Assignment operator (C++) (original) (raw)

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Operator (=) used for assigning values in C++

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_Array first; // initialization by default constructor My_Array second(first); // initialization by copy constructor My_Array third = first; // Also initialization by copy constructor second = third; // assignment by copy assignment operator

Return value of overloaded assignment operator

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The language permits an overloaded assignment operator to have an arbitrary return type (including [void](/wiki/Void%5Ftype "Void type")). 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

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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

class My_Array{ int* array; int count; public: My_Array& operator=(const My_Array& other) { if (this != &other) { // protect against invalid self-assignment // 1: allocate new memory and copy the elements int* new_array = new int[other.count]; std::copy(other.array, other.array + other.count, new_array);

        // 2: deallocate old memory
        delete[] array;

        // 3: assign the new memory to the object
        array = new_array;
        count = other.count;
    }
    // by convention, always return *this
    return *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:

void swap(My_Array& other) // the swap member function (should never fail!)
{
    // swap all the members (and base subobject, if applicable) with other
    using std::swap; // because of ADL the compiler will use 
    // custom swap for members if it exists
    // falling back to std::swap
    swap(array, other.array);
    swap(count, other.count);
}

My_Array& operator = (My_Array other) // note: argument passed by value!
{
    // swap this with other
    swap(other);

    // by convention, always return *this
    return *this;

    // other is destroyed, releasing the memory
}

Assignment between different classes

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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:

class Ancestor { public: int a; };

class Descendant : public Ancestor { public: int b; };

int main() { Descendant d; Ancestor a(d); Ancestor b(d); a = d; }

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

  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