CS253: Software Development with C++
Spring 2022, self-assignment.
Show Lecture.Self-assignment as a slide show.
CS253 Self-assignment
This illustrates the danger of self-assignment.
That is, when an object containing pointers or other handles to external resources is assigned to itself, you’ve got to be careful in operator= , or you might destroy your data before you copy it to yourself.
Copy Constructor General Outline
When a class contains a handle to external data, the copy constructor generally follows this pattern:
- Copy the resource from the other object to this object.
That is, don’t just copy the handle (pointer, file descriptor, etc.). Instead you have to make an actual copy of the resource (copy the memory from new , create a new temporary file and copy the contents, etc.).
Of course, this is a copy constructor, not an assignment operator, so there was no previous value. We are constructing .
Assignment Operator General Outline
When a class contains a handle to external data, the assignment operator ( operator= ) generally follows this pattern:
- Get rid of the old external data ( delete memory, close network socket, delete temporary file, unlock semaphore, etc.).
- This is a silly class that stores a float in dynamic memory.
- It works—so far.
Try Copy Constructor
- That failed because the default copy ctor just copied the pointer.
- Both objects’ dtors tried to free the same pointer … boom!
- We deserve that, because we didn’t follow the general outline for a copy constructor. We got the default copy ctor.
Fixed Copy Constructor
- The new copy ctor allocated more space and copied the data, not just the pointer.
Try Assignment Operator
- The default assignment operator has the same problem.
- Also, relying upon the compiler-created operator= when you've written the copy ctor is deprecated.
Fix Assignment Operator
- The assignment operator now allocates space & copies the data.
- Whaaaaaaaaaat?
Explanation
What happens inside a = a ?
- First, delete the old data.
- Second, copy the new data (that we just freed).
- We must copy the data, but, sometimes, we can’t !
- If this is self-assignment, don’t do anything.
- How to detect that?
- See if the address of the left-hand-side ( this ) equals the address of the right-hand-side ( &rhs ).
- The & in the declaration const HeapFloat &rhs means “reference to”, whereas the & in the expression this != &rhs means ”address of”.
This Sounds Silly
- Yeah, but, … so what? Nobody will ever really do that.
- Not explicitly, no. But it might happen indirectly.
- Find the smallest element.
- Swap it with the first element.
- What if the first element is the smallest element?
- You’ll try to assign [0] to [0] ! Self-assignment!
Not very DRY
- new float (*rhs.data) appears twice— WET !
The copy ctor calls operator= ; as complex objects often do. Initialize data = nullptr so the initial delete data is ok.
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assignment operators
Assignment operators, what is “self assignment”.
Self assignment is when someone assigns an object to itself. For example,
Obviously no one ever explicitly does a self assignment like the above, but since more than one pointer or reference can point to the same object (aliasing), it is possible to have self assignment without knowing it:
This is only valid for copy assignment. Self-assignment is not valid for move assignment.
Why should I worry about “self assignment”?
If you don’t worry about self assignment , you’ll expose your users to some very subtle bugs that have very subtle and often disastrous symptoms. For example, the following class will cause a complete disaster in the case of self-assignment:
If someone assigns a Fred object to itself, line #1 deletes both this->p_ and f.p_ since *this and f are the same object. But line #2 uses *f.p_ , which is no longer a valid object. This will likely cause a major disaster.
The bottom line is that you the author of class Fred are responsible to make sure self-assignment on a Fred object is innocuous . Do not assume that users won’t ever do that to your objects. It is your fault if your object crashes when it gets a self-assignment.
Aside: the above Fred::operator= (const Fred&) has a second problem: If an exception is thrown while evaluating new Wilma(*f.p_) (e.g., an out-of-memory exception or an exception in Wilma ’s copy constructor ), this->p_ will be a dangling pointer — it will point to memory that is no longer valid. This can be solved by allocating the new objects before deleting the old objects.
Okay, okay, already; I’ll handle self-assignment. How do I do it?
