Throw by value, catch exceptions from a hierarchy by reference

Reason

Throwing by value (not by pointer) and catching by reference prevents copying, especially slicing base subobjects.

Example; bad

void f()
{
    try {
        // ...
        throw new widget{}; // don't: throw by value, not by raw pointer
        // ...
    }
    catch (base_class e) {  // don't: might slice
        // ...
    }
}

Instead, use a reference:

catch (base_class& e) { /* ... */ }

or - typically better still - a const reference:

catch (const base_class& e) { /* ... */ }

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

Note

Catch by value can be appropriate for a small value type such as an enum value.

Note

To rethrow a caught exception use throw; not throw e;. Using throw e; would throw a new copy of e (sliced to the static type std::exception, when the exception is caught by catch (const std::exception& e)) instead of rethrowing the original exception of type std::runtime_error. (But keep Don't try to catch every exception in every function and Minimize the use of explicit try/catch in mind.)

Enforcement

• Flag catching by value of a type that has a virtual function.
• Flag throwing raw pointers.