define assignment operators in c

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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 −

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 −

Home » Learn C Programming from Scratch » C Assignment Operators

C Assignment Operators

Summary : in this tutorial, you’ll learn about the C assignment operators and how to use them effectively.

Introduction to the C assignment operators

An assignment operator assigns the vale of the right-hand operand to the left-hand operand. The following example uses the assignment operator (=) to assign 1 to the counter variable:

After the assignmment, the counter variable holds the number 1.

The following example adds 1 to the counter and assign the result to the counter:

The = assignment operator is called a simple assignment operator. It assigns the value of the left operand to the right operand.

Besides the simple assignment operator, C supports compound assignment operators. A compound assignment operator performs the operation specified by the additional operator and then assigns the result to the left operand.

The following example uses a compound-assignment operator (+=):

The expression:

is equivalent to the following expression:

The following table illustrates the compound-assignment operators in C:

• A simple assignment operator assigns the value of the left operand to the right operand.
• A compound assignment operator performs the operation specified by the additional operator and then assigns the result to the left operand.

Assignment Operators in C

Assignment Operators in C Example

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1. Simple Assignment Operator (=)

Example of simple assignment operator.

2. Compound Assignment Operators

Example of Augmented Arithmetic and Assignment Operators

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, 2. after executing the following code, what is the value of the number variable, benefits of using assignment operators, best practices and tips for using the assignment operator, live classes schedule.

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C Programming Tutorial

• Assignment Operator in C

Last updated on July 27, 2020

We have already used the assignment operator ( = ) several times before. Let's discuss it here in detail. The assignment operator ( = ) is used to assign a value to the variable. Its general format is as follows:

The operand on the left side of the assignment operator must be a variable and operand on the right-hand side must be a constant, variable or expression. Here are some examples:

The precedence of the assignment operator is lower than all the operators we have discussed so far and it associates from right to left.

We can also assign the same value to multiple variables at once.

here x , y and z are initialized to 100 .

Since the associativity of the assignment operator ( = ) is from right to left. The above expression is equivalent to the following:

Note that expressions like:

are called assignment expression. If we put a semicolon( ; ) at the end of the expression like this:

then the assignment expression becomes assignment statement.

Compound Assignment Operator #

Assignment operations that use the old value of a variable to compute its new value are called Compound Assignment.

Consider the following two statements:

Here the second statement adds 5 to the existing value of x . This value is then assigned back to x . Now, the new value of x is 105 .

To handle such operations more succinctly, C provides a special operator called Compound Assignment operator.

The general format of compound assignment operator is as follows:

where op can be any of the arithmetic operators ( + , - , * , / , % ). The above statement is functionally equivalent to the following:

Note : In addition to arithmetic operators, op can also be >> (right shift), << (left shift), | (Bitwise OR), & (Bitwise AND), ^ (Bitwise XOR). We haven't discussed these operators yet.

After evaluating the expression, the op operator is then applied to the result of the expression and the current value of the variable (on the RHS). The result of this operation is then assigned back to the variable (on the LHS). Let's take some examples: The statement:

is equivalent to x = x + 5; or x = x + (5); .

Similarly, the statement:

is equivalent to x = x * 2; or x = x * (2); .

Since, expression on the right side of op operator is evaluated first, the statement:

is equivalent to x = x * (y + 1) .

The precedence of compound assignment operators are same and they associate from right to left (see the precedence table ).

The following table lists some Compound assignment operators:

The following program demonstrates Compound assignment operators in action:

Expected Output:

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Assignment operator in c.

Last Updated on June 23, 2023 by Prepbytes

This type of operator is employed for transforming and assigning values to variables within an operation. In an assignment operation, the right side represents a value, while the left side corresponds to a variable. It is essential that the value on the right side has the same data type as the variable on the left side. If this requirement is not fulfilled, the compiler will issue an error.

What is Assignment Operator in C language?

In C, the assignment operator serves the purpose of assigning a value to a variable. It is denoted by the equals sign (=) and plays a vital role in storing data within variables for further utilization in code. When using the assignment operator, the value present on the right-hand side is assigned to the variable on the left-hand side. This fundamental operation allows developers to store and manipulate data effectively throughout their programs.

