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.. title:: clang-tidy - bugprone-unhandled-self-assignment
bugprone-unhandled-self-assignment
==================================
`cert-oop54-cpp` redirects here as an alias for this check. For the CERT alias,
the `WarnOnlyIfThisHasSuspiciousField` option is set to `false`.
Finds user-defined copy assignment operators which do not protect the code
against self-assignment either by checking self-assignment explicitly or
using the copy-and-swap or the copy-and-move method.
By default, this check searches only those classes which have any pointer or C array field
to avoid false positives. In case of a pointer or a C array, it's likely that self-copy
assignment breaks the object if the copy assignment operator was not written with care.
See also:
`OOP54-CPP. Gracefully handle self-copy assignment
<https://wiki.sei.cmu.edu/confluence/display/cplusplus/OOP54-CPP.+Gracefully+handle+self-copy+assignment>`_
A copy assignment operator must prevent that self-copy assignment ruins the
object state. A typical use case is when the class has a pointer field
and the copy assignment operator first releases the pointed object and
then tries to assign it:
.. code-block:: c++
class T {
int* p;
public:
T(const T &rhs) : p(rhs.p ? new int(*rhs.p) : nullptr) {}
~T() { delete p; }
// ...
T& operator=(const T &rhs) {
delete p;
p = new int(*rhs.p);
return *this;
}
};
There are two common C++ patterns to avoid this problem. The first is
the self-assignment check:
.. code-block:: c++
class T {
int* p;
public:
T(const T &rhs) : p(rhs.p ? new int(*rhs.p) : nullptr) {}
~T() { delete p; }
// ...
T& operator=(const T &rhs) {
if(this == &rhs)
return *this;
delete p;
p = new int(*rhs.p);
return *this;
}
};
The second one is the copy-and-swap method when we create a temporary copy
(using the copy constructor) and then swap this temporary object with ``this``:
.. code-block:: c++
class T {
int* p;
public:
T(const T &rhs) : p(rhs.p ? new int(*rhs.p) : nullptr) {}
~T() { delete p; }
// ...
void swap(T &rhs) {
using std::swap;
swap(p, rhs.p);
}
T& operator=(const T &rhs) {
T(rhs).swap(*this);
return *this;
}
};
There is a third pattern which is less common. Let's call it the copy-and-move method
when we create a temporary copy (using the copy constructor) and then move this
temporary object into ``this`` (needs a move assignment operator):
.. code-block:: c++
class T {
int* p;
public:
T(const T &rhs) : p(rhs.p ? new int(*rhs.p) : nullptr) {}
~T() { delete p; }
// ...
T& operator=(const T &rhs) {
T t = rhs;
*this = std::move(t);
return *this;
}
T& operator=(T &&rhs) {
p = rhs.p;
rhs.p = nullptr;
return *this;
}
};
.. option:: WarnOnlyIfThisHasSuspiciousField
When `true`, the check will warn only if the container class of the copy assignment operator
has any suspicious fields (pointer or C array). This option is set to `true` by default.
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