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

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
Astrée		exception-caught-by-earlier-handler	Fully checked
Axivion Bauhaus Suite		CertC++-ERR54
Clang		-Wexceptions
CodeSonar		LANG.STRUCT.UCTCH	Unreachable Catch
ECLAIR		CP1.ERR36	Fully implemented
Helix QAC		C++4030, C++4639
Klocwork		MISRA.CATCH.NOALL MISRA.CATCH.WRONGORD
LDRA tool suite		541 S, 556 S	Fully implemented
Parasoft C/C++test		CERT_CPP-ERR54-a	Where multiple handlers are provided in a single try-catch statement or function-try-block for a derived class and some or all of its bases, the handlers shall be ordered most-derived to base class
Polyspace Bug Finder		CERT C++: ERR54-CPP	Checks for: Exception handlers not ordered from most-derived to base class Incorrect order of ellipsis handler Rule fully covered.
PVS-Studio		V759
RuleChecker		exception-caught-by-earlier-handler	Fully checked
SonarQube C/C++ Plugin		S1045

Related Vulnerabilities

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

Related Guidelines

Bibliography