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.
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:
- 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.
- 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:
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).
- 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.
- 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.
- 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.
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