assignment operator syntax name

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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 −

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• Assignment operators

An assignment operator assigns a value to its left operand based on the value of its right operand.

The basic assignment operator is equal ( = ), which assigns the value of its right operand to its left operand. That is, x = y assigns the value of y to x . The other assignment operators are usually shorthand for standard operations, as shown in the following definitions and examples.

Simple assignment operator which assigns a value to a variable. The assignment operation evaluates to the assigned value. Chaining the assignment operator is possible in order to assign a single value to multiple variables. See the example.

Addition assignment

The addition assignment operator adds the value of the right operand to a variable and assigns the result to the variable. The types of the two operands determine the behavior of the addition assignment operator. Addition or concatenation is possible. See the addition operator for more details.

Subtraction assignment

The subtraction assignment operator subtracts the value of the right operand from a variable and assigns the result to the variable. See the subtraction operator for more details.

Multiplication assignment

The multiplication assignment operator multiplies a variable by the value of the right operand and assigns the result to the variable. See the multiplication operator for more details.

Division assignment

The division assignment operator divides a variable by the value of the right operand and assigns the result to the variable. See the division operator for more details.

Remainder assignment

The remainder assignment operator divides a variable by the value of the right operand and assigns the remainder to the variable. See the remainder operator for more details.

Exponentiation assignment

This is an experimental technology, part of the ECMAScript 2016 (ES7) proposal. Because this technology's specification has not stabilized, check the compatibility table for usage in various browsers. Also note that the syntax and behavior of an experimental technology is subject to change in future version of browsers as the spec changes.

The exponentiation assignment operator evaluates to the result of raising first operand to the power second operand. See the exponentiation operator for more details.

Left shift assignment

The left shift assignment operator moves the specified amount of bits to the left and assigns the result to the variable. See the left shift operator for more details.

Right shift assignment

The right shift assignment operator moves the specified amount of bits to the right and assigns the result to the variable. See the right shift operator for more details.

Unsigned right shift assignment

The unsigned right shift assignment operator moves the specified amount of bits to the right and assigns the result to the variable. See the unsigned right shift operator for more details.

Bitwise AND assignment

The bitwise AND assignment operator uses the binary representation of both operands, does a bitwise AND operation on them and assigns the result to the variable. See the bitwise AND operator for more details.

Bitwise XOR assignment

The bitwise XOR assignment operator uses the binary representation of both operands, does a bitwise XOR operation on them and assigns the result to the variable. See the bitwise XOR operator for more details.

Bitwise OR assignment

The bitwise OR assignment operator uses the binary representation of both operands, does a bitwise OR operation on them and assigns the result to the variable. See the bitwise OR operator for more details.

Left operand with another assignment operator

In unusual situations, the assignment operator (e.g. x += y ) is not identical to the meaning expression (here x = x + y ). When the left operand of an assignment operator itself contains an assignment operator, the left operand is evaluated only once. For example:

Specifications

Browser compatibility.

• Arithmetic operators

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Assignment operators (C# reference)

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The assignment operator = assigns the value of its right-hand operand to a variable, a property , or an indexer element given by its left-hand operand. The result of an assignment expression is the value assigned to the left-hand operand. The type of the right-hand operand must be the same as the type of the left-hand operand or implicitly convertible to it.

The assignment operator = is right-associative, that is, an expression of the form

is evaluated as

The following example demonstrates the usage of the assignment operator with a local variable, a property, and an indexer element as its left-hand operand:

The left-hand operand of an assignment receives the value of the right-hand operand. When the operands are of value types , assignment copies the contents of the right-hand operand. When the operands are of reference types , assignment copies the reference to the object.

This is called value assignment : the value is assigned.

ref assignment

Ref assignment = ref makes its left-hand operand an alias to the right-hand operand, as the following example demonstrates:

In the preceding example, the local reference variable arrayElement is initialized as an alias to the first array element. Then, it's ref reassigned to refer to the last array element. As it's an alias, when you update its value with an ordinary assignment operator = , the corresponding array element is also updated.

The left-hand operand of ref assignment can be a local reference variable , a ref field , and a ref , out , or in method parameter. Both operands must be of the same type.

Compound assignment

For a binary operator op , a compound assignment expression of the form

is equivalent to

except that x is only evaluated once.

Compound assignment is supported by arithmetic , Boolean logical , and bitwise logical and shift operators.

Null-coalescing assignment

You can use the null-coalescing assignment operator ??= to assign the value of its right-hand operand to its left-hand operand only if the left-hand operand evaluates to null . For more information, see the ?? and ??= operators article.

Operator overloadability

A user-defined type can't overload the assignment operator. However, a user-defined type can define an implicit conversion to another type. That way, the value of a user-defined type can be assigned to a variable, a property, or an indexer element of another type. For more information, see User-defined conversion operators .

A user-defined type can't explicitly overload a compound assignment operator. However, if a user-defined type overloads a binary operator op , the op= operator, if it exists, is also implicitly overloaded.

