The class gives us a virtual member function named what. We can overwrite it in determined classes to have an exception depiction. To learn more about programming and other related concepts, check out the courses on Great Learning Academy. Remember Me! Great Learning is an ed-tech company that offers impactful and industry-relevant programs in high-growth areas. Know More. Sign in. Log into your account. Forgot your password? Password recovery. Whenever an exception is thrown and caught, and control is returned outside of the function that threw the exception, stack unwinding takes place.
During stack unwinding, any automatic objects that were created within the scope of the block that was exited are safely destroyed via calls to their destructors. If a try block ends without an exception, all associated catch blocks are ignored. A return statement issued in this context returns from the function containing the catch block. Branching out of a try block or a handler is allowed. Branching into a catch block is not allowed, however, because that is equivalent to jumping past an initiation of the exception.
Nesting of exceptions, that is, throwing an exception while another remains unhandled, is allowed only in restricted circumstances. From the point when an exception is thrown to the point when the matching catch clause is entered, the exception is unhandled.
Functions that are called along the way, such as destructors of automatic objects being destroyed, may throw new exceptions, as long as the exception does not escape the function. If a function exits via an exception while another exception remains unhandled, the terminate function is called immediately.
Once an exception handler has been entered, the exception is considered handled, and exceptions may be thrown again. You can determine whether any exception has been thrown and is currently unhandled. See Section 5. A function declaration can include an exception specification , a list of exceptions that a function may throw, directly or indirectly. The two following declarations indicate to the caller that the function f1 generates only exceptions that can be caught by a handler of type X , and that the function f2 generates only exceptions that can be caught by handlers of type W , Y , or Z : void f1 int throw X ; void f2 int throw W,Y,Z ;.
This declaration guarantees that no exception is generated by the function f3. If a function exits through any exception that is not allowed by an exception specification, it results in a call to the predefined function unexpected. By default, unexpected calls terminate which by default exits the program. The check for unexpected exceptions is done at program execution time, not at compile time.
Even if it appears that a disallowed exception might be thrown, there is no error unless the disallowed exception is actually thrown at runtime. The compiler can, however, eliminate unnecessary checking in some simple cases.
For instance, no checking for f is generated in the following example. The absence of an exception specification allows any exception to be thrown. There are five runtime error messages associated with exceptions:. When errors are detected at runtime, the error message displays the type of the current exception and one of the five error messages.
By default, the predefined function terminate is called, which then calls abort. The compiler uses the information provided in the exception specification to optimize code production.
For example, table entries for functions that do not throw exceptions are suppressed, and runtime checking for exception specifications of functions is eliminated wherever possible.
Thus, declaring functions with correct exception specifications can lead to better code generation. For information about using these functions in a multithreaded environment, see Section 9. The terminate function is called in any of the following circumstances:. Such a function takes no parameters, returns no value, and must terminate the program or the current thread.
The unexpected function is called when a function attempts to exit through an exception not listed in its exception specification. The default version of unexpected calls terminate. A replacement version of unexpected might throw an exception permitted by the violated exception specification.
If it does so, exception handling continues as though the original function had really thrown the replacement exception. Such a function takes no parameters, returns no value, and must not return to its caller. An uncaught, or active, exception is an exception that has been thrown, but not yet accepted by a handler. This situation most commonly occurs when a destructor called during stack unwinding throws an exception. Another way to prevent such termination is to design your program so that destructors do not need to throw exceptions.
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