The C++ Standard, [except.handle], paragraph 4 [ISO/IEC 14882-2014], states:

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.

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

Noncompliant Code Example

In this noncompliant code example, the first handler catches all exceptions of class B, as well as exceptions of class D, since they are also of class B. Consequently, the second handler does not catch any exceptions.

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

void f() {
  try {
    // ...
  } catch (B &b) {
    // ...
  } catch (D &d) {
    // ...
  }
}

Compliant Solution

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

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

void f() {
  try {
    // ...
  } catch (D &d) {
    // ...
  } catch (B &b) {
    // ...
  }
}

Risk Assessment

Exception handlers with inverted priorities cause unexpected control flow when an exception of the derived type occurs.

Automated Detection

Tool	Version	Checker	Description
Clang		-Wexceptions
ECLAIR		CP1.ERR36	Fully implemented
Parasoft C/C++test	9.5	EXCEPT-17
PRQA QA-C++		4040, 4034

Related Vulnerabilities

Search for other vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Bibliography