CertC++-DCL57

Do not let exceptions escape from destructors or deallocation functions

Required inputs: IR, StaticSemanticAnalysis

Under certain circumstances, terminating a destructor, operator delete, or  operator delete[] by throwing an exception can trigger undefined behavior.

For instance, the C++ Standard, [basic.stc.dynamic.deallocation], paragraph 3 [ ISO/IEC 14882-2014], in part, states the following:

If a deallocation function terminates by throwing an exception, the behavior is undefined.

In these situations, the function must logically be declared  noexcept because throwing an exception from the function can never have well-defined behavior. The C++ Standard, [except.spec], paragraph 15, states the following:

A deallocation function with no explicit exception-specification is treated as if it were specified with noexcept(true).

As such, deallocation functions (object, array, and placement forms at either global or class scope) must not terminate by throwing an exception. Do not declare such functions to be  noexcept(false). However, it is acceptable to rely on the implicit  noexcept(true) specification or declare  noexcept explicitly on the function signature.

Object destructors are likely to be called during stack unwinding as a result of an exception being thrown. If the destructor itself throws an exception, having been called as the result of an exception being thrown, then the function  std::terminate() is called with the default effect of calling  std::abort() [ ISO/IEC 14882-2014]. When  std::abort() is called, no further objects are destroyed, resulting in an indeterminate program state and undefined behavior. Do not terminate a destructor by throwing an exception. 

The C++ Standard, [class.dtor], paragraph 3, states [ ISO/IEC 14882-2014] the following:

A declaration of a destructor that does not have an exception-specification is implicitly considered to have the same exception-specification as an implicit declaration.

An implicit declaration of a destructor is considered to be  noexcept(true) according to [except.spec], paragraph 14. As such, destructors must not be declared  noexcept(false) but may instead rely on the implicit  noexcept(true) or declare  noexcept explicitly.

Any noexcept function that terminates by throwing an exception violates  ERR55-CPP. Honor exception specifications.

Noncompliant Code Example

In this noncompliant code example, the class destructor does not meet the implicit  noexcept guarantee because it may throw an exception even if it was called as the result of an exception being thrown. Consequently, it is declared as  noexcept(false) but still can trigger undefined behavior.

#include <stdexcept>
 
class S {
  bool has_error() const;
 
public:
  ~S() noexcept(false) {
    // Normal processing
    if (has_error()) {
      throw std::logic_error("Something bad");
    }
  }
};

Noncompliant Code Example ( std::uncaught_exception())

Use of  std::uncaught_exception() in the destructor solves the termination problem by avoiding the propagation of the exception if an existing exception is being processed, as demonstrated in this noncompliant code example. However, by circumventing normal destructor processing, this approach may keep the destructor from releasing important resources.

#include <exception>
#include <stdexcept>

class S {
  bool has_error() const;

public:
  ~S() noexcept(false) {
    // Normal processing
    if (has_error() && !std::uncaught_exception()) {
      throw std::logic_error("Something bad");
    }
  }
};

Noncompliant Code Example ( function-try-block)

This noncompliant code example, as well as the following compliant solution, presumes the existence of a  Bad class with a destructor that can throw. Although the class violates this rule, it is presumed that the class cannot be modified to comply with this rule.

// Assume that this class is provided by a 3rd party and it is not something
// that can be modified by the user.
class Bad {
  ~Bad() noexcept(false);
};

To safely use the Bad class, the  SomeClass destructor attempts to handle exceptions thrown from the Bad destructor by absorbing them.

class SomeClass {
  Bad bad_member;
public:
  ~SomeClass()
  try {
    // ...
  } catch(...) {
    // Handle the exception thrown from the Bad destructor.
  }
};

However, the C++ Standard, [except.handle], paragraph 15 [ ISO/IEC 14882-2014], in part, states the following:

The currently handled exception is rethrown if control reaches the end of a handler of the function-try-block of a constructor or destructor.

Consequently, the caught exception will inevitably escape from the  SomeClass destructor because it is implicitly rethrown when control reaches the end of the function-try-block handler.

