9.15 — Shallow vs. deep copying

Shallow copying

Because C++ does not know much about your class, the default copy constructor and default assignment operators it provides use a copying method known as a memberwise copy (also known as a shallow copy. This means that C++ copies each member of the class individually (using the assignment operator for overloaded operator=, and direct initialization for the copy constructor). When classes are simple (e.g. do not contain any dynamically allocated memory), this works very well.

For example, let’s take a look at our Fraction class:

The default copy constructor and assignment operator provided by the compiler for this class look something like this:

Note that because these default versions work just fine for copying this class, there’s really no reason to write our own version of these functions in this case.

However, when designing classes that handle dynamically allocated memory, memberwise (shallow) copying can get us in a lot of trouble! This is because shallow copies of a pointer just copy the address of the pointer -- it does not allocate any memory or copy the contents being pointed to!

Let’s take a look at an example of this:

The above is a simple string class that allocates memory to hold a string that we pass in. Note that we have not defined a copy constructor or overloaded assignment operator. Consequently, C++ will provide a default copy constructor and default assignment operator that do a shallow copy. The copy constructor will look something like this:

Now, consider the following snippet of code:

While this code looks harmless enough, it contains an insidious problem that will cause the program to crash! Can you spot it? Don’t worry if you can’t, it’s rather subtle.

Let’s break down this example line by line:

This line is harmless enough. This calls the MyString constructor, which allocates some memory, sets cHello.m_data to point to it, and then copies the string “Hello, world!” into it.

This line seems harmless enough as well, but it’s actually the source of our problem! When this line is evaluated, C++ will use the default copy constructor (because we haven’t provided our own). This copy constructor will do a shallow copy, initializing copy.m_data to the same address of hello.m_data. As a result, copy.m_data and hello.m_data are now both pointing to the same piece of memory!

When copy goes out of scope, the MyString destructor is called on copy. The destructor deletes the dynamically allocated memory that both copy.m_data and hello.m_data are pointing to! Consequently, by deleting copy, we’ve also (inadvertently) affected hello. Variable copy then gets destroyed, but hello.m_data is left pointing to the deleted (invalid) memory!

Now you can see why this program has undefined behavior. We deleted the string that hello was pointing to, and now we are trying to print the value of memory that is no longer allocated.

The root of this problem is the shallow copy done by the copy constructor -- doing a shallow copy on pointer values in a copy constructor or overloaded assignment operator is almost always asking for trouble.

Deep copying

One answer to this problem is to do a deep copy on any non-null pointers being copied. A deep copy allocates memory for the copy and then copies the actual value, so that the copy lives in distinct memory from the source. This way, the copy and source are distinct and will not affect each other in any way. Doing deep copies requires that we write our own copy constructors and overloaded assignment operators.

Let’s go ahead and show how this is done for our MyString class:

As you can see, this is quite a bit more involved than a simple shallow copy! First, we have to check to make sure source even has a string (line 8). If it does, then we allocate enough memory to hold a copy of that string (line 11). Finally, we have to manually copy the string (lines 14 and 15).

Now let’s do the overloaded assignment operator. The overloaded assignment operator is slightly trickier:

Note that our assignment operator is very similar to our copy constructor, but there are three major differences:

  • We added a self-assignment check.
  • We return *this so we can chain the assignment operator.
  • We need to explicitly deallocate any value that the string is already holding (so we don’t have a memory leak when m_data is reallocated later).

When the overloaded assignment operator is called, the item being assigned to may already contain a previous value, which we need to make sure we clean up before we assign memory for new values. For non-dynamically allocated variables (which are a fixed size), we don’t have to bother because the new value just overwrite the old one. However, for dynamically allocated variables, we need to explicitly deallocate any old memory before we allocate any new memory. If we don’t, the code will not crash, but we will have a memory leak that will eat away our free memory every time we do an assignment!

A better solution

Classes in the standard library that deal with dynamic memory, such as std::string and std::vector, handle all of their memory management, and have overloaded copy constructors and assignment operators that do proper deep copying. So instead of doing your own memory management, you can just initialize or assign them like normal fundamental variables! That makes these classes simpler to use, less error-prone, and you don’t have to spend time writing your own overloaded functions!


