Notes and Examples on Exceptions

base class

children of exception

• logic_error

  logic_errors arise when a program obtains a state which it should never obtain. Logic errors may arise when assumptions for specific functions are violated. They may also arise when a function obtains a result, which it should never obtain.

• runtime_error

  runtime_errors occur outside the program's control and are difficult to predict.

children of logic_error

• domain_error

  A domain error expresses that a term satisfies the basic type expected, but is unacceptable to the restricted domain expected by some operation. For example, a function which calculates cosec(x) = 1/ sin(x), could through a domain error, for x=0.

• invalid_argument

  An invalid argument error, is similar to a domain error. For example, a function which expects to receive a symetric matrix, but which is passed a non-symetric matrix, could throw an invalid argument error.

• length_error

  A length error occurs, when an object, or set of objects is too large to fit into a previously defined length. For example, if a string variable is too large to fit into a static character array, length error, would be an appropriate exception to throw.

• out_of_range

  An out of range error is thrown when an argument is "out of range". For example i=1000, x[i], where the array x is only 10 long, could reasonably throw an out of range error.

children of runtime_error

• range_error

  A range error indicates a result is out of range, for the return variable type. For example, a routine which uses 32 bit local variables, which tries to return a number larger than 0x10000, could throw a range error. Alternatively this function could throw an overflow error.

• underflow_error

  The result from an arithmetic calculation that is too small to be expressed properly. For example, in floating point, a negative exponent can be generated that is too large (too small a number) to be stored in its allotted space.

• overflow_error

  The result from an arithmetic calculation that exceeds the space designated to hold it.

Unrelated Exceptions

io_base::failure

This is an unusual type of exception. It is an inner class, of the class io_base. The idea is that normally, a failure only occurs when an object of type io_base, has created that failure. failure is also a child of std::exception. So the decleration of failure looks like:
namespace std { class io_base { class failure : public std::exception {

ParentChildException.cpp demonstrates how to use dynamic_cast to determine if two classes are related. The relationship between most of the standard exceptions is demonstrated. A template function, TestInheritance, uses the dynamic cast operator to try to convert a pointer of two different exception types to one another. An upcast, returns a valid pointer. A downcast, or a cast to an unrelated type, returns a null. This program also shows how to get access to protected members of a class:

class hack_ios_base : protected ios_base {
public:
___hack_ios_base() {}

___static ios_base::failure FAIL;
};

ios_base::failure hack_ios_base::FAIL( "failure");

NestedClass.cpp is a trivial program which calls a constructor for a nested class, outside of the containing class. This was used as the template to write the code for hack_ios_base.

exception.cpp illustrates recasting of exceptions, and the set_unexpected function.

exception_what.cpcp illustrates calls to what

StrangeThrow.cpp test out trying throw(...) and it doesn't work. It also demonstrates that it is possible to use throw() in a creator.

Botch.cpp shows why it is a design mistake to throw an exception from a destructor, the stack can't unwind, because it causes two throws, while the program is designed to handle only one

exception_constructor.cpp Shows how constructors are called for thrown objects

bad_typeid.cpp shows how calling typeid on a null pointer causes an exception