C# language specification

For more information, see the Assignment operators section of the C# language specification .

• C# operators and expressions
• ref keyword
• Use compound assignment (style rules IDE0054 and IDE0074)

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Additional resources

21.12 — Overloading the assignment operator

The copy assignment operator (operator=) is used to copy values from one object to another already existing object .

Related content

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.

cppreference.com

Copy assignment operator.

A copy assignment operator is a non-template non-static member function with the name operator = that can be called with an argument of the same class type and copies the content of the argument without mutating the argument.

[ edit ] Syntax

For the formal copy assignment operator syntax, see function declaration . The syntax list below only demonstrates a subset of all valid copy assignment operator syntaxes.

[ edit ] Explanation

The copy assignment operator is called whenever selected by overload resolution , e.g. when an object appears on the left side of an assignment expression.

[ edit ] Implicitly-declared copy assignment operator

If no user-defined copy assignment operators are provided for a class type, the compiler will always declare one as an inline public member of the class. This implicitly-declared copy assignment operator has the form T & T :: operator = ( const T & ) if all of the following is true:

• each direct base B of T has a copy assignment operator whose parameters are B or const B & or const volatile B & ;
• each non-static data member M of T of class type or array of class type has a copy assignment operator whose parameters are M or const M & or const volatile M & .

Otherwise the implicitly-declared copy assignment operator is declared as T & T :: operator = ( T & ) .

Due to these rules, the implicitly-declared copy assignment operator cannot bind to a volatile lvalue argument.

A class can have multiple copy assignment operators, e.g. both T & T :: operator = ( T & ) and T & T :: operator = ( T ) . If some user-defined copy assignment operators are present, the user may still force the generation of the implicitly declared copy assignment operator with the keyword default . (since C++11)

The implicitly-declared (or defaulted on its first declaration) copy assignment operator has an exception specification as described in dynamic exception specification (until C++17) noexcept specification (since C++17)

Because the copy assignment operator is always declared for any class, the base class assignment operator is always hidden. If a using-declaration is used to bring in the assignment operator from the base class, and its argument type could be the same as the argument type of the implicit assignment operator of the derived class, the using-declaration is also hidden by the implicit declaration.

[ edit ] Implicitly-defined copy assignment operator

If the implicitly-declared copy assignment operator is neither deleted nor trivial, it is defined (that is, a function body is generated and compiled) by the compiler if odr-used or needed for constant evaluation (since C++14) . For union types, the implicitly-defined copy assignment copies the object representation (as by std::memmove ). For non-union class types, the operator performs member-wise copy assignment of the object's direct bases and non-static data members, in their initialization order, using built-in assignment for the scalars, memberwise copy-assignment for arrays, and copy assignment operator for class types (called non-virtually).

[ edit ] Deleted copy assignment operator

An implicitly-declared or explicitly-defaulted (since C++11) copy assignment operator for class T is undefined (until C++11) defined as deleted (since C++11) if any of the following conditions is satisfied:

• T has a non-static data member of a const-qualified non-class type (or possibly multi-dimensional array thereof).
• T has a non-static data member of a reference type.
• T has a potentially constructed subobject of class type M (or possibly multi-dimensional array thereof) such that the overload resolution as applied to find M 's copy assignment operator
• does not result in a usable candidate, or
• in the case of the subobject being a variant member , selects a non-trivial function.

[ edit ] Trivial copy assignment operator

The copy assignment operator for class T is trivial if all of the following is true:

• it is not user-provided (meaning, it is implicitly-defined or defaulted);
• T has no virtual member functions;
• T has no virtual base classes;
• the copy assignment operator selected for every direct base of T is trivial;
• the copy assignment operator selected for every non-static class type (or array of class type) member of T is trivial.

A trivial copy assignment operator makes a copy of the object representation as if by std::memmove . All data types compatible with the C language (POD types) are trivially copy-assignable.

[ edit ] Eligible copy assignment operator

Triviality of eligible copy assignment operators determines whether the class is a trivially copyable type .

[ edit ] Notes

If both copy and move assignment operators are provided, overload resolution selects the move assignment if the argument is an rvalue (either a prvalue such as a nameless temporary or an xvalue such as the result of std::move ), and selects the copy assignment if the argument is an lvalue (named object or a function/operator returning lvalue reference). If only the copy assignment is provided, all argument categories select it (as long as it takes its argument by value or as reference to const, since rvalues can bind to const references), which makes copy assignment the fallback for move assignment, when move is unavailable.

It is unspecified whether virtual base class subobjects that are accessible through more than one path in the inheritance lattice, are assigned more than once by the implicitly-defined copy assignment operator (same applies to move assignment ).

See assignment operator overloading for additional detail on the expected behavior of a user-defined copy-assignment operator.

[ edit ] Example

[ edit ] defect reports.