Compliant Solution

A destructor should perform the same way whether or not there is an active exception. Typically, this means that it should invoke only operations that do not throw exceptions, or it should handle all exceptions and not rethrow them (even implicitly). This compliant solution differs from the previous noncompliant code example by having an explicit return statement in the  SomeClass destructor. This statement prevents control from reaching the end of the exception handler. Consequently, this handler will catch the exception thrown by  Bad::~Bad() when  bad_member is destroyed. It will also catch any exceptions thrown within the compound statement of the function-try-block, but the SomeClass destructor will not terminate by throwing an exception.

class SomeClass {
  Bad bad_member;
public:
  ~SomeClass()
  try {
    // ...
  } catch(...) {
    // Catch exceptions thrown from noncompliant destructors of
    // member objects or base class subobjects.

    // NOTE: Flowing off the end of a destructor function-try-block causes
    // the caught exception to be implicitly rethrown, but an explicit
    // return statement will prevent that from happening.
    return;
  }
};

Noncompliant Code Example

In this noncompliant code example, a global deallocation is declared  noexcept(false) and throws an exception if some conditions are not properly met. However, throwing from a deallocation function results in undefined behavior.

#include <stdexcept>
 
bool perform_dealloc(void *);
 
void operator delete(void *ptr) noexcept(false) {
  if (perform_dealloc(ptr)) {
    throw std::logic_error("Something bad");
  }
}

Compliant Solution

The compliant solution does not throw exceptions in the event the deallocation fails but instead fails as gracefully as possible.

#include <cstdlib>
#include <stdexcept>

bool perform_dealloc(void *);
void log_failure(const char *);

void operator delete(void *ptr) noexcept(true) {
  if (perform_dealloc(ptr)) {
    log_failure("Deallocation of pointer failed");
    std::exit(1); // Fail, but still call destructors
  }
}

Risk Assessment

Attempting to throw exceptions from destructors or deallocation functions can result in undefined behavior, leading to resource leaks or denial-of-service attacks.

Rule	Severity	Likelihood	Remediation Cost	Priority	Level
DCL57-CPP	Low	Likely	Medium	P6	L2

Related Guidelines

SEI CERT C++ Coding Standard	ERR55-CPP. Honor exception specifications ERR50-CPP. Do not abruptly terminate the program
MISRA C++:2008	Rule 15-5-1 (Required)

Bibliography

[ Henricson 1997]	Recommendation 12.5, Do not let destructors called during stack unwinding throw exceptions
[ ISO/IEC 14882-2014]	Subclause 3.4.7.2, "Deallocation Functions" Subclause 15.2, "Constructors and Destructors" Subclause 15.3, "Handling an Exception" Subclause 15.4, "Exception Specifications"
[ Meyers 2005]	Item 8, "Prevent Exceptions from Leaving Destructors"
[ Sutter 2000]	"Never allow exceptions from escaping destructors or from an overloaded  operator delete()" (p. 29)

Excerpt from SEI CERT C++ Coding Standard [https://cmu-sei.github.io/secure-coding-standards/sei-cert-cpp-coding-standard/rules/declarations-and-initialization-dcl/dcl57-cpp], Copyright (C) 1995-2026 Carnegie Mellon University. See section 9.4. "3rd-Party Licenses" in the documentation for full details.

Possible Messages

Key	Text	Severity	Disabled
exception_escaping_constructor	Escaping exception from constructor.	None	False
exception_escaping_destructor	Escaping exception from destructor.	None	False

Options

• This rule shares the following common options: exclude_in_macros, exclude_messages_in_system_headers, excludes, extend_exclude_to_macro_invocations, includes, justification_checker, languages, post_processing, provider, report_at, severity

• The following places define options that affect this rule: Stylechecks, Analysis-GlobalOptions

allowed_exceptions

allowed_exceptions : set[str] = {'bad_alloc', 'bad_cast', 'failure', 'runtime_error', 'system_error'}

Exceptions that are allowed to escape from destructors.

 

constructors

constructors : bool = False

Whether to consider constructors.

 

destructors

destructors : bool = True

Whether to consider destructors.

 

exclude_exception_base_classes

exclude_exception_base_classes : set[bauhaus.analysis.config.QualifiedName] = set()

Exclude issues for the exception types mentioned in this set of qualified names. Also excludes classes derived from those class names as well as pointers or references to any of these class types.

 

generate_violation_path

generate_violation_path : bool = True

Whether to compute a trace for the exception. This improves the usability of the violation description, but requires additional computing which might slow down the rule.