Move assignment operator: Difference between revisions

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{{Short description|Operator used in C++}}

{{copy edit|date=February 2016}}

In the [[C++|C++ programming language]], the '''move assignment operator''' <code>=</code> is used for transferring a temporary object to an existing object. The move assignment operator, like most C++ operators, can be [[Operator overloading|overloaded]]. Like the [[copy assignment operator]] it is a [[special member functions|special member function]].

If the move assignment operator is not explicitly defined, the [[compiler]] generates an implicit move assignment operator ([[C++11]] and newer) provided that [[Copy constructor (C++)|copy]]/[[move constructor]]s, [[Assignment operator (C++)|copy assignment operator]] or [[Destructor (computer programming)|destructors]] have not been declared.<ref name=":0" /> The parameter of a move assignment operator is an [[rvalue reference]] ({{Mono|T&&}}) to type ''T'', where ''T'' is the object that defines the move assignment operator. The move assignment operator is different than a move constructor because a move assignment operator is called on an existing object, while a move constructor is called on an object created by the operation. Thereafter, the other object's data is no longer valid.

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== Overloading move assignment operator ==

To overload the move assignment operator, the signature of the function must be as follows:<ref name=":0">{{Cite web|title = Move assignment operator - cppreference.com|url = http://en.cppreference.com/w/cpp/language/move_assignment|website = en.cppreference.com|access-date = 2016-02-23}}</ref><syntaxhighlight lang="c++">

T& operator=(T&& data)

</syntaxhighlight>To successfully overload the move assignment operator, the following conditions must be met:

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* The current object's data is de-allocated.

* The object that is being moved from must have its data marked as [[nullptr]] (or something to signify the move)

* The operator returnsmust return a reference to "{{Mono|*this}}".

An implementation ofConsider the following move assignment operator for a simple string class:<ref>{{Cite web|title = Move Constructors and Move Assignment Operators (C++)|url = https://msdn.microsoft.com/en-us/library/dd293665.aspx|website = msdn.microsoft.com|access-date = 2016-02-23}}</ref><syntaxhighlight lang="c++">

class Resource {

public:

Resource& operator=(Resource&& other) {

if (this != &other) { // If the object isn't being called on itself

delete this->data; // Delete the object's data

this->data = other.data; // "Move" other's data into the current object

other.data = nullptr; // Mark the other object as "empty"

}

return *this; // return *this

}

void* data;

};

</syntaxhighlight>A more practical example:<syntaxhighlight lang="c++">

class String {

public:

ResourceString& operator=(ResourceString&& other) noexcept {

public:

// If we're not trying to move the object into itself...

String&if operator(this !=(String && otherStringother) {

delete[] this->datadata_; // DeleteFree thethis objectstring's original data.

this->datadata_ = other.datadata_; // "Move"Copy the other string's data pointer into thethis current objectstring.

if (this != &otherString) {

other.datadata_ = nullptr; // MarkFinally, reset the other objectstring's asdata "empty"pointer.

delete text;

this->text = otherString.text;

otherString.text = nullptr;

}

return *this;

}

return *this;

}

private:

char* textdata_;

};

</syntaxhighlight>

==References==

<references />

{{C++ programming language}}

{{uncategorised|date=February 2016}}

[[Category:C++]]

[[Category:Operators (programming)]]

[[Category:Assignment operations]]

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