You should worry about self assignment every time you create a class . This does not mean that you need to add extra code to all your classes: as long as your objects gracefully handle self assignment, it doesn’t matter whether you had to add extra code or not.
We will illustrate the two cases using the assignment operator in the previous FAQ :
If self-assignment can be handled without any extra code, don’t add any extra code. But do add a comment so others will know that your assignment operator gracefully handles self-assignment:
Example 1a:
Example 1b:
If you need to add extra code to your assignment operator, here’s a simple and effective technique:
Or equivalently:
By the way: the goal is not to make self-assignment fast. If you don’t need to explicitly test for self-assignment, for example, if your code works correctly (even if slowly) in the case of self-assignment, then do not put an if test in your assignment operator just to make the self-assignment case fast. The reason is simple: self-assignment is almost always rare, so it merely needs to be correct - it does not need to be efficient. Adding the unnecessary if statement would make a rare case faster by adding an extra conditional-branch to the normal case, punishing the many to benefit the few.
In this case, however, you should add a comment at the top of your assignment operator indicating that the rest of the code makes self-assignment is benign, and that is why you didn’t explicitly test for it. That way future maintainers will know to make sure self-assignment stays benign, or if not, they will need to add the if test.
I’m creating a derived class; should my assignment operators call my base class’s assignment operators?
Yes (if you need to define assignment operators in the first place).
If you define your own assignment operators, the compiler will not automatically call your base class’s assignment operators for you. Unless your base class’s assignment operators themselves are broken, you should call them explicitly from your derived class’s assignment operators (again, assuming you create them in the first place).
However if you do not create your own assignment operators, the ones that the compiler create for you will automatically call your base class’s assignment operators.
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C Programming Assignment Operators
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What is an assignment operator in c.
Assignment Operators in C are used to assign values to the variables. They come under the category of binary operators as they require two operands to operate upon. The left side operand is called a variable and the right side operand is the value. The value on the right side of the "=" is assigned to the variable on the left side of "=". The value on the right side must be of the same data type as the variable on the left side. Hence, the associativity is from right to left.
In this C tutorial , we'll understand the types of C programming assignment operators with examples. To delve deeper you can enroll in our C Programming Course .
Before going in-depth about assignment operators you must know about operators in C. If you haven't visited the Operators in C tutorial, refer to Operators in C: Types of Operators .
Types of Assignment Operators in C
There are two types of assignment operators in C:
| += | addition assignment | It adds the right operand to the left operand and assigns the result to the left operand. |
| -= | subtraction assignment | It subtracts the right operand from the left operand and assigns the result to the left operand. |
| *= | multiplication assignment | It multiplies the right operand with the left operand and assigns the result to the left operand |
| /= | division assignment | It divides the left operand with the right operand and assigns the result to the left operand. |
| %= | modulo assignment | It takes modulus using two operands and assigns the result to the left operand. |
Example of Augmented Arithmetic and Assignment Operators
There can be five combinations of bitwise operators with the assignment operator, "=". Let's look at them one by one.
| &= | bitwise AND assignment | It performs the bitwise AND operation on the variable with the value on the right |
| |= | bitwise OR assignment | It performs the bitwise OR operation on the variable with the value on the right |
| ^= | bitwise XOR assignment | It performs the bitwise XOR operation on the variable with the value on the right |
| <<= | bitwise left shift assignment | Shifts the bits of the variable to the left by the value on the right |
| >>= | bitwise right shift assignment | Shifts the bits of the variable to the right by the value on the right |
Example of Augmented Bitwise and Assignment Operators
Practice problems on assignment operators in c, 1. what will the value of "x" be after the execution of the following code.
The correct answer is 52. x starts at 50, increases by 5 to 55, then decreases by 3 to 52.
2. After executing the following code, what is the value of the number variable?
The correct answer is 144. After right-shifting 73 (binary 1001001) by one and then left-shifting the result by two, the value becomes 144 (binary 10010000).
Benefits of Using Assignment Operators
- Simplifies Code: For example, x += 1 is shorter and clearer than x = x + 1.
