# Boost Exception

### error_info

#include <boost/exception/info.hpp>

    namespace
    boost
        {
        template <class Tag,class T>
        class
        error_info
            {
            public:

            typedef T value_type;

            error_info( value_type const & v );
            value_type const & value() const;
            value_type & value();
            };
        }

#### Requirements:

T must have accessible copy constructor and must not be a reference (there is no requirement that T's copy constructor does not throw.)

#### Description:

This class template is used to associate a Tag type with a value type T. Objects of type error_info<Tag,T> can be passed to operator<< to be stored in objects of type boost::exception.

#### Usage:

The header <boost/exception/error_info.hpp> provides a declaration of the error_info template, which is sufficient for the purpose of typedefing an instance for specific Tag and T, for example:

    #include <boost/exception/error_info.hpp>

    struct tag_errno;
    typedef boost::error_info<tag_errno,int> errno_info;

Or, the shorter equivalent:

    #include <boost/exception/error_info.hpp>

    typedef boost::error_info<struct tag_errno,int> errno_info;

This errno_info typedef can be passed to operator<< (#include <boost/exception/info.hpp> first) to store an int named tag_errno in exceptions of types that derive from boost::exception:

    throw file_read_error() << errno_info(errno);

It can also be passed to get_error_info (#include <boost/exception/get_error_info.hpp> first) to retrieve the tag_errno int from a boost::exception:

    catch( boost::exception & x )
        {
        if( int const * e=boost::get_error_info<errno_info>(x) )
            ....
        }

For convenience and uniformity, Boost Exception defines the following commonly used error_info typedefs, ready for use with operator<<:

errinfo_api_function
errinfo_at_line
errinfo_errno
errinfo_file_handle
errinfo_file_name
errinfo_file_open_mode
errinfo_nested_exception
errinfo_type_info_name

See also: Boost Exception | boost/exception/error_info.hpp | boost/exception/exception.hpp | boost/exception/info.hpp | diagnostic_information | diagnostic_information_what | error_info::error_info | error_info::value | error_info::value_type | exception | exception/operator<< | exception_ptr | Frequently Asked Questions | get_error_info | Integrating Boost Exception in Existing Exception Class Hierarchies | Motivation | original_exception_type | tuple/operator<<