21.12 — Overloading the assignment operator
The copy assignment operator (operator=) is used to copy values from one object to another already existing object .
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As of C++11, C++ also supports “Move assignment”. We discuss move assignment in lesson 22.3 -- Move constructors and move assignment .
Copy assignment vs Copy constructor
The purpose of the copy constructor and the copy assignment operator are almost equivalent -- both copy one object to another. However, the copy constructor initializes new objects, whereas the assignment operator replaces the contents of existing objects.
The difference between the copy constructor and the copy assignment operator causes a lot of confusion for new programmers, but it’s really not all that difficult. Summarizing:
- If a new object has to be created before the copying can occur, the copy constructor is used (note: this includes passing or returning objects by value).
- If a new object does not have to be created before the copying can occur, the assignment operator is used.
Overloading the assignment operator
Overloading the copy assignment operator (operator=) is fairly straightforward, with one specific caveat that we’ll get to. The copy assignment operator must be overloaded as a member function.
This prints:
This should all be pretty straightforward by now. Our overloaded operator= returns *this, so that we can chain multiple assignments together:
Issues due to self-assignment
Here’s where things start to get a little more interesting. C++ allows self-assignment:
This will call f1.operator=(f1), and under the simplistic implementation above, all of the members will be assigned to themselves. In this particular example, the self-assignment causes each member to be assigned to itself, which has no overall impact, other than wasting time. In most cases, a self-assignment doesn’t need to do anything at all!
However, in cases where an assignment operator needs to dynamically assign memory, self-assignment can actually be dangerous:
First, run the program as it is. You’ll see that the program prints “Alex” as it should.
Now run the following program:
You’ll probably get garbage output. What happened?
Consider what happens in the overloaded operator= when the implicit object AND the passed in parameter (str) are both variable alex. In this case, m_data is the same as str.m_data. The first thing that happens is that the function checks to see if the implicit object already has a string. If so, it needs to delete it, so we don’t end up with a memory leak. In this case, m_data is allocated, so the function deletes m_data. But because str is the same as *this, the string that we wanted to copy has been deleted and m_data (and str.m_data) are dangling.
Later on, we allocate new memory to m_data (and str.m_data). So when we subsequently copy the data from str.m_data into m_data, we’re copying garbage, because str.m_data was never initialized.
Detecting and handling self-assignment
Fortunately, we can detect when self-assignment occurs. Here’s an updated implementation of our overloaded operator= for the MyString class:
By checking if the address of our implicit object is the same as the address of the object being passed in as a parameter, we can have our assignment operator just return immediately without doing any other work.
Because this is just a pointer comparison, it should be fast, and does not require operator== to be overloaded.
When not to handle self-assignment
Typically the self-assignment check is skipped for copy constructors. Because the object being copy constructed is newly created, the only case where the newly created object can be equal to the object being copied is when you try to initialize a newly defined object with itself:
In such cases, your compiler should warn you that c is an uninitialized variable.
Second, the self-assignment check may be omitted in classes that can naturally handle self-assignment. Consider this Fraction class assignment operator that has a self-assignment guard:
If the self-assignment guard did not exist, this function would still operate correctly during a self-assignment (because all of the operations done by the function can handle self-assignment properly).
Because self-assignment is a rare event, some prominent C++ gurus recommend omitting the self-assignment guard even in classes that would benefit from it. We do not recommend this, as we believe it’s a better practice to code defensively and then selectively optimize later.
The copy and swap idiom
A better way to handle self-assignment issues is via what’s called the copy and swap idiom. There’s a great writeup of how this idiom works on Stack Overflow .
The implicit copy assignment operator
Unlike other operators, the compiler will provide an implicit public copy assignment operator for your class if you do not provide a user-defined one. This assignment operator does memberwise assignment (which is essentially the same as the memberwise initialization that default copy constructors do).
Just like other constructors and operators, you can prevent assignments from being made by making your copy assignment operator private or using the delete keyword:
Note that if your class has const members, the compiler will instead define the implicit operator= as deleted. This is because const members can’t be assigned, so the compiler will assume your class should not be assignable.
If you want a class with const members to be assignable (for all members that aren’t const), you will need to explicitly overload operator= and manually assign each non-const member.
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Default Assignment Operator and References in C++
We have discussed assignment operator overloading for dynamically allocated resources here . In this article, we discussed that when we don’t write our own assignment operator, the compiler creates an assignment operator itself that does shallow copy and thus causes problems. The difference between shallow copy and deep copy becomes visible when the class has pointers as member fields. However, there is no difference when pointers are not used. What happens when we have references in our class and there is no user-defined assignment operator.
For example, predict the output of the following program,
The compiler doesn’t create default assignment operator in the following cases:
1. Class has a non-static data member of a const type or a reference type. 2. Class has a non-static data member of a type that has an inaccessible copy assignment operator. 3. Class is derived from a base class with an inaccessible copy assignment operator.
When any of the above conditions is true, the user must define the assignment operator. For example, if we add an assignment operator to the above code, the code works fine without any error.
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