Example of Assignment Operator in C

For example, consider the following line of code:

Types of Assignment Operators in C

Here is a list of the assignment operators that you can find in the C language:

Simple assignment operator (=): This is the basic assignment operator, which assigns the value on the right-hand side to the variable on the left-hand side.

Addition assignment operator (+=): This operator adds the value on the right-hand side to the variable on the left-hand side and assigns the result back to the variable.

x += 3; // Equivalent to x = x + 3; (adds 3 to the current value of "x" and assigns the result back to "x")

Subtraction assignment operator (-=): This operator subtracts the value on the right-hand side from the variable on the left-hand side and assigns the result back to the variable.

x -= 4; // Equivalent to x = x – 4; (subtracts 4 from the current value of "x" and assigns the result back to "x")

* Multiplication assignment operator ( =):** This operator multiplies the value on the right-hand side with the variable on the left-hand side and assigns the result back to the variable.

x = 2; // Equivalent to x = x 2; (multiplies the current value of "x" by 2 and assigns the result back to "x")

Division assignment operator (/=): This operator divides the variable on the left-hand side by the value on the right-hand side and assigns the result back to the variable.

x /= 2; // Equivalent to x = x / 2; (divides the current value of "x" by 2 and assigns the result back to "x")

Bitwise AND assignment (&=): The bitwise AND assignment operator "&=" performs a bitwise AND operation between the value on the left-hand side and the value on the right-hand side. It then assigns the result back to the left-hand side variable.

x &= 3; // Binary: 0011 // After bitwise AND assignment: x = 1 (Binary: 0001)

Bitwise OR assignment (|=): The bitwise OR assignment operator "|=" performs a bitwise OR operation between the value on the left-hand side and the value on the right-hand side. It then assigns the result back to the left-hand side variable.

x |= 3; // Binary: 0011 // After bitwise OR assignment: x = 7 (Binary: 0111)

Bitwise XOR assignment (^=): The bitwise XOR assignment operator "^=" performs a bitwise XOR operation between the value on the left-hand side and the value on the right-hand side. It then assigns the result back to the left-hand side variable.

x ^= 3; // Binary: 0011 // After bitwise XOR assignment: x = 6 (Binary: 0110)

Left shift assignment (<<=): The left shift assignment operator "<<=" shifts the bits of the value on the left-hand side to the left by the number of positions specified by the value on the right-hand side. It then assigns the result back to the left-hand side variable.

x <<= 2; // Binary: 010100 (Shifted left by 2 positions) // After left shift assignment: x = 20 (Binary: 10100)

Right shift assignment (>>=): The right shift assignment operator ">>=" shifts the bits of the value on the left-hand side to the right by the number of positions specified by the value on the right-hand side. It then assigns the result back to the left-hand side variable.

x >>= 2; // Binary: 101 (Shifted right by 2 positions) // After right shift assignment: x = 5 (Binary: 101)

Conclusion The assignment operator in C, denoted by the equals sign (=), is used to assign a value to a variable. It is a fundamental operation that allows programmers to store data in variables for further use in their code. In addition to the simple assignment operator, C provides compound assignment operators that combine arithmetic or bitwise operations with assignment, allowing for concise and efficient code.

FAQs related to Assignment Operator in C

Q1. Can I assign a value of one data type to a variable of another data type? In most cases, assigning a value of one data type to a variable of another data type will result in a warning or error from the compiler. It is generally recommended to assign values of compatible data types to variables.

Q2. What is the difference between the assignment operator (=) and the comparison operator (==)? The assignment operator (=) is used to assign a value to a variable, while the comparison operator (==) is used to check if two values are equal. It is important not to confuse these two operators.

Q3. Can I use multiple assignment operators in a single statement? No, it is not possible to use multiple assignment operators in a single statement. Each assignment operator should be used separately for assigning values to different variables.

Q4. Are there any limitations on the right-hand side value of the assignment operator? The right-hand side value of the assignment operator should be compatible with the data type of the left-hand side variable. If the data types are not compatible, it may lead to unexpected behavior or compiler errors.

