Copy-and-Swap Idiom in C++

Before going into deep let us first have a look on the normal ‘overloaded assignment operator’ that we use.

// Simple C++ program to demonstrate overloading of
// assignment operator.
#include <iostream>
#include <cstring>
using namespace std;

class anyArrayClass
{
    int size;
    int *ptr;
public:

    // Initializer list
    anyArrayClass(int s=0):size(s),
        ptr(size? new int[size]:nullptr) {}

    // Copy constructor
    anyArrayClass(const anyArrayClass& obj):size(obj.size),
        ptr(size? new int[size]:nullptr)
    {
        memmove(ptr, obj.ptr, size*sizeof(int));
    }

    // Overloaded assignment operator
    anyArrayClass& operator=(const anyArrayClass& obj)
    {
        // self assignment check
        if (this != &obj)
        {
            delete ptr;
            size = obj.size;
            ptr = size? new int[size]: nullptr;
            memmove(ptr, obj.ptr, size*sizeof(int));
            return *this;
        }
    }
    ~anyArrayClass()
    {
        delete[] ptr;
    }
}

The above assignment operator does the following things:
1. Self assignment check.
2. If there assignment is not to self, then it does following.
    a) Deallocating memory assigned to this->ptr
    b) Allocating new memory to this->ptr and copying the values
    c) Returning *this

Drawbacks of above approach:

1. The self assignment check: Self allocation is done very rarely so the self assignment check is not relevant in most of the scenarios. This just slows down the code.
2. Memory deallocation and allocation: As it can be seen that first the memory is deallocated (leaving the pointer dangling) and then the new chunk of memory is allocated. Now if due to some reason the memory is not allocated and an exception is thrown than the ‘this->ptr’ will be left dangling pointing to a deallocated memory. The scenario should be either the assignment is successful or the object should not be altered at all.

Here comes the role of copy-and-swap approach. This approach elegantly solves the above issue and also provides a scope for code re-usability. Lets see what exactly is it.

Consider the following code:

// Simple C++ program to demonstrate use of copy-and-swap
// idiom by improving above code.
#include <iostream>
#include <cstring>
using namespace std;

class anyArrayClass
{
    int size;
    int *ptr;
public:
    anyArrayClass(int s=0):size(s),
     ptr(size? new int[size]:nullptr) {}

    // Copy constructor
    anyArrayClass(const anyArrayClass& obj):size(obj.size),
                           ptr(size? new int[size]:nullptr)
    {
        memmove(ptr, obj.ptr, size*sizeof(int));
    }

    friend void swap(anyArrayClass& obj1, anyArrayClass& obj2)
    {
        std::swap(obj1.size, obj2.size);
        std::swap(obj1.ptr, obj2.ptr);
    }

    // overloaded assignment operator
    // argument passed by value. calls copy ctor
    anyArrayClass& operator=(anyArrayClass obj)
    {
        // calling friend function
        swap(*this, obj);
        return *this;
    }

    ~anyArrayClass()
    {
        delete[] ptr;
    }
}

In the above example, the parameter to the ‘operator=()’ is passed by value which calls copy constructor to create a anyArrayClass object local to the ‘operator=()’. Than the value of the temp object is swapped with ‘*this’ object (LHS of assignment operator call).

Advantages:

1. No more self assignment check needed as the parameter is passed by value (This means no more memory deallocation is needed). Moreover as self allocation is very rare so the overhead of copying in case of self assignment should not be a problem.
2. Now as copy constructor is used to create the temp object therefore, the swapping will only be done if the temp object is at all created. Basically what we were doing manually there, the compiler is doing it for us in here.
3. Code re-useability: as we can see the ‘operator=()’ does not have much code in its body rather, we are using the copy constructor and swap function to do the job.

This article is contributed by Bishwa Nath. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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