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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;
}

...

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.

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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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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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