BOOST_ERROR_INFO
#include <boost/exception/info.hpp>
namespace @@ -34,6 +37,9 @@ boost
See also:
+ + +
diff --git a/doc/adding_data_at_throw.html b/doc/adding_data_at_throw.html deleted file mode 100644 index 7e4af8a..0000000 --- a/doc/adding_data_at_throw.html +++ /dev/null @@ -1,73 +0,0 @@ - - - - -
-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.
-See also:
- --
-
-
-
-Copyright (c) 2006-2008 by Emil Dotchevski and Reverge Studios, Inc.
-Distributed under the Boost Software License, Version 1.0.
-
-Boost Exception
-Adding of Arbitrary Data to Active Exception Objects
-Sometimes the throw site does not have all the information that is needed at the catch site to make sense of what went wrong. Here is an example:
-#include <stdio.h>
-#include <string>
-
-class
-file_read_error
- {
- public:
-
- explicit
- file_read_error( std::string const & fn ):
- fn_(fn)
- {
- };
-
- std::string const &
- file_name() const
- {
- return fn_;
- }
-
- private:
-
- std::string fn_;
- };
-
-void
-file_read( FILE * f, void * buffer, size_t size )
- {
- if( size!=fread(buffer,1,size,f) )
- throw file_read_error("????");
- }
-We have defined an exception class file_read_error which can store a file name, so that when we catch a file_read_error object, we know which file the failure is related to. However, the file_read function does not have the file name at the time of the throw; all it has is a FILE handle.
-One possible solution is to not use FILE handles directly. We could have our own class file which stores both a FILE handle and a file name, and pass that to file_read(). However, this could be problematic if we communicate with 3rd party code that does not use our class file (probably because they have their own similar class.)
-A better solution is to make class file_read_error derive (possibly indirectly) from boost::exception, and free the file_read() function from the burden of storing the file name in exceptions it throws:
-#include <boost/exception.hpp> -#include <stdio.h> -#include <errno.h> - -typedef boost::error_info<struct tag_errno,int> errno_info; - -class file_read_error: public boost::exception { }; - -void -file_read( FILE * f, void * buffer, size_t size ) - { - if( size!=fread(buffer,1,size,f) ) - throw file_read_error() << errno_info(errno); - }-
If file_read() detects a failure, it throws an exception which contains the information that is available at the time, namely the errno. Other relevant information, such as the file name, can be added in a context higher up the call stack, where it is known naturally:
-#include <boost/exception.hpp> -#include <boost/shared_ptr.hpp> -#include <stdio.h> -#include <string> - -typedef boost::error_info<struct tag_file_name,std::string> file_name_info; - -boost::shared_ptr<FILE> file_open( char const * file_name, char const * mode ); -void file_read( FILE * f, void * buffer, size_t size ); - -void -parse_file( char const * file_name ) - { - boost::shared_ptr<FILE> f = file_open(file_name,"rb"); - assert(f); - try - { - char buf[1024]; - file_read( f.get(), buf, sizeof(buf) ); - } - catch( - boost::exception & e ) - { - e << file_name_info(file_name); - throw; - } - }-
The above function is (almost) exception-neutral -- if an exception is emitted by any function call within the try block, parse_file() does not need to do any real work, but it intercepts any boost::exception object, stores the file name, and re-throws using a throw-expression with no operand (15.1.6). The rationale for catching any boost::exception object is that the file name is relevant to any failure that occurs in parse_file(), even if the failure is unrelated to file I/O.
-As usual, the stored data can be retrieved using get_error_info().
-See also:
- --
-
-
-
-Copyright (c) 2006-2008 by Emil Dotchevski and Reverge Studios, Inc.
-Distributed under the Boost Software License, Version 1.0.
-