  • The default copy constructor and default assignment operators do shallow copies, which is fine for classes that contain no dynamically allocated variables.
  • Classes with dynamically allocated variables need to have a copy constructor and assignment operator that do a deep copy.
  • Favor using classes in the standard library over doing your own memory management.
9.x -- Chapter 9 comprehensive quiz
9.14 -- Overloading the assignment operator

28 comments to 9.15 — Shallow vs. deep copying

  • sergk

    I’d like to note, to avoid problems with inherited classes, one have to make destructor and assignment operators to be virtual.

    -- serg.

  • Phil Braun

    In the following example pulled from the article, there seems to be two problems that can occur.

    The first problem is the line “delete[] m_pchString;”. If “m_pchString” is not allocated, could this cause an exception? Would it not be better to check if “m_pchString” is valid before attempting to delete the memory?

    The second problem occurs if “cSource.m_pchString” is a zero length string. Would it not be better to assign NULL to m_pchString if “cSource.m_pchString” has zero length? Oh, and what happens when an attempt is made to create a zero length string in the “new” statement and when “strncpy” attempts to copy a zero length string?

    Otherwise this code is a good learning tool.


    • Alex

      if m_pchString is pointing to null, deleting it won’t do anything (good or bad). If it’s pointing to deallocated memory, then you’ve got bigger problems already.

      I added a null string check to the constructor, so it’s no longer possible to even create a 0-length string. The smallest string you could create would be length 1 (just a null terminator).

  • prc

    My question is, assuming we didn’t use c style strings, and instead just used std::string (or for that matter, assumed we just use any objected in the heap that wasn’t an array), would the proper way to deep copy it be this:

    or is there a better way that we should go about this?

    • Alex

      If you’re using std::string you can just do a direct assignment:

      The overloaded assignment operator that std::string comes with will handle all of the memory management for you.

  • really very good one and nicely descriptive Thanks

  • enoquick

    An interesting technical for operator=() is :

    T& T::operator=(const T& x) {
    T t(x); // copy constructor
    swap(t); // exception safe
    return *this;

    void T::swap(T&)throw(); // exchange the single components -- no throw exception

  • KM

    Very nice explanation. Made many things clear. Thank you!

  • gulfam

    Very good help in learning OPP

  • Quang

    MyString cHello("Hello, world!");

        MyString cCopy = cHello; // use default copy constructor
    } // cCopy goes out of scope here

    std::cout << cHello.GetString() << std::endl; // this will crash

    I dont understand why there is a block here and what is it use for?
    Thank you for reading Alex!

  • Pramod Gaikwad

    Thank you very much Alex !!!! 🙂

    It is nice C++ tutorial and learning tool ever i have seen. Many OOP’s concept get cleared so far.  much more to come ……….

  • Gopal

    Hi Alex,

    "However, for dynamically allocated variables, we need to explicitly deallocate any old memory before we allocate any new memory. If we don’t, the code will not crash, but we will have a memory leak that will eat away our free memory every time we do an assignment!"

    Can you explain how memory leak happens? Actually it should not as we are overwriting it.

    • Alex

      When we allocate dynamically allocated memory, we assign the address to a pointer so we can access that memory, right? We can also use that pointer to deallocate the memory. If the pointer holding the address of the dynamically allocated memory goes out of scope or gets assigned a different value, then we no longer have any way to deallocate that memory. The memory is thus lost (leaked) until the application closes and the operating system cleans up.

      Thus, we need to deallocate the memory before we assign a new value to the pointer.

  • Mato

    In your examples we see copying of the member variable, what about of copying member functions?

  • Kattencrack Kledge


    In this part of the constructor of the MyString class:

    You accidentally wrote "\n" instead of "\0", which leads to garbage chars when printed out!

  • sitaram chhimpa

    There is error in first copy constructor example

    how can you assign m_numerator directly I think this is typo error.

  • umair

    Can you explain these lines(13-15) in your program under deep copying

    shouldn’t this be source.m_data[i]?

  • The MyString code example that is supposed to produce bad output will not compile because the
    source.m_data[i]; should actually read

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