# Boost Exception

### diagnostic_information

#include <boost/exception/diagnostic_information.hpp>
#include <boost/exception_ptr.hpp>

    namespace
    boost
        {
        template <class E>
        std::string diagnostic_information( E const & e );

        std::string diagnostic_information( exception_ptr const & p );
        }

#### Returns:

A string value that contains varying amount of implementation-specific diagnostic information about the passed object:

- If E can be statically converted to boost::exception, the returned value contains the string representations of all error_info objects stored in the boost::exception through operator<<, along with other diagnostic information relevant to the exception. If e can be dynamically converted to std::exception, the returned value also contains the what() string.
- Otherwise, if E can be statically converted to std::exception:
  - if e can be dynamically converted to boost::exception, the returned value is the same as if E could be statically converted to boost::exception;
  - otherwise the returned value contains the what() string.
- Otherwise, the boost::diagnostic_information template is not available.

The string representation of each error_info object is deduced by a function call that is bound at the time the error_info<Tag,T> template is instantiated. The following overload resolutions are attempted in order:

1. Unqualified call to to_string(x), where x is of type error_info<Tag,T> (the return value is expected to be of type std::string.)
2. Unqualified call to to_string(x.value()) (the return value is expected to be of type std::string.)
3. Unqualified call to s << x.value(), where s is a std::ostringstream.

The first successfully bound function is used at the time diagnostic_information is called; if all 3 overload resolutions are unsuccessful, the system is unable to convert the error_info object to string, and an unspecified stub string value is used without issuing a compile error.

The exception_ptr overload of diagnostic_information is equivalent to:

    if( p )
        try
            {
            rethrow_exception(p);
            }
        catch(...)
            {
            return current_exception_diagnostic_information();
            }
    else return <unspecified-string-value>;

#### Notes:

- The format of the returned string is unspecified.
- The returned string is not user-friendly.
- The returned string may include additional platform-specific diagnostic information.

#### Example:

this is a possible output from the diagnostic_information function, as used in libs/exception/example/example_io.cpp:

    example_io.cpp(70): Throw in function class boost::shared_ptr<struct _iobuf> __cdecl my_fopen(const char *,const char *)
    Dynamic exception type: class boost::exception_detail::clone_impl<struct fopen_error>
    std::exception::what: example_io error
    [struct boost::errinfo_api_function_ *] = fopen
    [struct boost::errinfo_errno_ *] = 2, "No such file or directory"
    [struct boost::errinfo_file_name_ *] = tmp1.txt
    [struct boost::errinfo_file_open_mode_ *] = rb

See also: BOOST_THROW_EXCEPTION | Boost Exception | boost/exception/diagnostic_information.hpp | Configuration Macros | current_exception_diagnostic_information | Diagnostic Information | diagnostic_information_what | Frequently Asked Questions | Motivation | throw_exception