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

[ edit ] See also

• converting constructor
• copy constructor
• copy elision
• default constructor
• aggregate initialization
• constant initialization
• copy initialization
• default initialization
• direct initialization
• initializer list
• list initialization
• reference initialization
• value initialization
• zero initialization
• move assignment
• move constructor
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Assignment operators assign values to JavaScript variables.

Shift Assignment Operators

Bitwise assignment operators, logical assignment operators, the = operator.

The Simple Assignment Operator assigns a value to a variable.

Simple Assignment Examples

The += operator.

The Addition Assignment Operator adds a value to a variable.

Addition Assignment Examples

The -= operator.

The Subtraction Assignment Operator subtracts a value from a variable.

Subtraction Assignment Example

The *= operator.

The Multiplication Assignment Operator multiplies a variable.

Multiplication Assignment Example

The **= operator.

The Exponentiation Assignment Operator raises a variable to the power of the operand.

Exponentiation Assignment Example

The /= operator.

The Division Assignment Operator divides a variable.

Division Assignment Example

The %= operator.

The Remainder Assignment Operator assigns a remainder to a variable.

Remainder Assignment Example

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The <<= Operator

The Left Shift Assignment Operator left shifts a variable.

Left Shift Assignment Example

The >>= operator.

The Right Shift Assignment Operator right shifts a variable (signed).

Right Shift Assignment Example

The >>>= operator.

The Unsigned Right Shift Assignment Operator right shifts a variable (unsigned).

Unsigned Right Shift Assignment Example

The &= operator.

The Bitwise AND Assignment Operator does a bitwise AND operation on two operands and assigns the result to the the variable.

Bitwise AND Assignment Example

The |= operator.

The Bitwise OR Assignment Operator does a bitwise OR operation on two operands and assigns the result to the variable.

Bitwise OR Assignment Example

The ^= operator.

The Bitwise XOR Assignment Operator does a bitwise XOR operation on two operands and assigns the result to the variable.

Bitwise XOR Assignment Example

The &&= operator.

The Logical AND assignment operator is used between two values.

If the first value is true, the second value is assigned.

Logical AND Assignment Example

The &&= operator is an ES2020 feature .

The ||= Operator

The Logical OR assignment operator is used between two values.

If the first value is false, the second value is assigned.

Logical OR Assignment Example

The ||= operator is an ES2020 feature .

The ??= Operator

The Nullish coalescing assignment operator is used between two values.

If the first value is undefined or null, the second value is assigned.

Nullish Coalescing Assignment Example

The ??= operator is an ES2020 feature .

Test Yourself With Exercises

Use the correct assignment operator that will result in x being 15 (same as x = x + y ).

Start the Exercise

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A ssignment Operators

Assignment operators are used to assign values to variables in JavaScript.

Assignment Operators List

There are so many assignment operators as shown in the table with the description.

Below we have described each operator with an example code:

Addition assignment operator(+=).

The Addition assignment operator adds the value to the right operand to a variable and assigns the result to the variable. Addition or concatenation is possible. In case of concatenation then we use the string as an operand.

Subtraction Assignment Operator(-=)

The Substraction Assignment Operator subtracts the value of the right operand from a variable and assigns the result to the variable.

Multiplication Assignment Operator(*=)

The Multiplication Assignment operator multiplies a variable by the value of the right operand and assigns the result to the variable.

Division Assignment Operator(/=)

The Division Assignment operator divides a variable by the value of the right operand and assigns the result to the variable.

Remainder Assignment Operator(%=)

The Remainder Assignment Operator divides a variable by the value of the right operand and assigns the remainder to the variable.

Exponentiation Assignment Operator

The Exponentiation Assignment Operator raises the value of a variable to the power of the right operand.

Left Shift Assignment Operator(<<=)

This Left Shift Assignment O perator moves the specified amount of bits to the left and assigns the result to the variable.

Right Shift Assignment O perator(>>=)

The Right Shift Assignment Operator moves the specified amount of bits to the right and assigns the result to the variable.

Bitwise AND Assignment Operator(&=)

The Bitwise AND Assignment Operator uses the binary representation of both operands, does a bitwise AND operation on them, and assigns the result to the variable.

Btwise OR Assignment Operator(|=)

The Btwise OR Assignment Operator uses the binary representation of both operands, does a bitwise OR operation on them, and assigns the result to the variable.

Bitwise XOR Assignment Operator(^=)

The Bitwise XOR Assignment Operator uses the binary representation of both operands, does a bitwise XOR operation on them, and assigns the result to the variable.

Logical AND Assignment Operator(&&=)

The Logical AND Assignment assigns the value of  y  into  x  only if  x  is a  truthy  value.

Logical OR Assignment Operator( ||= )

The Logical OR Assignment Operator is used to assign the value of y to x if the value of x is falsy.

Nullish coalescing Assignment Operator(??=)

The Nullish coalescing Assignment Operator assigns the value of y to x if the value of x is null.

Supported Browsers: The browsers supported by all JavaScript Assignment operators are listed below:

• Google Chrome
• Microsoft Edge
• Internet Explorer

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