Q5. Can I assign the result of an expression to a variable using the assignment operator? Yes, it is possible to assign the result of an expression to a variable using the assignment operator. For example, x = y + z; assigns the sum of y and z to the variable x.

Q6. What happens if I assign a value to an uninitialized variable? Assigning a value to an uninitialized variable will initialize it with the assigned value. However, it is considered good practice to explicitly initialize variables before using them to avoid potential bugs or unintended behavior.

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Operators in C

In C language, operators are symbols that represent operations to be performed on one or more operands. They are the basic components of the C programming. In this article, we will learn about all the built-in operators in C with examples.

What is a C Operator?

An operator in C can be defined as the symbol that helps us to perform some specific mathematical, relational, bitwise, conditional, or logical computations on values and variables. The values and variables used with operators are called operands. So we can say that the operators are the symbols that perform operations on operands.

For example,

Here, ‘+’ is the operator known as the addition operator, and ‘a’ and ‘b’ are operands. The addition operator tells the compiler to add both of the operands ‘a’ and ‘b’.

Types of Operators in C

C language provides a wide range of operators that can be classified into 6 types based on their functionality:

• Arithmetic Operators
• Relational Operators
• Logical Operators
• Bitwise Operators
• Assignment Operators
• Other Operators

1. Arithmetic Operations in C

The arithmetic operators are used to perform arithmetic/mathematical operations on operands. There are 9 arithmetic operators in C language:

Example of C Arithmetic Operators

2. relational operators in c.

The relational operators in C are used for the comparison of the two operands. All these operators are binary operators that return true or false values as the result of comparison.

These are a total of 6 relational operators in C:

Example of C Relational Operators

Here, 0 means false and 1 means true.

3. Logical Operator in C

Logical Operators are used to combine two or more conditions/constraints or to complement the evaluation of the original condition in consideration. The result of the operation of a logical operator is a Boolean value either true or false .

Example of Logical Operators in C

4. bitwise operators in c.

The Bitwise operators are used to perform bit-level operations on the operands. The operators are first converted to bit-level and then the calculation is performed on the operands. Mathematical operations such as addition, subtraction, multiplication, etc. can be performed at the bit level for faster processing.

There are 6 bitwise operators in C:

Example of Bitwise Operators

5. assignment operators in c.

Assignment operators are used to assign value to a variable. The left side operand of the assignment operator is a variable and the right side operand of the assignment operator is a value. The value on the right side must be of the same data type as the variable on the left side otherwise the compiler will raise an error.

The assignment operators can be combined with some other operators in C to provide multiple operations using single operator. These operators are called compound operators.

In C, there are 11 assignment operators :

Example of C Assignment Operators

6. other operators.

Apart from the above operators, there are some other operators available in C used to perform some specific tasks. Some of them are discussed here: 

sizeof Operator

• sizeof is much used in the C programming language.
• It is a compile-time unary operator which can be used to compute the size of its operand.
• The result of sizeof is of the unsigned integral type which is usually denoted by size_t.
• Basically, the sizeof the operator is used to compute the size of the variable or datatype.

To know more about the topic refer to this article.

Comma Operator ( , )

• The comma operator (represented by the token) is a binary operator that evaluates its first operand and discards the result, it then evaluates the second operand and returns this value (and type).
• The comma operator has the lowest precedence of any C operator.
• Comma acts as both operator and separator. 

Conditional Operator ( ? : )

• The conditional operator is the only ternary operator in C++.
• Here, Expression1 is the condition to be evaluated. If the condition(Expression1) is True then we will execute and return the result of Expression2 otherwise if the condition(Expression1) is false then we will execute and return the result of Expression3.
• We may replace the use of if..else statements with conditional operators.

dot (.) and arrow (->) Operators

• Member operators are used to reference individual members of classes, structures, and unions.
• The dot operator is applied to the actual object. 
• The arrow operator is used with a pointer to an object.

To know more about dot operators refer to this article and to know more about arrow(->) operators refer to this article.

