C++ provides an error detection/recovery mechanism called exception handling. Exception handling, while not always easy to program correctly is well suited to object oriented software development.
Exceptions in C++ are objects that are thrown in search of catchers. As each call frame is searched for a catcher, and the exception propagates up the call chain. Object destructors for non-dynamically allocated objects get called ensuring proper object cleanup. If an exception can find no catcher, the application exits with an error.
The linguistic construction for emitting, or throwing and exception is the throw statement. For example:
throw SomeObject;
The linguistic mechanism for specifying exception catchers is the try/catch block:
try {
…
}
catch (SomeType variable) {
…
}
…
catch (...) {
…
}
If an exception is thrown within the block body of the catch statement, the catch blocks are searched for a matching data type, and the textually first matching block is executed. If none match and a catch (...) is specified it is invoked.
You can also re-throw an exception after processing using an empty throw statement. This is used in the following example to destroy some dynamically allocated memory.
SomeType* pType;
try {
pType = new SomeType;
…
}
catch (...) {
delete pType;
throw;
}
Exception catching does allow polymorphism. For example, suppose we have a base class
MyBase and a derived class Derived.
try {
…
throw Derived;
…
}
catch (MyBase& reason) {
…
}
The catch block will execute, and if virtual methods are called, the Derived
implementation will be used.
So much for an introduction to C++ exception handling. Based on the presentation so far, however there are clear advantages to defining a hierarchy of exceptions that derive from a common base class that provides a common set of reporting facilities. The NSCL Exception class library in SpecTcl and nscldaq does this. This library named libException.so that can be found in the lib directory of the NSCL DAQ or SpecTcl installations. Linking with that library is a matter of e.g.:
g++ … -L/usr/opt/spectcl/3.2/lib -lException -Wl,"-rpath=/usr/opt/spectcl/3.2/lib"
Be sure to substitute the correct top level directory for /usr/opt/spectcl/3.2 in both places in the example above.
The base class of the library is called CException. All NSCL
frameworks and applications enclose essentially the entire program or the body of each thread
in a try/catch block that can catch and report the error information from a
CException prior to exiting. Your code can catch this or
derived class issues either for error reporting or error recovery purposes.