CppCopyAssignmentOperator.md

 

 

 

 

 

(C++) copy assignment operator

 

The copy assignment operator is an operator used in assignment and one of The Big Four. It differs from a constructor, because the copy assignment operator might have to clean up the previous (to-be-overwritten) instance its resources.

 

There are multiple idioms for overloading a copy assignment operator:

• the 'classic way', which checks for self-assignment
• the 'swap idiom'

 

 

 

 

 

 

The classic way

 

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struct MyClass {   //Default constructor   MyClass(const int x = 0) : m_x(x) {}   //Canonical form of copy assignment operator   MyClass& operator=(const MyClass& other)   {     //Prevent self-assignment     if (this != &other)     {       //Copy member variables       m_x = other.m_x;     }     //Return *this by convention     return *this;   }   ///Read the member variable   int GetX() const { return m_x; }   private:   //Member variables   int m_x; };

 

 

 

 

 

The swap idiom

 

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#include <algorithm> struct MyClass {   //Default constructor   MyClass(const int x = 0) : m_x(x) {}   //Canonical form of copy assignment operator   MyClass& operator=(const MyClass& other)   {     MyClass temp(other);     Swap(temp);     //Return *this by convention     return *this;   }   ///Read the member variable   int GetX() const { return m_x; }   private:   //Member variables   int m_x;   void Swap(MyClass& other)   {     std::swap(m_x,other.m_x);   } };

 

 

 

 

 

Complete example

 

• Download the Qt Creator project 'CppCopyAssignmentOperator' (zip)

 

 

 

 

Technical facts

 

Application type(s)

• Desktop application

Operating system(s) or programming environment(s)

• Lubuntu 12.10 (quantal)

IDE(s):

• Qt Creator 2.5.2

Project type:

• Console application

C++ standard:

• C++98

Compiler(s):

• G++ 4.7.2

Libraries used:

• STL: GNU ISO C++ Library, version 4.7.2

 

 

 

 

 

Qt project file: CppCopyAssignmentOperator.pro

 

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TEMPLATE = app CONFIG += console CONFIG -= qt SOURCES += main.cpp

 

 

 

 

 

main.cpp

 

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struct MyClassClassic {   //Default constructor   MyClassClassic(const int x = 0) : m_x(x) {}   //Canonical form of copy assignment operator   MyClassClassic& operator=(const MyClassClassic& other)   {     //Prevent self-assignment     if (this != &other)     {       //Copy member variables       m_x = other.m_x;     }     //Return *this by convention     return *this;   }   ///Read the member variable   int GetX() const { return m_x; }   private:   //Member variables   int m_x; }; #include <algorithm> struct MyClassSwap {   //Default constructor   MyClassSwap(const int x = 0) : m_x(x) {}   //Canonical form of copy assignment operator   MyClassSwap& operator=(const MyClassSwap& other)   {     MyClassSwap temp(other);     Swap(temp);     //Return *this by convention     return *this;   }   ///Read the member variable   int GetX() const { return m_x; }   private:   //Member variables   int m_x;   void Swap(MyClassSwap& other)   {     std::swap(m_x,other.m_x);   } }; #include <cassert> int main() {   {     MyClassClassic x(1);     MyClassClassic y(2);     assert(x.GetX() != y.GetX());     x = y;     assert(x.GetX() == y.GetX());   }   {     MyClassSwap x(1);     MyClassSwap y(2);     assert(x.GetX() != y.GetX());     x = y;     assert(x.GetX() == y.GetX());   } }