Boost Exception

Adding of Arbitrary Data at the Point of the Throw

The following example demonstrates how errno can be stored in exception objects using Boost Exception:

#include <boost/exception.hpp>
#include <errno.h>
#include <iostream>

typedef boost::error_info<struct tag_errno,int> errno_info; //(1)

class my_error: public boost::exception, public std::exception { }; //(2)

void
f()
    {
    throw my_error() << errno_info(errno); //(3)
    }

First, we instantiate the error_info template using a unique identifier -- tag_errno, and the type of the info it identifies -- int. This provides compile-time type safety for the various values stored in exception objects.

Second, we define class my_error, which derives from boost::exception.

Finally, (3) illustrates how the typedef from (1) can be used with operator<<() to store values in exception objects at the point of the throw.

The stored errno value can be recovered at a later time like this:

// ...continued

void
g()
    {
    try
        {
        f();
        }
    catch(
    my_error & x )
        {
        if( boost::shared_ptr<int const> err=boost::get_error_info<errno_info>(x) )
            std::cerr << "Error code: " << *err;
        }
    }

The get_error_info() function template is instantiated with the typedef from (1), and is passed an exception object of any type that derives publicly from boost::exception. If the exception object contains the requested value, the returned shared_ptr will point to it; otherwise an empty shared_ptr is returned.