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In C++ consider the two scenario,

  1. Returning object by reference.
  2. Returning object as it is.

In which of the above cases deep copying is done and Why?

Thanks in advance.

doggie style
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    Returning a reference does no copying. Returning the object uses its copy constructor. Whether that does a deep copy depends on how it's written. – Barmar Jan 02 '15 at 06:36
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    None of the methods are, inherently, deep copying. Returning by value *can* do deep copying, if the class have a proper copy-constructor that does the actual deep copying. – Some programmer dude Jan 02 '15 at 06:36
  • @Barmar Depends on what? Can you give an example, or can you guide me on what I should read on to get a better understanding? – doggie style Jan 02 '15 at 06:40
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    It depends on whether the copy constructor and assignment operator makes copies of all the objects that it has pointers to or just copies the pointers. – Barmar Jan 02 '15 at 06:42
  • If you understand what deep copying is, it should be pretty obvious what it depends on. So maybe you need to go back to the books and study the concept. – Barmar Jan 02 '15 at 06:43
  • possible duplicate of [What is the difference between a deep copy and a shallow copy?](http://stackoverflow.com/questions/184710/what-is-the-difference-between-a-deep-copy-and-a-shallow-copy) – Shahriar Jan 02 '15 at 06:49

2 Answers2

1

When an object is returned by reference, there is no copying of objects.

When an object is returned by value, a copy will be made. Whether that will be a shallow copy or a deep copy depends on the copy constructor.

Example 1

Simple struct:

struct Point
{
    double x;
    double y;
    double z;
};

You don't need to implement a copy constructor for such a struct. The compiler will generate a correctly working copy constructor for it.

Example 2

struct Edge;

struct Vertex
{
    std::list<Edge*> edges;
};

For Vertex, the copy constructor generated by the compiler will copy the list of Edges. This happens because the copy constructor of std::list makes a deep copy. However, the deep copy ends there. It won't create new Edge objects when making a copy of a Vertex. If that is not adequate, you'll have to implement a copy constructor for Vertex and do the right thing based on your need.

R Sahu
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0

If an object has pointers to dynamically allocated memory, and the dynamically allocated memory needs to be copied when the original object is copied, then a deep copy is required.

A class that requires deep copies generally needs:

  1. A constructor to either make an initial allocation or set the pointer to NULL.
  2. A destructor to delete the dynamically allocated memory.
  3. A copy constructor to make a copy of the dynamically allocated memory.
  4. An overloaded assignment operator to make a copy of the dynamically allocated memory.

To make a deep copy, you must write a copy constructor and overload the assignment operator, otherwise the copy will point to the original, with disastrous consequences.

Read this for more - Shallow vs. deep copying and How and When to Make Deep Copies in C++

Shahriar
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