Response> ptr;

    // called if the operation succeeds
    virtual void success(const std::string& data) = 0;

    // called if the operation fails
    virtual void error(AsyncAPIBase::errorcode ec) = 0;
};
//]

// the actual async API
class AsyncAPI: public AsyncAPIBase
{
public:
    // constructor acquires some resource that can be read
    AsyncAPI(const std::string& data);

//[method_init_read
    // derive Response subclass, instantiate, pass Response::ptr
    void init_read(Response::ptr);
//]

    // ... other operations ...
    void inject_error(errorcode ec);

private:
    std::string data_;
    errorcode injected_;
};

/*****************************************************************************
*   fake AsyncAPI implementation... pay no attention to the little man behind
*   the curtain...
*****************************************************************************/
AsyncAPI::AsyncAPI(const std::string& data):
    data_(data),
    injected_(0)
{}

void AsyncAPI::inject_error(errorcode ec)
{
    injected_ = ec;
}

void AsyncAPI::init_read(Response::ptr response)
{
    // make a local copy of injected_
    errorcode injected(injected_);
    // reset it synchronously with caller
    injected_ = 0;
    // local copy of data_ so we can capture in lambda
    std::string data(data_);
    // Simulate an asynchronous I/O operation by launching a detached thread
    // that sleeps a bit before calling either completion method.
    std::thread([injected, response, data](){
        std::this_thread::sleep_for(std::chrono::milliseconds(100));
        if (! injected)
        {
            // no error, call success()
            response->success(data);
        }
        else
        {
            // injected error, call error()
            response->error(injected);
        }            
    }).detach();
}

/*****************************************************************************
*   adapters
*****************************************************************************/
// helper function
std::runtime_error make_exception(const std::string& desc, AsyncAPI::errorcode);

//[PromiseResponse
class PromiseResponse: public Response
{
public:
    // called if the operation succeeds
    virtual void success(const std::string& data)
    {
        promise_.set_value(data);
    }

    // called if the operation fails
    virtual void error(AsyncAPIBase::errorcode ec)
    {
        promise_.set_exception(std::make_exception_ptr(make_exception("read", ec)));
    }

    boost::fibers::future<std::string> get_future()
    {
        return promise_.get_future();
    }

private:
    boost::fibers::promise<std::string> promise_;
};
//]

//[method_read
std::string read(AsyncAPI& api)
{
    // Because init_read() requires a shared_ptr, we must allocate our
    // ResponsePromise on the heap, even though we know its lifespan.
    auto promisep(std::make_shared<PromiseResponse>());
    boost::fibers::future<std::string> future(promisep->get_future());
    // Both 'promisep' and 'future' will survive until our lambda has been
    // called.
    api.init_read(promisep);
    return future.get();
}
//]

/*****************************************************************************
*   helpers
*****************************************************************************/
std::runtime_error make_exception(const std::string& desc, AsyncAPI::errorcode ec)
{
    std::ostringstream buffer;
    buffer << "Error in AsyncAPI::" << desc << "(): " << ec;
    return std::runtime_error(buffer.str());
}

/*****************************************************************************
*   driving logic
*****************************************************************************/
int main(int argc, char *argv[])
{
    // prime AsyncAPI with some data
    AsyncAPI api("abcd");

    // successful read(): retrieve it
    std::string data(read(api));
    assert(data == "abcd");

    // read() with error
    std::string thrown;
    api.inject_error(1);
    try
    {
        data = read(api);
    }
    catch (const std::exception& e)
    {
        thrown = e.what();
    }
    assert(thrown == make_exception("read", 1).what());

    return EXIT_SUCCESS;
}