Cast Operator

• Casting operators convert one data type to another. For example, int(2.2000) would return 2.
• A cast is a special operator that forces one data type to be converted into another. 
• The most general cast supported by most of the C compilers is as follows −   [ (type) expression ] .

addressof (&) and Dereference (*) Operators

• Pointer operator & returns the address of a variable. For example &a; will give the actual address of the variable.
• The pointer operator * is a pointer to a variable. For example *var; will pointer to a variable var. 

Example of Other C Operators

Unary, binary and ternary operators in c.

Operators can also be classified into three types on the basis of the number of operands they work on:

• Unary Operators: Operators that work on single operand.
• Binary Operators: Operators that work on two operands.
• Ternary Operators: Operators that work on three operands.

Operator Precedence and Associativity in C

In C, it is very common for an expression or statement to have multiple operators and in these expression, there should be a fixed order or priority of operator evaluation to avoid ambiguity.

Operator Precedence and Associativity is the concept that decides which operator will be evaluated first in the case when there are multiple operators present in an expression.

The below table describes the precedence order and associativity of operators in C. The precedence of the operator decreases from top to bottom. 

To know more about operator precedence and associativity, refer to this article – Operator Precedence and Associativity in C

In this article, the points we learned about the operator are as follows:

• Operators are symbols used for performing some kind of operation in C.
• There are six types of operators, Arithmetic Operators, Relational Operators, Logical Operators, Bitwise Operators, Assignment Operators, and Miscellaneous Operators.
• Operators can also be of type unary, binary, and ternary according to the number of operators they are using.
• Every operator returns a numerical value except logical, relational, and conditional operator which returns a boolean value (true or false).
• There is a Precedence in the operators means the priority of using one operator is greater than another operator.

FAQs on C Operators

Q1. what are operators in c.

Operators in C are certain symbols in C used for performing certain mathematical, relational, bitwise, conditional, or logical operations for the user.

Q2. What are the 7 types of operators in C?

There are 7 types of operators in C as mentioned below: Unary operator Arithmetic operator Relational operator Logical operator Bitwise operator Assignment operator Conditional operator

Q3. What is the difference between the ‘=’ and ‘==’ operators?

‘=’ is a type of assignment operator that places the value in right to the variable on left, Whereas ‘==’ is a type of relational operator that is used to compare two elements if the elements are equal or not.

Q4. What is the difference between prefix and postfix operators in C?

In prefix operations, the value of a variable is incremented/decremented first and then the new value is used in the operation, whereas, in postfix operations first the value of the variable is used in the operation and then the value is incremented/decremented. Example: b=c=10; a=b++; // a==10 a=++c; // a==11

Q5. What is the Modulo operator?

The Modulo operator(%) is used to find the remainder if one element is divided by another. Example: a % b (a divided by b) 5 % 2 == 1

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Assignment Operators in C

Assignment operators are used to assigning the result of an expression to a variable. Up to now, we have used the shorthand assignment operator “=”, which assigns the result of a right-hand expression to the left-hand variable. For example, in the expression x = y + z, the sum of y and z is assigned to x.

Another form of assignment operator is variable operator_symbol= expression ; which is equivalent to variable = variable operator_symbol expression;

We have the following different types of assignment and assignment short-hand operators.

Expected Output:

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C structures, c reference, c operators.

Operators are used to perform operations on variables and values.

In the example below, we use the + operator to add together two values:

Although the + operator is often used to add together two values, like in the example above, it can also be used to add together a variable and a value, or a variable and another variable:

C divides the operators into the following groups:

• Arithmetic operators
• Assignment operators
• Comparison operators
• Logical operators
• Bitwise operators

Arithmetic Operators

Arithmetic operators are used to perform common mathematical operations.

Assignment Operators

Assignment operators are used to assign values to variables.

In the example below, we use the assignment operator ( = ) to assign the value 10 to a variable called x :

The addition assignment operator ( += ) adds a value to a variable:

A list of all assignment operators:

Comparison Operators

Comparison operators are used to compare two values (or variables). This is important in programming, because it helps us to find answers and make decisions.

The return value of a comparison is either 1 or 0 , which means true ( 1 ) or false ( 0 ). These values are known as Boolean values , and you will learn more about them in the Booleans and If..Else chapter.

