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How Catch Blocks are Evaluated (C++)

C++ enables you to throw exceptions of any type, although in general it is recommended to throw types that are derived from std::exception. A C++ exception can be caught by a catch handler that specifies the same type as the thrown exception, or by a handler that can catch any type of exception.

If the type of thrown exception is a class, which also has a base class (or classes), it can be caught by handlers that accept base classes of the exception's type, as well as references to bases of the exception's type. Note that when an exception is caught by a reference, it is bound to the actual thrown exception object; otherwise, it is a copy (much the same as an argument to a function).

When an exception is thrown, it may be caught by the following types of catch handlers:

  • A handler that can accept any type (using the ellipsis syntax).

  • A handler that accepts the same type as the exception object; because it is a copy, const and volatile modifiers are ignored.

  • A handler that accepts a reference to the same type as the exception object.

  • A handler that accepts a reference to a const or volatile form of the same type as the exception object.

  • A handler that accepts a base class of the same type as the exception object; since it is a copy, const and volatile modifiers are ignored. The catch handler for a base class must not precede the catch handler for the derived class.

  • A handler that accepts a reference to a base class of the same type as the exception object.

  • A handler that accepts a reference to a const or volatile form of a base class of the same type as the exception object.

  • A handler that accepts a pointer to which a thrown pointer object can be converted via standard pointer conversion rules.

The order in which catch handlers appear is significant, because handlers for a given try block are examined in order of their appearance. For example, it is an error to place the handler for a base class before the handler for a derived class. After a matching catch handler is found, subsequent handlers are not examined. As a result, an ellipsis catch handler must be the last handler for its try block. For example:

// ...
try
{
    // ...
}
catch( ... )
{
    // Handle exception here.
}
// Error: the next two handlers are never examined.
catch( const char * str )
{
    cout << "Caught exception: " << str << endl;
}
catch( CExcptClass E )
{
    // Handle CExcptClass exception here.
}

In this example, the ellipsis catch handler is the only handler that is examined.

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