Self-copy assignment can occur in situations of varying complexity, but essentially, all self-copy assignments entail some variation of the following.
User-provided copy operators must properly handle self-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
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 (STL) 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-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.
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, then
operator= does nothing; otherwise, the copy proceeds as in the original example.
This solution does not provide a strong exception guarantee for the copy assignment. Specifically, if an exception is called when evaluating the
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.
Compliant Solution (Move and Swap)
This compliant solution uses the same classes
T from the previous compliant solution, but adds the following public constructor and methods:
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 constructors.
Allowing a copy assignment operator to corrupt an object could lead to undefined behavior.
|Checked by |
|Check for assignment to self in operator=|
|Polyspace Bug Finder|
|CERT C++: OOP54-CPP||Checks for copy assignment operators where self-assignment is not tested (rule partially covered)|
|4072, 4073, 4075, 4076|
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).
|[Henricson 1997]||Rule 5.12, Copy assignment operators should be protected from doing destructive actions if an object is assigned to itself|
|[ISO/IEC 14882-2014]||Subclause 184.108.40.206, "Template Argument Requirements"|
Subclause 220.127.116.11, "Function Arguments"
|[Meyers 2005]||Item 11, "Handle Assignment to Self in |