Comparison operators are used to compare two values.

Note: The return value of a comparison is either true ( 1 ) or false ( 0 ).

In the following example, we use the greater than operator ( > ) to find out if 5 is greater than 3:

A list of all comparison operators:

Logical Operators

You can also test for true or false values with logical operators.

Logical operators are used to determine the logic between variables or values, by combining multiple conditions:

C Exercises

Test yourself with exercises.

Fill in the blanks to multiply 10 with 5 , and print the result:

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Assignment operators

• 8 contributors

expression assignment-operator expression

assignment-operator : one of   =   *=   /=   %=   +=   -=   <<=   >>=   &=   ^=   |=

Assignment operators store a value in the object specified by the left operand. There are two kinds of assignment operations:

simple assignment , in which the value of the second operand is stored in the object specified by the first operand.

compound assignment , in which an arithmetic, shift, or bitwise operation is performed before storing the result.

All assignment operators in the following table except the = operator are compound assignment operators.

Assignment operators table

Operator keywords

Three of the compound assignment operators have keyword equivalents. They are:

C++ specifies these operator keywords as alternative spellings for the compound assignment operators. In C, the alternative spellings are provided as macros in the <iso646.h> header. In C++, the alternative spellings are keywords; use of <iso646.h> or the C++ equivalent <ciso646> is deprecated. In Microsoft C++, the /permissive- or /Za compiler option is required to enable the alternative spelling.

Simple assignment

The simple assignment operator ( = ) causes the value of the second operand to be stored in the object specified by the first operand. If both objects are of arithmetic types, the right operand is converted to the type of the left, before storing the value.

Objects of const and volatile types can be assigned to l-values of types that are only volatile , or that aren't const or volatile .

Assignment to objects of class type ( struct , union , and class types) is performed by a function named operator= . The default behavior of this operator function is to perform a member-wise copy assignment of the object's non-static data members and direct base classes; however, this behavior can be modified using overloaded operators. For more information, see Operator overloading . Class types can also have copy assignment and move assignment operators. For more information, see Copy constructors and copy assignment operators and Move constructors and move assignment operators .

An object of any unambiguously derived class from a given base class can be assigned to an object of the base class. The reverse isn't true because there's an implicit conversion from derived class to base class, but not from base class to derived class. For example:

Assignments to reference types behave as if the assignment were being made to the object to which the reference points.

For class-type objects, assignment is different from initialization. To illustrate how different assignment and initialization can be, consider the code

The preceding code shows an initializer; it calls the constructor for UserType2 that takes an argument of type UserType1 . Given the code

the assignment statement

can have one of the following effects:

Call the function operator= for UserType2 , provided operator= is provided with a UserType1 argument.

Call the explicit conversion function UserType1::operator UserType2 , if such a function exists.

Call a constructor UserType2::UserType2 , provided such a constructor exists, that takes a UserType1 argument and copies the result.

Compound assignment

The compound assignment operators are shown in the Assignment operators table . These operators have the form e1 op = e2 , where e1 is a non- const modifiable l-value and e2 is:

an arithmetic type

a pointer, if op is + or -

a type for which there exists a matching operator *op*= overload for the type of e1

The built-in e1 op = e2 form behaves as e1 = e1 op e2 , but e1 is evaluated only once.

Compound assignment to an enumerated type generates an error message. If the left operand is of a pointer type, the right operand must be of a pointer type, or it must be a constant expression that evaluates to 0. When the left operand is of an integral type, the right operand must not be of a pointer type.

Result of built-in assignment operators

The built-in assignment operators return the value of the object specified by the left operand after the assignment (and the arithmetic/logical operation in the case of compound assignment operators). The resultant type is the type of the left operand. The result of an assignment expression is always an l-value. These operators have right-to-left associativity. The left operand must be a modifiable l-value.

In ANSI C, the result of an assignment expression isn't an l-value. That means the legal C++ expression (a += b) += c isn't allowed in C.

Expressions with binary operators C++ built-in operators, precedence, and associativity C assignment operators

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cppreference.com

Assignment operators.

Assignment operators modify the value of the object.

