Error Handling

Table of Contents

• ErrorOr and Exceptions
• Error Codes
• Failed Preconditions

This page describes how error handling is done via ErrorOr.

ErrorOr and Exceptions

Whenever an asynchronous wamp::Session operation fails, it will emit a wamp::ErrorOr that contains a std::error_code instead of an actual result.

The wamp::ErrorOr::value method would normally be used to retrieve the underlying result of the asynchronous operation. If the ErrorOr instead contains an error, ErrorOr::value will throw a wamp::error::Failure exception. Since Failure derives from std::system_error, it contains a std::error_code that represents the cause of the error.

Example using error codes:

using namespace wamp;
AsioContext ioctx;
auto tcp = TcpConnector::create<Json>(ioctx, {"localhost", 8001});
 
boost::asio::spawn(ioctx, [&](boost::asio::yield_context yield)
{
    auto session = Session::create(ioctx, tcp);
    auto index = session->connect(yield);
    if (!index)
    {
        std::cerr << "Connect failed: " << index.error() << "\n";
        return;
    }
 
    auto info = session->join(Realm("somerealm"), yield);
    if (!info)
    {
        std::cerr << "Join failed: " << info.error() << "\n";
        return;
    }
 
    // etc.
});
 
ioctx.run();

Example using exceptions:

using namespace wamp;
AsioContext ioctx;
auto tcp = TcpConnector::create<Json>(ioctx, {"localhost", 8001});
 
boost::asio::spawn(ioctx, [&](boost::asio::yield_context yield)
{
    auto session = Session::create(ioctx, tcp);
    try
    {
        session->connect(yield).value(); // Note the .value()
        session->join(Realm("somerealm"), yield).value(); // Note the .value()
        // etc.
    }
    catch (const error::Failure& e)
    {
        // Print a message describing the error
        std::cerr << e.what() << "\n";
 
        // Obtain the std::error_code associated with the exception
        auto ec = e.code();
        if (ec == SessionErrc::noSuchRealm)
            std::cerr << "The realm doesn't exist\n";
        else
            std::cerr << "Unexpected error: " << ec << "\n";
    }
});
 
ioctx.run();

Error Codes

An error code can either belong to std::generic_category, or to one of the error categories defined by the library in <cppwamp/error.hpp>:

Error category	Values	Used for reporting
wamp::SessionCategory	wamp::SessionErrc	WAMP session errors
wamp::ProtocolCategory	wamp::ProtocolErrc	invalid WAMP messages
wamp::TransportCategory	wamp::TransportErrc	general transport layer errors
wamp::RawsockCategory	wamp::RawsockErrc	raw socket transport errors
std::generic_category	std::errc	OS-level socket errors

Failed Preconditions

Note that Session will always throw wamp::error::Logic exceptions whenever preconditions are not met. Preconditions for API functions are listed in this reference documentation.

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Next: Remote Procedure Calls