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Assignment Operators in C

In C, 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 = 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 the most frequently used operator in C. As per 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.

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 produces the following result −

Learn C++

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.

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Move Constructors and Move Assignment Operators (C++)

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This topic describes how to write a move constructor and a move assignment operator for a C++ class. A move constructor enables the resources owned by an rvalue object to be moved into an lvalue without copying. For more information about move semantics, see Rvalue Reference Declarator: && .

This topic builds upon the following C++ class, MemoryBlock , which manages a memory buffer.

The following procedures describe how to write a move constructor and a move assignment operator for the example C++ class.

To create a move constructor for a C++ class

Define an empty constructor method that takes an rvalue reference to the class type as its parameter, as demonstrated in the following example:

In the move constructor, assign the class data members from the source object to the object that is being constructed:

Assign the data members of the source object to default values. This prevents the destructor from freeing resources (such as memory) multiple times:

To create a move assignment operator for a C++ class

Define an empty assignment operator that takes an rvalue reference to the class type as its parameter and returns a reference to the class type, as demonstrated in the following example:

In the move assignment operator, add a conditional statement that performs no operation if you try to assign the object to itself.

In the conditional statement, free any resources (such as memory) from the object that is being assigned to.

The following example frees the _data member from the object that is being assigned to:

Follow steps 2 and 3 in the first procedure to transfer the data members from the source object to the object that is being constructed:

Return a reference to the current object, as shown in the following example:

Example: Complete move constructor and assignment operator

The following example shows the complete move constructor and move assignment operator for the MemoryBlock class:

Example Use move semantics to improve performance

The following example shows how move semantics can improve the performance of your applications. The example adds two elements to a vector object and then inserts a new element between the two existing elements. The vector class uses move semantics to perform the insertion operation efficiently by moving the elements of the vector instead of copying them.

This example produces the following output:

Before Visual Studio 2010, this example produced the following output:

The version of this example that uses move semantics is more efficient than the version that does not use move semantics because it performs fewer copy, memory allocation, and memory deallocation operations.

Robust Programming

To prevent resource leaks, always free resources (such as memory, file handles, and sockets) in the move assignment operator.

To prevent the unrecoverable destruction of resources, properly handle self-assignment in the move assignment operator.

If you provide both a move constructor and a move assignment operator for your class, you can eliminate redundant code by writing the move constructor to call the move assignment operator. The following example shows a revised version of the move constructor that calls the move assignment operator:

The std::move function converts the lvalue other to an rvalue.

Rvalue Reference Declarator: && std::move

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cppreference.com

Union declaration.

A union is a special class type that can hold only one of its non-static data members at a time.

[ edit ] Syntax

The class specifier for a union declaration is similar to class or struct declaration:

A union can have member functions (including constructors and destructors), but not virtual functions.

A union cannot have base classes and cannot be used as a base class.

A union cannot have non-static data members of reference types.

Just like in struct declaration, the default member access in a union is public .

[ edit ] Explanation

The union is at least as big as necessary to hold its largest data member, but is usually not larger. The other data members are intended to be allocated in the same bytes as part of that largest member. The details of that allocation are implementation-defined, except that all non-static data members have the same address. It is undefined behavior to read from the member of the union that wasn't most recently written. Many compilers implement, as a non-standard language extension, the ability to read inactive members of a union.

Possible output:

Each member is allocated as if it is the only member of the class.

If two union members are standard-layout types, it's well-defined to examine their common subsequence on any compiler.

[ edit ] Member lifetime

The lifetime of a union member begins when the member is made active. If another member was active previously, its lifetime ends.

When active member of a union is switched by an assignment expression of the form E1 = E2 that uses either the built-in assignment operator or a trivial assignment operator, for each union member X that appears in the member access and array subscript subexpressions of E1 that is not a class with non-trivial or deleted default constructors, if modification of X would have undefined behavior under type aliasing rules, an object of the type of X is implicitly created in the nominated storage; no initialization is performed and the beginning of its lifetime is sequenced after the value computation of the left and right operands and before the assignment.

Trivial move constructor, move assignment operator, (since C++11) copy constructor and copy assignment operator of union types copy object representations. If the source and the destination are not the same object, these special member functions start lifetime of every object (except for objects that are neither subobjects of the destination nor of implicit-lifetime type ) nested in the destination corresponding to the one nested in the source before the copy is performed. Otherwise, they do nothing. Two union objects have the same corresponding active member (if any) after construction or assignment via trivial special functions.

[ edit ] Anonymous unions

An anonymous union is an unnamed union definition that does not simultaneously define any variables (including objects of the union type, references, or pointers to the union).

Anonymous unions have further restrictions: they cannot have member functions, cannot have static data members, and all their data members must be public. The only declarations allowed are non-static data members and static_assert declarations (since C++11) .