- Reduces Errors: They break complex expressions into simpler, more manageable parts thus reducing errors.
- Improves Readability: They make the code easier to read and understand by succinctly expressing common operations.
- Enhances Performance: They often operate in place, potentially reducing the need for additional memory or temporary variables.
Best Practices and Tips for Using the Assignment Operator
While performing arithmetic operations with the same variable, use compound assignment operators
- Initialize Variables When Declaring int count = 0 ; // Initialization
- Avoid Complex Expressions in Assignments a = (b + c) * (d - e); // Consider breaking it down: int temp = b + c; a = temp * (d - e);
- Avoid Multiple Assignments in a Single Statement // Instead of this a = b = c = 0 ; // Do this a = 0 ; b = 0 ; c = 0 ;
- Consistent Formatting int result = 0 ; result += 10 ;
When mixing assignments with other operations, use parentheses to ensure the correct order of evaluation.
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Assignment Operators in C
In C language, the assignment operator stores a certain value in an already declared variable. A variable in C can be assigned the value in the form of a literal, another variable, or an expression.
The value to be assigned forms the right-hand operand, whereas the variable to be assigned should be the operand to the left of the " = " symbol, which is defined as a simple assignment operator in C.
In addition, C has several augmented assignment operators.
The following table lists the assignment operators supported by the C language −
| Operator | Description | Example |
|---|---|---|
| = | Simple assignment operator. Assigns values from right side operands to left side operand | C = A + B will assign the value of A + B to C |
| += | Add AND assignment operator. It adds the right operand to the left operand and assign the result to the left operand. | C += A is equivalent to C = C + A |
| -= | Subtract AND assignment operator. It subtracts the right operand from the left operand and assigns the result to the left operand. | C -= A is equivalent to C = C - A |
| *= | Multiply AND assignment operator. It multiplies the right operand with the left operand and assigns the result to the left operand. | C *= A is equivalent to C = C * A |
| /= | Divide AND assignment operator. It divides the left operand with the right operand and assigns the result to the left operand. | C /= A is equivalent to C = C / A |
| %= | Modulus AND assignment operator. It takes modulus using two operands and assigns the result to the left operand. | C %= A is equivalent to C = C % A |
| <<= | Left shift AND assignment operator. | C <<= 2 is same as C = C << 2 |
| >>= | Right shift AND assignment operator. | C >>= 2 is same as C = C >> 2 |
| &= | Bitwise AND assignment operator. | C &= 2 is same as C = C & 2 |
| ^= | Bitwise exclusive OR and assignment operator. | C ^= 2 is same as C = C ^ 2 |
| |= | Bitwise inclusive OR and assignment operator. | C |= 2 is same as C = C | 2 |
Simple Assignment Operator (=)
The = operator is one of the most frequently used operators in C. As per the ANSI C standard, all the variables must be declared in the beginning. Variable declaration after the first processing statement is not allowed.
You can declare a variable to be assigned a value later in the code, or you can initialize it at the time of declaration.
You can use a literal, another variable, or an expression in the assignment statement.
Once a variable of a certain type is declared, it cannot be assigned a value of any other type. In such a case the C compiler reports a type mismatch error.
In C, the expressions that refer to a memory location are called "lvalue" expressions. A lvalue may appear as either the left-hand or right-hand side of an assignment.
On the other hand, the term rvalue refers to a data value that is stored at some address in memory. A rvalue is an expression that cannot have a value assigned to it which means an rvalue may appear on the right-hand side but not on the left-hand side of an assignment.
Variables are lvalues and so they may appear on the left-hand side of an assignment. Numeric literals are rvalues and so they may not be assigned and cannot appear on the left-hand side. Take a look at the following valid and invalid statements −
Augmented Assignment Operators
In addition to the = operator, C allows you to combine arithmetic and bitwise operators with the = symbol to form augmented or compound assignment operator. The augmented operators offer a convenient shortcut for combining arithmetic or bitwise operation with assignment.
