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E.15: using reference forms to catch exceptions in the inheritance system

Reason (reason)

To prevent slicing.

To avoid truncation.

Example (sample)

Void f ()

{

Try {

/ /...

}

Catch (exception e) {/ / don't: may slice

/ /...

}

}

Instead, use a reference:

Use references instead of:

Catch (exception& e) {/ *... * /}

Or-typically better still-a const reference:

Or-- generally better-- use constant references:

Catch (const exception& e) {/ *... * /}

Most handlers do not modify their exception and in general we recommend use of const.

Most handlers do not change the contents of the exception, so we usually recommend using the constant form.

Note (Note)

Use throw; to rethrow the caught exception; not throw e;. Using throw e; throws a new copy of e (the truncated result of the static type std::exception) instead of rethrowing the original exception of type std::runtime_error. (but stick to it: don't try to catch all exceptions in every function and don't forget to use explicit try/catch as little as possible. )

Enforcement (implementation recommendations)

Flag by-value exceptions if their types are part of a hierarchy (could require whole-program analysis to be perfect).

If the exception type is part of the class hierarchy, mark-passing values are used (which requires more perfect parsing of the entire program).

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