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It is important that resources are reclaimed Reclaiming resources when exceptions are thrown is important. Throwing an An exception being thrown may result in cleanup code being bypassed . As a result, it is the responsibility of the exception handler to properly clean up. This may be problematic if the exception is to be caught in a different function or module. Instead, it is preferable if resources are reclaimed automatically when objects go out of scope.

Non-Compliant Code Example

or an object being left in a partially initialized state. Such a partially initialized object would violate basic exception safety, as described in ERR56-CPP. Guarantee exception safety. It is preferable that resources be reclaimed automatically, using the RAII design pattern [Stroustrup 2001], when objects go out of scope. This technique avoids the need to write complex cleanup code when allocating resources.

However, constructors do not offer the same protection. Because a constructor is involved in allocating resources, it does not automatically free any resources it allocates if it terminates prematurely. The C++ Standard, [except.ctor], paragraph 2 [ISO/IEC 14882-2014], states the following:

An object of any storage duration whose initialization or destruction is terminated by an exception will have destructors executed for all of its fully constructed subobjects (excluding the variant members of a union-like class), that is, for subobjects for which the principal constructor (12.6.2) has completed execution and the destructor has not yet begun execution. Similarly, if the non-delegating constructor for an object has completed execution and a delegating constructor for that object exits with an exception, the object’s destructor will be invoked. If the object was allocated in a new-expression, the matching deallocation function (3.7.4.2, 5.3.4, 12.5), if any, is called to free the storage occupied by the object.

It is generally recommended that constructors that cannot complete their job should throw exceptions rather than exit normally and leave their object in an incomplete state [Cline 2009].

Resources must not be leaked as a result of throwing an exception, including during the construction of an object.

This rule is a subset of MEM51-CPP. Properly deallocate dynamically allocated resources, as all failures to deallocate resources violate that rule.

Noncompliant Code Example

In this noncompliant code example, pst is not properly released when process_item throws an exception, In this non-compliant code example, the resources associated with the object pointed to by pst are not recovered in the event that processItem throws an exception, thereby potentially causing a resource leak.

while (moreToDo#include <new>
 
struct SomeType {
  SomeType() noexcept; // Performs nontrivial initialization.
  ~SomeType(); // Performs nontrivial finalization.
  void process_item() noexcept(false);
};
 
void f() {
  SomeType *pst = new (std::nothrow) SomeType();
  if (!pst) {
    // Handle error
    return;
  }
 
  try {
    pst->processItem>process_item();
  }
  catch (...) {
    // deal with exception Process error, but do not recover from it; rethrow.
    throw;
  }
  delete pst;
}

Compliant Solution (delete)

In this codecompliant solution, the exception handler recovers the resources associated with the object pointed to by pstfrees pst by calling delete.

while (moreToDo#include <new>

struct SomeType {
  SomeType() noexcept; // Performs nontrivial initialization.
  ~SomeType(); // Performs nontrivial finalization.

  void process_item() noexcept(false);
};

void f() {
  SomeType *pst = new (std::nothrow) SomeType();
  if (!pst) {
    // Handle error
    return;
  }
  try {
    pst->processItem>process_item();
  }
  catch (...) {
    // deal with exceptionProcess error, but do not recover from it; rethrow.
    delete pst;
    throw;
  }
  delete pst;
}

Handling resource cleanup in While this compliant solution properly releases its resources using catch clauses does work, but can this approach can have several some disadvantages:

• Each distinct cleanup requires its own try & and catch blocks.
• The cleanup operation must not throw any exceptions.

Compliant Solution (RAII Design Pattern)

A better approach would be is to employ RAII. This pattern forces every object to ' clean up after itself ' in the face of abnormal behavior, preventing the programmer from having to do so. A judicious unique_ptr would free the resource whether an error occurs or not. Another benefit of this approach is that it does not require statements to handle resource allocation errors, in conformance with MEM52-CPP. Detect and handle memory allocation errors.

struct SomeType {
  SomeType() noexcept; // Performs nontrivial initialization.
  ~SomeType(); // Performs nontrivial finalization.

  void process_item() noexcept(false);
};

void f() {
  SomeType st;
  try {
    st.process_item();
  } catch (...) {
    // Process error, but do not recover from it; rethrow.
    throw;
  } // After re-throwing the exception, the destructor is run for st.
} // If f() exits without throwing an exception, the destructor is run for st.

Noncompliant Code Example

In this noncompliant code example, the C::C() constructor might fail to allocate memory for a, might fail to allocate memory for b, or might throw an exception in the init() method. If init() throws an exception, neither a nor b will be released. Likewise, if the allocation for b fails, a will not be released.

struct A {/* ... */};
struct B {/* ... */};

class C {
  A *a;
  B *b;
protected:
  void init() noexcept(false);
public:
  C() : a(new A()), b(new B()) {
    init();
  }
};

Compliant Solution (try/catch)

This compliant solution mitigates the potential failures by releasing a and b if an exception is thrown during their allocation or during init().

struct A {/* ... */};
struct B {/* ... */};
 
class C {
  A *a;
  B *b;
protected:
  void init() noexcept(false);
public:
  C() : a(nullptr), b(nullptr) {
    try {
      awhile (moreToDo) {
  std::unique_ptr<SomeType> pst = new SomeTypeA();
      b = try {
new B();
     pst->processItem init();
    }
  catch (...) {
      delete a;
    // deal withdelete exceptionb;
      throw; // pst automatically freed
  }
  // pst automatically freed
}

    }
  }
};

Compliant Solution (std::unique_ptr)

This compliant solution uses std::unique_ptr to create objects that clean up after themselves should anything go wrong in the C::C() constructor. The std::unique_ptr applies the principles of RAII to pointers.

#include <memory>
 
struct A {/* ... */};
struct B {/* ... */};

class C {
  std::unique_ptr<A> a;
  std::unique_ptr<B> b;
protected:
  void init() noexcept(false);
public:
  C() : a(new A()), b(new B()) {
    init();
  }
};

Risk Assessment

Memory and other resource leaks will eventually cause a program to crash. If an attacker can provoke repeated resource leaks by forcing an exception to be thrown through the submission of suitably crafted data, then the attacker can mount a denial-of-service attack.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography

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