For example, the expression "a += b" has the same effect of performing "a + b" first and then assigning the result back to the variable "a".
Run the code and check its output −
Similarly, the expression "a <<= b" has the same effect of performing "a << b" first and then assigning the result back to the variable "a".
Here is a C program that demonstrates the use of assignment operators in C −
When you compile and execute the above program, it will produce the following result −
cppreference.com
Assignment operators.
| (C11) | ||||
| Miscellaneous | ||||
| General | ||||
| (C11) | ||||
| (C99) | ||||
Assignment and compound assignment operators are binary operators that modify the variable to their left using the value to their right.
| Operator | Operator name | Example | Description | Equivalent of |
|---|---|---|---|---|
| = | basic assignment | a = b | becomes equal to | |
| += | addition assignment | a += b | becomes equal to the addition of and | a = a + b |
| -= | subtraction assignment | a -= b | becomes equal to the subtraction of from | a = a - b |
| *= | multiplication assignment | a *= b | becomes equal to the product of and | a = a * b |
| /= | division assignment | a /= b | becomes equal to the division of by | a = a / b |
| %= | modulo assignment | a %= b | becomes equal to the remainder of divided by | a = a % b |
| &= | bitwise AND assignment | a &= b | becomes equal to the bitwise AND of and | a = a & b |
| |= | bitwise OR assignment | a |= b | becomes equal to the bitwise OR of and | a = a | b |
| ^= | bitwise XOR assignment | a ^= b | becomes equal to the bitwise XOR of and | a = a ^ b |
| <<= | bitwise left shift assignment | a <<= b | becomes equal to left shifted by | a = a << b |
| >>= | bitwise right shift assignment | a >>= b | becomes equal to right shifted by | a = a >> b |
| Simple assignment Notes Compound assignment References See Also See also |
[ edit ] Simple assignment
The simple assignment operator expressions have the form
| lhs rhs | |||||||||
| lhs | - | expression of any complete object type |
| rhs | - | expression of any type to lhs or with lhs |
Assignment performs implicit conversion from the value of rhs to the type of lhs and then replaces the value in the object designated by lhs with the converted value of rhs .
Assignment also returns the same value as what was stored in lhs (so that expressions such as a = b = c are possible). The value category of the assignment operator is non-lvalue (so that expressions such as ( a = b ) = c are invalid).
rhs and lhs must satisfy one of the following:
- both lhs and rhs have compatible struct or union type, or..
- rhs must be implicitly convertible to lhs , which implies
- both lhs and rhs have arithmetic types , in which case lhs may be volatile -qualified or atomic (since C11)
- both lhs and rhs have pointer to compatible (ignoring qualifiers) types, or one of the pointers is a pointer to void, and the conversion would not add qualifiers to the pointed-to type. lhs may be volatile or restrict (since C99) -qualified or atomic (since C11) .
- lhs is a (possibly qualified or atomic (since C11) ) pointer and rhs is a null pointer constant such as NULL or a nullptr_t value (since C23)
| has type (possibly qualified or atomic(since C11)) _Bool and rhs is a pointer or a value(since C23) | (since C99) |
| has type (possibly qualified or atomic) and rhs has type | (since C23) |
[ edit ] Notes
If rhs and lhs overlap in memory (e.g. they are members of the same union), the behavior is undefined unless the overlap is exact and the types are compatible .
Although arrays are not assignable, an array wrapped in a struct is assignable to another object of the same (or compatible) struct type.
The side effect of updating lhs is sequenced after the value computations, but not the side effects of lhs and rhs themselves and the evaluations of the operands are, as usual, unsequenced relative to each other (so the expressions such as i = ++ i ; are undefined)
Assignment strips extra range and precision from floating-point expressions (see FLT_EVAL_METHOD ).
In C++, assignment operators are lvalue expressions, not so in C.
