ERR36-CPP. Multiple catch handlers to a try block should order their exceptions from most derived to most basic

C++2004, section 15.3 "Handling an Exception", says:

The handlers for a try block are tried in order of appearance. That makes it possible to write handlers that can never be executed, for example by placing a handler for a derived class after a handler for a corresponding base class.

A ... in a handlers exception-declaration functions similarly to ... in a function parameter declaration; it specifies a match for any exception. If present, a ... handler shall be the last handler for its try block.

Consequently if two handlers catch exceptions that are derived from the same base class (such as std::exception), the most derived exception should come first.

Non-Compliant Code Example

In this non-compliant code example, the first handler will catch all exceptions of class B, as well as exceptions of class D, since they are also of class B. Consequently the second handler will not catch any exceptions.

// classes used for exception handling
class B {};
class D: public B {};

// ... Using the classes from above
try {
 // ...
} catch (B &b) {
 // ...
} catch (D &d) {
 // ...
}

Compliant Solution

In this compliant solution, the first handler will catch all exceptions of class D, and the second handler will catch all the other exceptions of class B.

// classes used for exception handling
class B {};
class D: public B {};

// ... Using the classes from above
try {
 // ...
} catch (D &d) {
 // ...
} catch (B &b) {
 // ...
}

Risk Assessment

Badly ordering exception handlers can cause unexpected control flow.

Bibliography

[ISO/IEC 14882-2003]
[MISRA 08] Rule 15-3-6 & 15-3-7

ERR35-CPP. A handler in a constructor or destructor's function-try-block should not reference class data      12. Exceptions and Error Handling (ERR)      ERR37-CPP. A function with an exception-specification should not throw any other exceptions