# Boost Exception

## Frequently Asked Questions

### What is the cost of calling boost::throw_exception?

The cost is that boost::exception is added as a base of the exception emitted by boost::throw_exception (unless the passed type already derives from boost::exception.)

Calling boost::throw_exception does not cause dynamic memory allocations.

### What is the cost of BOOST_THROW_EXCEPTION?

In addition to calling boost::throw_exception, BOOST_THROW_EXCEPTION invokes __FILE__, __LINE__ and the BOOST_THROW_EXCEPTION_CURRENT_FUNCTION macros. The space required to store the information is already included in sizeof(boost::exception).

Calling BOOST_THROW_EXCEPTION does not cause dynamic memory allocations.

### Should I use boost::throw_exception or BOOST_THROW_EXCEPTION or just throw?

The benefit of calling boost::throw_exception instead of using throw directly is that it ensures that the emitted exception derives from boost::exception and that it is compatible with boost::current_exception.

The BOOST_THROW_EXCEPTION macro also results in a call to boost::throw_exception, but in addition it records in the exception object the __FILE__ and __LINE__ of the throw, as well as the pretty name of the function that throws. This enables boost::diagnostic_information to compose a more useful, if not user-friendly message.

Typical use of boost::diagnostic_information is:

    catch(...)
        {
        std::cerr <<
            "Unexpected exception, diagnostic information follows:\n" <<
            current_exception_diagnostic_information();
        }

This is a possible message it may display -- the information in the first line is only available if BOOST_THROW_EXCEPTION was used to throw:

    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<class 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

In some development environments, the first line in that message can be clicked to show the location of the throw in the debugger, so it's easy to set a break point and run again to see the unexpected throw in the context of its call stack.

### Why doesn't boost::exception derive from std::exception?

Despite that virtual inheritance should be used in deriving from base exception types, quite often exception types (including the ones defined in the standard library) don't derive from std::exception virtually.

If boost::exception derives from std::exception, using the enable_error_info function with such user-defined types would introduce dangerous ambiguity which would break all catch(std::exception &) statements.

Of course, boost::exception should not be used to replace std::exception as a base type in exception type hierarchies. Instead, it should be included as a virtual base, in addition to std::exception (which should probably also be derived virtually.)

### Why is boost::exception abstract?

To prevent exception-neutral contexts from erroneously erasing the type of the original exception when adding error_info to an active exception object:

    catch( boost::exception & e )
        {
        e << foo_info(foo);
        throw e; //Compile error: boost::exception is abstract
        }

The correct code is:

    catch( boost::exception & e )
        {
        e << foo_info(foo);
        throw; //Okay, re-throwing the original exception object.
        }

### Why use operator<< overload for adding info to exceptions?

Before throwing an object of type that derives from boost::exception, it is often desirable to add one or more error_info objects in it. The syntactic sugar provided by operator<< allows this to be done directly in a throw expression:

    throw error() << foo_info(foo) << bar_info(bar);

### Why is operator<< allowed to throw?

This question is referring to the following issue. Consider this throw statement example:

    throw file_open_error() << file_name(fn);

The intention here is to throw a file_open_error, however if operator<< fails to copy the std::string contained in the file_name error_info wrapper, a std::bad_alloc could propagate instead. This behavior seems undesirable to some programmers.

Bjarne Stroustrup, The C++ Programming Language, 3rd Edition, page 371:

> "Throwing an exception requires an object to throw. A C++ implementation is required to have enough spare memory to be able to throw bad_alloc in case of memory exhaustion. However, it is possible that throwing some other exception will cause memory exhaustion."

Therefore, the language itself does not guarantee that an attempt to throw an exception is guaranteed to throw an object of the specified type; propagating a std::bad_alloc seems to be a possibility even outside of the scope of Boost Exception.

See also: Boost Exception | Motivation