Reason

catch-clauses are evaluated in the order they appear and one clause can hide another.

Example, bad

void f()
{
    // ...
    try {
            // ...
    }
    catch (Base& b) { /* ... */ }
    catch (Derived& d) { /* ... */ }
    catch (...) { /* ... */ }
    catch (std::exception& e) { /* ... */ }
}

If Derivedis derived from Base the Derived-handler will never be invoked. The "catch everything" handler ensured that the std::exception-handler will never be invoked.

Enforcement

Flag all "hiding handlers".

Con: Constants and immutability

You can't have a race condition on a constant. It is easier to reason about a program when many of the objects cannot change their values. Interfaces that promise "no change" of objects passed as arguments greatly increase readability.

Constant rule summary:

• Con.1: By default, make objects immutable
• Con.2: By default, make member functions const
• Con.3: By default, pass pointers and references to consts
• Con.4: Use const to define objects with values that do not change after construction
• Con.5: Use constexpr for values that can be computed at compile time