[ edit ] Definitions

Copy assignment replaces the contents of the object a with a copy of the contents of b ( b is not modified). For class types, this is performed in a special member function, described in copy assignment operator .

For non-class types, copy and move assignment are indistinguishable and are referred to as direct assignment .

Compound assignment replace the contents of the object a with the result of a binary operation between the previous value of a and the value of b .

[ edit ] Assignment operator syntax

The assignment expressions have the form

• ↑ target-expr must have higher precedence than an assignment expression.
• ↑ new-value cannot be a comma expression, because its precedence is lower.

[ edit ] Built-in simple assignment operator

For the built-in simple assignment, the object referred to by target-expr is modified by replacing its value with the result of new-value . target-expr must be a modifiable lvalue.

The result of a built-in simple assignment is an lvalue of the type of target-expr , referring to target-expr . If target-expr is a bit-field , the result is also a bit-field.

[ edit ] Assignment from an expression

If new-value is an expression, it is implicitly converted to the cv-unqualified type of target-expr . When target-expr is a bit-field that cannot represent the value of the expression, the resulting value of the bit-field is implementation-defined.

If target-expr and new-value identify overlapping objects, the behavior is undefined (unless the overlap is exact and the type is the same).

In overload resolution against user-defined operators , for every type T , the following function signatures participate in overload resolution:

For every enumeration or pointer to member type T , optionally volatile-qualified, the following function signature participates in overload resolution:

For every pair A1 and A2 , where A1 is an arithmetic type (optionally volatile-qualified) and A2 is a promoted arithmetic type, the following function signature participates in overload resolution:

[ edit ] Built-in compound assignment operator

The behavior of every built-in compound-assignment expression target-expr   op   =   new-value is exactly the same as the behavior of the expression target-expr   =   target-expr   op   new-value , except that target-expr is evaluated only once.

The requirements on target-expr and new-value of built-in simple assignment operators also apply. Furthermore:

• For + = and - = , the type of target-expr must be an arithmetic type or a pointer to a (possibly cv-qualified) completely-defined object type .
• For all other compound assignment operators, the type of target-expr must be an arithmetic type.

In overload resolution against user-defined operators , for every pair A1 and A2 , where A1 is an arithmetic type (optionally volatile-qualified) and A2 is a promoted arithmetic type, the following function signatures participate in overload resolution:

For every pair I1 and I2 , where I1 is an integral type (optionally volatile-qualified) and I2 is a promoted integral type, the following function signatures participate in overload resolution:

For every optionally cv-qualified object type T , the following function signatures participate in overload resolution:

[ edit ] Example

Possible output:

[ edit ] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[ edit ] See also

Operator precedence

Operator overloading

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Class Assignment Operators

I made the following operator overloading test:

The assignment operator behaves as-expected, outputting the address of the other instance.

Now, how would I actually assign something from the other instance? For example, something like this:

• operator-overloading
• assignment-operator

• You don't need either, but it still looks odd that you have an assignment operator and a destructor, but no copy constructor. As per the Rule of Three, if you need either, you'll likely need all three. –  sbi Commented Dec 22, 2010 at 14:54
• @sbi Of course. This is just some test code, though. –  Maxpm Commented Dec 22, 2010 at 15:24
• Still, reflexes kick in when I see that. I also noted that you pass a std::string object per copy instead of const reference. You might want to read this . –  sbi Commented Dec 22, 2010 at 15:32

5 Answers 5

The code you've shown would do it. No one would consider it to be a particularly good implementation, though.

This conforms to what is expected of an assignment operator:

BTW, you talk about "other class", but you have only one class, and multiple instances of that class.

• Ben, actually it would be better to provide a swap() member function and call that. Nevertheless, this is better than assigning. –  sbi Commented Dec 22, 2010 at 14:21

The traditional canonical form of the assignment operator looks like this:

(you don't want to invoke the copy constructor for assignment, too) and it returns a reference to *this .

A naive implementation would assign each data member individually:

(Note that this is exactly what the compiler-generated assignment operator would do, so it's pretty useless to overload it. I take it that this is for exercising, though.)

