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

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

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

Copy 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 will 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.

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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 (STL) functions. Since objects of Standard Template Library STL types are used in contexts where CopyAssignable is required, Standard Template Library STL types are required to gracefully handle self-copy assignment.

Move Assignment

The postconditions required for a move assignment are specified by the C++ Standard, [utility.arg.requirements], Table 22 [ISO/IEC 14882-2014], which states that for x = std::move(y), the value of y is left in a valid but unspecified state. When &x == &y, this postcondition translates into the values of both x and y remaining in a valid but unspecified state. Leaving the values in an unspecified state may result in vulnerabilities leading to exploitable code.

A user-provided move assignment operator must prevent self-move assignment from leaving the object in a valid but unspecified state. Akin to copy assignment, it can be accomplished by self-assignment tests, move-and-swap, or other idiomatic design patterns.

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Noncompliant Code Example

In this noncompliant code example, the copy and move assignment operators do operator does not protect against self-copy 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. If self-move assignment occurs, it is dependent on the implementation of std::swap as to whether s1 is left in a valid but unspecified state.

#include <new>
 
struct S { S(const S &) noexcept; /* ... */ }; // 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(T &&rhs) noexcept : n(rhs.n), s1(rhs.s1) { rhs.s1 = nullptr; }
  ~T() { delete s1; }
 
  // ...
 
  T& operator=(const T &rhs) {
    n = rhs.n;
    delete s1;
    s1 = new S(*rhs.s1);
    return *this;
  }
 
  T& operator=(T &&rhs) noexcept {
    n = rhs.n;
    s1 = rhs.s1;
    return *this;
  }
};

Compliant Solution (Self-Test)

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

#include <new>
 
struct S { S(const S &) noexcept; /* ... */ }; // 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(T &&rhs) noexcept : n(rhs.n), s1(rhs.s1) { 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;
  }
 
  T& operator=(T &&rhs) noexcept {
    if (this != &rhs) {
      n = rhs.n;
      s1 = rhs.s1;
    }
    return *this;
  }
};

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 rhs that is then swapped with *this. This compliant solution can provide 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. It avoids self-move assignment by testing for self-assignment, as in the previous compliant solution.

#include <new>
#include <utility>
 
struct S { S(const S &) noexcept; /* ... */ }; // 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(T &&rhs) noexcept : n(rhs.n), s1(rhs.s1) { 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;
  }
 
};

Compliant Solution (Move and Swap)

This compliant solution uses the same classes S and T from the previous compliant solution, but adds the following public constructor and methods:

  T& operator=(T &&rhs) noexcept { { *this = std::move(rhs); }

  //  if (&rhs != this) {
  ... everything except operator= ..

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

Exceptions

The copy assignment operator uses std::move() rather than swap() to achieve safe self-assignment and a strong exception guarantee. The move assignment operator uses a move (via the method parameter) and swap.

The move constructor is not strictly necessary, but defining a move constructor along with a move assignment operator is conventional for classes that support move operations.

Note that unlike copy assignment operators, the signature of a move assignment operator accepts a non-const reference to its object with the expectation that the moved-from object will be left in an unspecified, but valid state. Move constructors have the same difference from copy constructorsOOP54-CPP-EX1: Self-swap is trivially self-move assignment safe. Calling std::swap(x, x) results in an operation that moves the resources from x into a temporary object, performs a self-move assignment that should have no ill-effect on the already-moved-from state of x, and finally moves the resources from the temporary object back into x with the end result that the resources stored within x should be unchanged. Because of how pervasive the use of std::swap() is within the Standard Template Library and the unlikelihood that a self-swap would cause undefined behavior in practice, guarding against self-move assignment through self-swap is not required.

Risk Assessment

Allowing a copy assignment operator to corrupt an object could lead to undefined behavior. Allowing a move assignment operator to leave an object in a valid but unspecified state could lead to abnormal program execution.

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-copy assignment, because the copy operation may mutate both the source and destination objects (due to them being the same object).

Bibliography

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