[ edit ] Compound assignment
The compound assignment operator expressions have the form
| lhs op rhs | |||||||||
| op | - | one of *=, /= %=, += -=, <<=, >>=, &=, ^=, |= |
| lhs, rhs | - | expressions with (where lhs may be qualified or atomic), except when op is += or -=, which also accept pointer types with the same restrictions as + and - |
The expression lhs @= rhs is exactly the same as lhs = lhs @ ( rhs ) , except that lhs is evaluated only once.
| If lhs has type, the operation behaves as a single atomic read-modify-write operation with memory order . For integer atomic types, the compound assignment @= is equivalent to: addr = &lhs; T2 val = rhs; T1 old = *addr; T1 new; do { new = old @ val } while (! (addr, &old, new); | (since C11) |
[ edit ] References
- C17 standard (ISO/IEC 9899:2018):
- 6.5.16 Assignment operators (p: 72-73)
- C11 standard (ISO/IEC 9899:2011):
- 6.5.16 Assignment operators (p: 101-104)
- C99 standard (ISO/IEC 9899:1999):
- 6.5.16 Assignment operators (p: 91-93)
- C89/C90 standard (ISO/IEC 9899:1990):
- 3.3.16 Assignment operators
[ edit ] See Also
Operator precedence
| Common operators | ||||||
|---|---|---|---|---|---|---|
| a = b | ++a | +a | !a | a == b | a[b] | a(...) |
[ edit ] See also
| for Assignment operators |
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To answer my rhetorical question: It means that a well-designed assignment operator should not need to check for self-assignment. Assigning an object to itself should work correctly (i.e. have the end-effect of "doing nothing") without performing an explicit check for self-assignment.
Prerequisite: Operator Overloading The assignment operator,"=", is the operator used for Assignment. It copies the right value into the left value. Assignment Operators are predefined to operate only on built-in Data types. Assignment operator overloading is binary operator overloading.Overloading assignment operator in C++ copies all values of one
Assignment Operator General Outline. When a class contains a handle to external data, the assignment operator ( operator=) generally follows this pattern: Get rid of the old external data ( delete memory, close network socket, delete temporary file, unlock semaphore, etc.). Copy the resource from the other object to this object.
If self-assignment can be handled without any extra code, don't add any extra code. But do add a comment so others will know that your assignment operator gracefully handles self-assignment: Example 1a: Fred& Fred::operator= (const Fred& f) {. // This gracefully handles self assignment. *p_ = *f.p_; return *this;
Handling self assignment Understanding the return value. Copy Assignment I don't want to go through the gory details of how hard it is to write the truly optimal copy assignment operator. Instead, let's use the "copy and swap" idiom to do save ourselves the trouble!
Assignment Operators in C are used to assign values to the variables. They come under the category of binary operators as they require two operands to operate upon. The left side operand is called a variable and the right side operand is the value. The value on the right side of the "=" is assigned to the variable on the left side of "=".
for assignments to class type objects, the right operand could be an initializer list only when the assignment is defined by a user-defined assignment operator. removed user-defined assignment constraint. CWG 1538. C++11. E1 ={E2} was equivalent to E1 = T(E2) ( T is the type of E1 ), this introduced a C-style cast. it is equivalent to E1 = T{E2}
For example, adding the above if test to the Fred assignment operator would make the non-self assignment case slightly less efficient (an extra (and unnecessary) conditional branch). If self assignment actually occured once in a thousand times, the if would waste cycles 99.9% of the time.
C Structures. The structure in C is a user-defined data type that can be used to group items of possibly different types into a single type. The struct keyword is used to define the structure in the C programming language. The items in the structure are called its member and they can be of any valid data type.
1. "=": This is the simplest assignment operator. This operator is used to assign the value on the right to the variable on the left. Example: a = 10; b = 20; ch = 'y'; 2. "+=": This operator is combination of '+' and '=' operators. This operator first adds the current value of the variable on left to the value on the right and ...
Simple assignment operator. Assigns values from right side operands to left side operand. C = A + B will assign the value of A + B to C. +=. Add AND assignment operator. It adds the right operand to the left operand and assign the result to the left operand. C += A is equivalent to C = C + A. -=.
