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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 092009].

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

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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 (...) {
    // Process error, but do not recover from it; rethrow.
    throw;
  }
  delete pst;
}

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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 (...) {
    // Process error, but do not recover from it; rethrow.
    delete pst;
    throw;
  }
  delete pst;
}

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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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Bibliography

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