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Reclaiming resources when exceptions are thrown is important. An exception being thrown may result in cleanup code being bypassed 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.

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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, causing a resource leak:.

#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->process_item();
  } catch (...) {
    // HandleProcess error, but do not recover from it; rethrow.
    throw;
  }
  delete pst;
}

Compliant Solution (delete)

In this compliant solution, the exception handler frees pst by calling delete:.

#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->process_item();
  } catch (...) {
    // HandleProcess error, but do not recover from it; rethrow.
    delete pst;
    throw;
  }
  delete pst;
}

This compliant solution complies with MEM51-CPP. Properly deallocate dynamically allocated resources. However, although handling resource cleanup in catch clauses works, it can While this compliant solution properly releases its resources using catch clauses, this approach can have some disadvantages:

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

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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 (...) {
    // HandleProcess 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

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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 {
      a = new A();
      b = new B();
      init();
    } catch (...) {
      delete a;
      delete b;
      throw;
    }
  }
};

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This compliant solution uses std::unique_ptr to create objects that clean up after themselves should anything go wrong in the C::C() constructor (see MSC17-CPP. Do not use deprecated or obsolescent functionality for more information on . The std::unique_ptr).

...

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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();
  }
};

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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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