(C++) operator=

operator= is an operator, that is called the assign operator and assignment operator.


struct MyClass

{

  MyClass& operator=(const MyClass& rhs)

  {

    //Copy the class members

    return *this;

  }

};

Handle assignment to self: Identity test


struct MyClass

{

  MyClass& operator=(const MyClass& rhs)

  {

    //'Identity test'

    if (this == &rhs) return *this;

 

    //Copy

    //...

 

    //Return *this

    return *this;

  }

};

Handle assignment to self: Copy and swap


struct MyClass

{

  MyClass& operator=(MyClass tempCopy) //'Copy' (by passing by value)

  {

    //'Swap'

    Swap(tempCopy); //Swaps the data of temp with *this

 

    return *this;

  }

 

  void Swap(MyClass& m)

  {

    //Swap data of m with this

    std::swap(mX, m.mX);

    //Other data...

  }



  int mX; //Just to have a member variable

};

Advice

In class design, have assignment operators return a reference to *this [1,3].
In class design, also handle assignment to self [2,4]. To handle assignment to self there are two techniques: 'identity test' or 'copy and swap'.
Prefer the {}-initializer syntax for declarations with a named type [7]. Prefer the = syntax for the initialization in declarations using auto [6]
Reversing the order of the pair of symbols in the operators !=, >= and <= (by writing them as =!, => and =< respectively) is normally a syntax error [5]

syntax error

logic error

References

Scott Meyers. Effective C++ (3rd edition).ISBN: 0-321-33487-6. Item 10: Have assignment operators return a reference to *this.
Scott Meyers. Effective C++ (3rd edition).ISBN: 0-321-33487-6. Item 11: Handle assignment to self in operator=.
Joint Strike Fighter Air Vehicle C++ Coding Standards for the System Development and Demonstration Program. Document Number 2RDU00001 Rev C. December 2005. AV Rule 82: 'An assignment operator shall return a reference to *this.'
Joint Strike Fighter Air Vehicle C++ Coding Standards for the System Development and Demonstration Program. Document Number 2RDU00001 Rev C. December 2005. AV Rule 81: 'The assignment operator shall handle self-assignment correctly'
Paul Deitel, Harvey Deitel. C++11 for programmers (2nd edition). 2014. ISBN: 978-0-13-343985-4. Chapter 2.4, Common Programming Error 2.2. page 32: 'Reversing the order of the pair of symbols in the operators !=, >= and < (by writing them as =!, => and =< respectively) is normally a syntax error. In some cases, writing != as =! will not be a syntax error, but almost certainly will be a logic error that has an effect at execution time.'
Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 6.6. Advice, page 169: '[20] Prefer the = syntax for the initialization in declarations using auto'
Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 6.6. Advice, page 169: '[19] Prefer the {}-initializer syntax for declarations with a named type'

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