Assignment performs implicit conversion from the value of rhs to the type of lhs and then replaces the value in the object designated by lhs with the converted value of rhs . Assignment also returns the same value as what was stored in lhs (so that expressions such as a = b = c are possible). The value category of the assignment operator is non ...
From Marshall Cline: Bjarne Stroustrup, Herb Sutter, Andrei Alexandrescu, Pearson / Addison-Wesley Publishers and I collaborated to create a new C++ Super-FAQ!It's a team effort, with huge contributions from each of us and with amazing support from dozens of brilliant editors.
From Marshall Cline: Bjarne Stroustrup, Herb Sutter, Andrei Alexandrescu, Pearson / Addison-Wesley Publishers and I collaborated to create a new C++ Super-FAQ! It's a team effort, with huge contributions from each of us and with amazing support from dozens of brilliant editors. The result is "awesomer" than ever!
The first approach is more standard so if you see the second in practice it is likely a good developer will question if this is self-assignment safe or not. If you had a different structure: struct Foo {. std::shared_ptr<Bar> m_p; }; That would also be self-assignment safe if you just use m_p = src.m_p; .
Writing copy-assignment operators that are safe for self-assignment is in the C++ core guidelines and for a good reason. Running into a self-assignment situation by accident is much easier than some of the sarcastic comments here suggest, e.g. when iterating over STL containers without giving it much thought:
Policy Name Policy Description Database Resource Data Set Condition Name Action; Grant on Business Unit. Requisition Self Service user can manage requisition in primary BU by REST resources for themselves for the financial business unit associated with their primary assignment.
Self Referential structures are those structures that have one or more pointers which point to the same type of structure, as their member. In other words, structures pointing to the same type of structures are self-referential in nature. Example: In the above example 'link' is a pointer to a structure of type 'node'.
self,canonical. onlyData: boolean The resource item payload will be filtered in order to contain only data (no links section, for example). Header Parameters ... The following example shows how to retrieve a task assignment by submitting a GET request on the REST resource using cURL.
The sourcing assignment levels that you select when you create sourcing assignments in an assignment set formulate a sourcing hierarchy for that assignment set. The backlog planning process uses the sourcing hierarchy to determine which sourcing rule or bill of distribution to follow to find a source for a specific item. It always uses the most specific sourcing rule or bill of distribution ...
Assignment copies the reference to an object, not the object contents. No customizable code runs as part of an assignment to a variable holding an object reference, ever. This is also true for structs. In C++ assignment is customizable, in C# it is not. It is safe to assign the same object reference to a variable already holding it:
Get a standard California noncommercial Class C driver's license (DL) (a temporary/interim DL is acceptable). Complete an online CDL Application. Visit a DMV office, where you will: Submit a completed 10 Year History Record Check (DL 939) (if you have been issued a DL of any kind in another state or jurisdiction in the last 10 years).
2. Self assignment checking is done by comparing addresses of the two objects. C++ has a built in operator== for pointers of the same type. Your code is comparing the values of the two objects, and clearly you haven't defined operator== for your Person class. But comparing the values is not the right thing to do because there's no special ...
One of Mr. Kubitschek's essays, is called "Self Trivialization." It lays out a strategy for attracting supporters. The first step is to make verbal "bridgeheads" by using controversial ...
self,canonical. onlyData: boolean The resource item payload will be filtered in order to contain only data (no links section, for example). Header Parameters ... The following example shows how to retrieve a user task assignment by submitting a GET request on the REST resource using cURL.
The manager is common for all of the worker's assignments. You define this manager as a line manager. The worker also has a temporary project manager who manages a particular assignment but may not manage all of the worker's assignments. You define the project manager as an additional manager (for example, as a project manager) in relevant ...
There is a (narrow) exception to the rule above: The case of your move-assignment operator being 'idempotent' for self-assignment. For example, if your assignment operator only involves assignment of the members - then it's safe to self-assign just like a regular assignment (trusting that the members' self-assignment implementations are valid). ...