OOP54-CPP. Gracefully handle self-copy assignment

Self-assignment can occur in situations of varying complexity, but essentially, all self-assignments entail some variation of the following:

#include <utility>
 
struct S { /* ... */ }
 
void f() {
  S s;
  s = s; // Self-copy assignment
}

User-provided copy operators must properly handle self-assignment.

The postconditions required for copy assignment are specified by the C++ Standard, [utility.arg.requirements], Table 23 [ISO/IEC 14882-2014], which states that for x = y, the value of y is unchanged. When &x == &y, this postcondition translates into the values of both x and y remaining unchanged. A naive implementation of copy assignment could destroy object-local resources in the process of copying resources from the given parameter. If the given parameter is the same object as the local object, the act of destroying object-local resources will invalidate them. The subsequent copy of those resources will be left in an indeterminate state, which violates the postcondition.

A user-provided copy assignment operator must prevent self-copy assignment from leaving the object in an indeterminate state. This can be accomplished by self-assignment tests, copy-and-swap, or other idiomatic design patterns.

The C++ Standard, [copyassignable], specifies that types must ensure that self-copy assignment leave the object in a consistent state when passed to Standard Template Library functions. Since objects of STL types are used in contexts where CopyAssignable is required, STL types are required to gracefully handle self-copy assignment.

Noncompliant Code Example

In this noncompliant code example, the copy assignment operator does not protect against self-assignment. If self-copy assignment occurs, this->s1 is deleted, which results in rhs.s1 also being deleted. The invalidated memory for rhs.s1 is then passed into the copy constructor for S, which can result in dereferencing an invalid pointer.

#include <new>
 
struct S { /* ... */ }; // Has nonthrowing copy constructor
 
class T {
  int n;
  S *s1;
 
public:
  T(const T &rhs) : n(rhs.n), s1(rhs.s1 ? new S(*rhs.s1) : nullptr) {}
  ~T() { delete s1; }
 
  // ...
 
  T& operator=(const T &rhs) {
    n = rhs.n;
    delete s1;
    s1 = new S(*rhs.s1);
    return *this;
  }
};

Compliant Solution (Self-Test)

This compliant solution guards against self-assignment by testing whether the given parameter is the same as this. If self-assignment occurs, then operator= does nothing; otherwise, the copy proceeds as in the original example.

#include <new>
 
struct S { /* ... */ }; // Has nonthrowing copy constructor
 
class T {
  int n;
  S *s1;
 
public:
  T(const T &rhs) : n(rhs.n), s1(rhs.s1 ? new S(*rhs.s1) : nullptr) {}
  ~T() { delete s1; }

  // ...
 
  T& operator=(const T &rhs) {
    if (this != &rhs) {
      n = rhs.n;
      delete s1;
      try {
        s1 = new S(*rhs.s1);
      } catch (std::bad_alloc &) {
        s1 = nullptr; // For basic exception guarantees
        throw;
      }
    }
    return *this;
  }
};

Note that this solution does not provide a strong exception guarantee for the copy assignment. Specifically, if an exception is called when evaluating the new expression, this has already been modified. However, this solution does provide a basic exception guarantee because no resources are leaked and all data members contain valid values. Consequently this code complies with ERR56-CPP. Guarantee exception safety.

Compliant Solution (Copy and Swap)

This compliant solution avoids self-copy assignment by constructing a temporary object from rhs that is then swapped with *this. This compliant solution provides a strong exception guarantee because swap() will never be called if resource allocation results in an exception being thrown while creating the temporary object.

#include <new>
#include <utility>
 
struct S { /* ... */ }; // Has nonthrowing copy constructor
 
class T {
  int n;
  S *s1;
 
public:
  T(const T &rhs) : n(rhs.n), s1(rhs.s1 ? new S(*rhs.s1) : nullptr) {}
  ~T() { delete s1; }

  // ...
 
  void swap(T &rhs) noexcept {
    using std::swap;
    swap(n, rhs.n);
    swap(s1, rhs.s1);
  }
 
  T& operator=(const T &rhs) noexcept {
    T(rhs).swap(*this);
    return *this;
  }
};

Risk Assessment

Allowing a copy assignment operator to corrupt an object could lead to undefined behavior.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

This rule is a partial subset of OOP58-CPP. Copy operations must not mutate the source object when copy operations do not gracefully handle self-assignment, because the copy operation may mutate both the source and destination objects (due to them being the same object).

Bibliography