Exception Safe Assignment Operator In Java

Many languages do choose the route of making assignment a statement rather than an expression, including Python:

and Golang:

Other languages don't have assignment, but rather scoped bindings, e.g. OCaml:

However, is an expression itself.

The advantage of allowing assignment is that we can directly check the return value of a function inside the conditional, e.g. in this Perl snippet:

Perl additionally scopes the declaration to that conditional only, which makes it very useful. It will also warn if you assign inside a conditional without declaring a new variable there – will warn, will not.

Making the assignment in another statement is usually sufficient, but can bring scoping problems:

Golang heavily relies on return values for error checking. It therefore allows a conditional to take an initialization statement:

Other languages use a type system to disallow non-boolean expressions inside a conditional:

Of course that fails when using a function that returns a boolean.

We now have seen different mechanisms to defend against accidental assignment:

  • Disallow assignment as an expression
  • Use static type checking
  • Assignment doesn't exist, we only have bindings
  • Allow an initialization statement, disallow assignment otherwise
  • Disallow assignment inside a conditional without declaration

I've ranked them in order of ascending preference – assignments inside expressions can be useful (and it's simple to circumvent Python's problems by having an explicit declaration syntax, and a different named argument syntax). But it's ok to disallow them, as there are many other options to the same effect.

Bug-free code is more important than terse code.

answered Feb 13 '14 at 12:48

In the C++programming language, the assignment operator, , is the operator used for assignment. Like most other operators in C++, it can be overloaded.

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. It is one of the special member functions, which means that a default version of it is generated automatically by the compiler if the programmer does not declare one. The default version performs a memberwise copy, where each member is copied by its own copy assignment operator (which may also be programmer-declared or compiler-generated).

The copy assignment operator differs from the copy constructor in that it must clean up the data members of the assignment's target (and correctly handle self-assignment) whereas the copy constructor assigns values to uninitialized data members.[1] For example:

My_Arrayfirst;// initialization by default constructorMy_Arraysecond(first);// initialization by copy constructorMy_Arraythird=first;// Also initialization by copy constructorsecond=third;// assignment by copy assignment operator

Return value of overloaded assignment operator[edit]

The language permits an overloaded assignment operator to have an arbitrary return type (including ). However, the operator is usually defined to return a reference to the assignee. This is consistent with the behavior of assignment operator for built-in types (returning the assigned value) and allows for using the operator invocation as an expression, for instance in control statements or in chained assignment. Also, the C++ Standard Library requires this behavior for some user-supplied types.[2]

Overloading copy assignment operator[edit]

When deep copies of objects have to be made, exception safety should be taken into consideration. One way to achieve this when resource deallocation never fails is:

  1. Acquire new resources
  2. Release old resources
  3. Assign the new resources' handles to the object
classMy_Array{int*array;intcount;public:My_Array&operator=(constMy_Array&other){if(this!=&other)// protect against invalid self-assignment{// 1: allocate new memory and copy the elementsint*new_array=newint[other.count];std::copy(other.array,other.array+other.count,new_array);// 2: deallocate old memorydelete[]array;// 3: assign the new memory to the objectarray=new_array;count=other.count;}// by convention, always return *thisreturn*this;}// ...};

However, if a no-fail (no-throw) swap function is available for all the member subobjects and the class provides a copy constructor and destructor (which it should do according to the rule of three), the most straightforward way to implement copy assignment is as follows:[3]

public:voidswap(My_Array&other)// the swap member function (should never fail!){// swap all the members (and base subobject, if applicable) with otherusingstd::swap;// because of ADL the compiler will use // custom swap for members if it exists// falling back to std::swapswap(array,other.array);swap(count,other.count);}My_Array&operator=(My_Arrayother)// note: argument passed by value!{// swap this with otherswap(other);// by convention, always return *thisreturn*this;// other is destroyed, releasing the memory}

Assignment between different classes[edit]

C++ supports assignment between different classes, both via implicit copy constructor and assignment operator, if the destination instance class is the ancestor of the source instance class:

classAncestor{public:inta;};classDescendant:publicAncestor{public:intb;};intmain(){Descendantd;Ancestora(d);Ancestorb(d);a=d;return0;}

Copying from ancestor to descendant objects, which could leave descendant's fields uninitialized, is not permitted.

See also[edit]

References[edit]

External links[edit]

  1. ^Stroustrup, Bjarne (2000). The C++ Programming Language (3 ed.). Addison-Wesley. p. 244. ISBN 978-0-201-70073-2. 
  2. ^Working Draft, Standard for Programming Language C++, Section 17.6.3.1, Table 23; http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3337.pdf
  3. ^Sutter, H.; Alexandrescu, A. (October 2004), C++ Coding Standards, Addison-Wesley, ISBN 0-321-11358-6 

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