A better approach would be to employ the Copy-And-Swap idiom . (If you find GMan's answer too overwhelming, try mine , which is less exhaustive. :) ) Note that C&S employs the copy constructor and destructor to do assignment and therefore requires the object to be passed per copy, as you had in your question:

• I know you know about copy-and-swap, why did you declare the parameter as a reference? –  Ben Voigt Commented Dec 22, 2010 at 13:51
• @Ben: Thanks. I've added a note that, using c&s, the object should be copied. Old habits die hard, I guess. (Oh, and I'm not sure what's a "ninja edit", BTW.) –  sbi Commented Dec 22, 2010 at 13:56
• In this case, it was a ninja edit because you made the changes Ben was suggesting as he was suggesting them. –  Bill Commented Dec 22, 2010 at 18:00

almost all said, a few notes:

• check for self-assignment, i.e. if (&other != this) // assign
• look here for an excellent guide on operator overloading

• 1 If your assignment operator needs a check for self-assignment, chances are there's a better implementation. Good implementations (like Copy-And-Swap) don't need that test (which puts the burden of checking for the rare case on every assignment). –  sbi Commented Dec 22, 2010 at 14:06
• 2 <shameless_plug> We also have an operator overloading FAQ here on SO now: stackoverflow.com/questions/4421706/operator-overloading . </shameless_plug> –  sbi Commented Dec 22, 2010 at 14:06
• @sbi: thanks for the ref, I'll read it one day ;). The one I mention is short and easy for beginners, giving just bare essentials. I'll also read up the C&S one day, but as for self-test overhead - seems that C&S has an overhead of copying and in many cases memory allocation (if your class contains strings, vectors etc.), so it should have a "handle with care" label, isn't it? –  davka Commented Dec 22, 2010 at 14:21
• 1 @davka: The one you linked to is questionable, though. Also, C&S has no overhead. I have explained why it doesn't. . In short: assignment is tearing down old state, and building up new state by copying data from another object. That's exactly what copy-constructor and destructor do, and C&S manages to employ them in the right order to be exception-safe. –  sbi Commented Dec 22, 2010 at 14:26
• 1 @davka: When swapping, you allocate for the new data, copy the new data, swap old and new data, and deallocate the old data. When assigning, you deallocate the old data, allocate for the new data, and copy the data (and you pray allocation won't fail and catch you with your pants down). But swapping is supposed to be O(1) and non-throwing, so it doesn't factor into the runtime. (For example, with std::vector swapping will swap two pointers. Comparing to the O(N) of copying and the O(VeryLooong) of allocation, this is neglectable.) –  sbi Commented Dec 22, 2010 at 16:30

Traditionnaly the assignment operator and the copy constructor are defined passing a const reference, and not with a copy by value mechanism.

EDIT: I corrected because I had put code that didnt return the TestClass& (c.f. @sbi 's answer)

• 1 The new common practice actually does pass the RHS by value. It's called the copy-and-swap idiom . –  Ben Voigt Commented Dec 22, 2010 at 13:50
• And just an instictive automatic repulsion about RHS by value... ( without having looked a single second at the thourough SO subject about copy and swap idiom)... RHS by value while using polymorphism has meant such a hundred of bugs in my career... it will take me hours to be convinced to using RHS by value ;-) –  Stephane Rolland Commented Dec 22, 2010 at 14:12

You are correct about how to copy the contents from the other class. Simple objects can just be assigned using operator= .

However, be wary of cases where TestClass contains pointer members -- if you just assign the pointer using operator= , then both objects will have pointers pointing to the same memory, which may not be what you want. You may instead need to make sure you allocate some new memory and copy the pointed-to data into it so both objects have their own copy of the data. Remember you also need to properly deallocate the memory already pointed to by the assigned-to object before allocating a new block for the copied data.

By the way, you should probably declare your operator= like this:

This is the general convention used when overloading operator= . The return statement allows chaining of assignments (like a = b = c ) and passing the parameter by const reference avoids copying Other on its way into the function call.

• 1 The new common practice actually does pass the RHS by value. It's called the copy-and-swap idiom . –  Ben Voigt Commented Dec 22, 2010 at 13:51

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21.12 — Overloading the assignment operator