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The C++ 2004, section 15.3 "Handling an Exception", saysStandard, [except.handle], paragraph 4, states [ISO/IEC 14882-2014]:

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

...

Noncompliant Code Example

In this non-compliant noncompliant 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
void f() {
  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
void f() {
  try {
    // ...
  } catch (D &d) {
    // ...
  } catch (B &b) {
    // ...
  }

}

Risk Assessment

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

Automated Detection

Bibliography

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Related Vulnerabilities

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

Related Guidelines

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Bibliography