Members of an anonymous union are injected in the enclosing scope (and must not conflict with other names declared there).

Namespace-scope anonymous unions must be declared static unless they appear in an unnamed namespace.

[ edit ] Union-like classes

A union-like class is either a union, or a (non-union) class that has at least one anonymous union as a member. A union-like class has a set of variant members :

  • the non-static data members of its member anonymous unions;
  • in addition, if the union-like class is a union, its non-static data members that are not anonymous unions.

Union-like classes can be used to implement tagged union .

[ edit ] Defect reports

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

[ edit ] See also

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  1. Assignment Operators in C

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  1. USELESS Return from a Function in C++

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  6. Assignment Operator in C Programming

COMMENTS

  1. Copy assignment operator

     parameter-list-no-default  (since C++11) ) = delete; (since C++11)   operator= (parameter-list-no-default ) = default; (since C++11) the class whose copy assignment operator is being declared, the class type is given as in the descriptions below of only one parameter, which is of type T& parameter-list-no-default

  2. Default assignment operator= in c++ is a shallow copy?

    8 Answers Sorted by: 60 I'd say, default operator= is a copy. It copies each member. The distinction between a shallow copy and a deep copy doesn't arise unless the members being copied are some kind of indirection such as a pointer.

  3. Default Assignment Operator and References in C++

    Default Assignment Operator and References in C++ - GeeksforGeeks Default Assignment Operator and References in C++ Read Courses Practice We have discussed assignment operator overloading for dynamically allocated resources here.

  4. Assignment Operators in C

    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.

  5. Copy constructors and copy assignment operators (C++)

    Use an assignment operator operator= that returns a reference to the class type and takes one parameter that's passed by const reference—for example ClassName& operator= (const ClassName& x);. Use the copy constructor. If you don't declare a copy constructor, the compiler generates a member-wise copy constructor for you.

  6. 21.13

    Consequently, C++ will provide a default copy constructor and default assignment operator that do a shallow copy. The copy constructor will look something like this: Note that m_data is just a shallow pointer copy of source.m_data, meaning they now both point to the same thing. #include <iostream> int main() { MyString hello { "Hello, world!"

  7. Assignment operator (C++)

    The copy assignment operator, often just called the "assignment operator", is a special case of assignment operator where the source (right-hand side) and destination (left-hand side) are of the same class type.

  8. Assignment Operators in C

    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. -=.

  9. 21.12

    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.

  10. Copy constructors, assignment operators,

    6. 7. template< typename T > swap ( T& one, T& two ) { T tmp ( one ); one = two; two = tmp; } The first line runs the copy constructor of T, which can throw; the. remaining lines are assignment operators which can also throw. HOWEVER, if you have a type T for which the default std::swap () may result.

  11. Assignment operators

    The assignment operators return the value of the object specified by the left operand after the assignment. 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.

  12. What Is A Forced (Default) Copy Assignment Operator In Modern C++

    The forced copy assignment operator forces a copy assignment operator to be generated by the compiler and it is default in any class declarations. This means you don't need to declare it as above, let's give examples without using it. Let's give a simple C++ example to copy assignment operator with default option, here is a simple class. 1.

  13. Move Constructors and Move Assignment Operators (C++)

    This topic describes how to write a move constructor and a move assignment operator for a C++ class. A move constructor enables the resources owned by an rvalue object to be moved into an lvalue without copying. For more information about move semantics, see Rvalue Reference Declarator: &&. This topic builds upon the following C++ class ...

  14. When should we write our own assignment operator in C++?

    Since there is no user defined assignment operator in the above program, compiler creates a default assignment operator, which copies 'ptr' of right hand side to left hand side. So both 'ptr's start pointing to the same location. We can handle the above problem in two ways. 1) Do not allow assignment of one object to other object.

  15. compiler errors

    c++ default assignment operator Ask Question Asked 10 years, 6 months ago Modified 10 years, 6 months ago Viewed 3k times 7 int a [10]; int b [10]; a = b; // struct test { int a [10]; }; test a,b; a = b; First code doesn't compile, since we cant assign array, but the second does.

  16. Union declaration

    Trivial move constructor, move assignment operator, (since C++11) copy constructor and copy assignment operator of union types copy object representations. If the source and the destination are not the same object, these special member functions start lifetime of every object (except for objects that are neither subobjects of the destination nor of implicit-lifetime type) nested in the ...

  17. Non-static const member, can't use default assignment operator

    1 just put the literal code. you've pasted some pseudo code again. also paste the other iterator's type and where you get it from... do not try to annotate the code.

  18. c++

    Language-lawyered Explanation. class.mem states:. The member-specification in a class definition declares the full set of members of the class; no member can be added elsewhere.A direct member of a class X is a member of X that was first declared within the member-specification of X, including anonymous union members ([class.union.anon]) and direct members thereof.