In object-oriented programming, object copying is the act of creating and initializing a new object based on an existing object's state. The various ways to implement copy have implications that a programmer needs to understand in order to write a computer program that is correct and performant.
Copying allows for the emergent state of the original object – represented by its internal state – to be used and even modified without affecting the original object.
Generally, an object resembles a monolithic concept while having an internal structure that is composite data – a tree of state. Several strategies have been developed to copy this internal state based on program needs and runtime cost.
A similar distinction holds for comparing objects for equality. For two objects to be equal, their state must be the same in a meaningful way. Two objects could be considered equal if their fields are equal without traversing into sub-objects (shallow). Or maybe they are considered equal only if the state is equal throughout the object tree (deep).[clarification needed]
If two variables contain the same reference value, then clearly they refer to the same object which is even more specific than equal.
Even shallower than shallow copy, copying a reference is a form of object copying. This strategy is commonly employed when passing an object to a method. The reference is passed by value – a copy of the reference value (probably an address).
Shallow copy involves creating a new, uninitialized object, B, and copying each field value from the original, A.  Due to this procedure, this is also known as a field-by-field copy, field-for-field copy, or field copy. If the field value is a primitive type (such as int), the value is copied such that changes to the value in B do not affect the value in A. If the field value is a reference to an object (e.g., a memory address) the reference is copied, hence referring to the same object that A does. Changing the state of the inner object affects the emergent state of both A and B since the objects are shared. In a language without primitive types (where everything is an object), all fields of the copy are reference the same objects as the fields of the original.
A shallow copy is often relatively simple to implement and computationally cheap to perform. It can usually be implemented by simply copying a contiguous block of memory.
Deep copy involves copying the state of all subordinate objects – reclusively dereferencing object references at each level of the tree that is the state of the original object and creating new objects and copying fields. A modification of either the original or copied object, including their inner objects, does not affect the other since they share no content.
In more complex cases, some fields in a copy should have shared values with the original object (as in a shallow copy), corresponding to an association relationship; and some fields should have copies (as in a deep copy), corresponding to an aggregation relationship. In these cases a custom implementation of copying is generally required; this issue and solution dates to Smalltalk-80. Alternatively, fields can be marked as requiring a shallow copy or deep copy, and copy operations automatically generated (likewise for comparison operations). This is not implemented in most object-oriented languages, however, though there is partial support in Eiffel.
Lazy copy, related to copy-on-write, is an implementation of a deep copy. When initially copying an object, a relatively fast shallow copy is performed. A counter is also used to track how many objects share the data. When the program wants to modify an object, it can determine if the data is shared (by examining the counter) and can perform a deep copy if needed.
Lazy copy provides the semantics of a deep copy, but takes advantage of the speed of a shallow copy when possible. The downside are rather high but constant base costs because of the counter. Circular references can cause problems.
Generally, an object-oriented programming language provides a way to copy an object. A programmer must define how an custom-defined object is copied, just as they must define if two objects are equal, comparable and so on.
Some languages support one of both of the shallow or deep strategies, defining either one copy operation or separate shallow and deep operations. Many languages provide some default behavior.
In Java, an object is always accessed indirectly – via a reference. An object is never created implicitly but instead is always passed or assigned by a reference variable.
Parameters are passed by value, however, it is the value of the reference that is passed.
The language provides no automatic way to copy an object.
Copying is usually performed by a clone() method. This method usually calls the clone() method of its parent class to obtain a copy, and then does any custom copying procedures. Eventually, this gets to the clone() method of the uppermost object (
Object), which creates a new instance of the same class as the object and copies all the fields to the new instance (a shallow copy). If this method is used, the class must implement the
Cloneable interface, or else it will throw a "Clone Not Supported Exception". After obtaining a copy from the parent class, a class' own clone() method may then provide custom cloning capability, like deep copying (i.e. duplicate some of the structures referred to by the object) or giving the new instance a new unique ID.
The return type of clone() is
Object, but implementers of a clone method could write the type of the object being cloned instead due to Java's support for covariant return types. One advantage of using clone() is that since it is an overridable method, we can call clone() on any object, and it will use the clone() method of its class, without the calling code needing to know what that class is (which would be needed with a copy constructor).
A disadvantage is that one often cannot access the clone() method on an abstract type. Most interfaces and abstract classes in Java do not specify a public clone() method. Thus, often the only way to use the clone() method is if the class of an object is known, which is contrary to the abstraction principle of using the most generic type possible. For example, if one has a List reference in Java, one cannot invoke clone() on that reference because List specifies no public clone() method. Implementations of List like Array List and Linked List all generally have clone() methods, but it is inconvenient and bad abstraction to carry around the class type of an object.
Another way to copy objects in Java is to serialize them through the
Serializable interface. This is typically used for persistence and wire protocol purposes, but it does create copies of objects and, unlike clone, a deep copy that gracefully handles cycled graphs of objects is readily available with minimal effort from a programmer.
Both of these methods suffer from a notable problem: the constructor is not used for objects copied with clone or serialization. This can lead to bugs with improperly initialized data, prevents the use of
final member fields, and makes maintenance challenging. Some utilities attempt to overcome these issues by using reflection to deep copy objects, such as the deep-cloning library.
Runtime objects in Eiffel are accessible either indirectly through references or as expanded objects which fields are embedded within the objects that use them. That is, fields of an object are stored either externally or internally.
The Eiffel class
ANY contains features for shallow and deep copying and cloning of objects. All Eiffel classes inherit from
ANY, so these features are available within all classes, and are applicable both to reference and expanded objects.
copy feature effects a shallow, field-by-field copy from one object to another. In this case no new object is created. If
y were copied to
x, then the same objects referenced by
y before the application of
copy, will also be referenced by
x after the
copy feature completes.
To effect the creation of a new object which is a shallow duplicate of
y, the feature
twin is used. In this case, one new object is created with its fields identical to those of the source.
twin relies on the feature
copy, which can be redefined in descendants of
ANY, if needed. The result of
twin is of the anchored type
Deep copying and creating deep twins can be done using the features
deep_twin, again inherited from class
ANY. These features have the potential to create many new objects, because they duplicate all the objects in an entire object structure. Because new duplicate objects are created instead of simply copying references to existing objects, deep operations will become a source of performance issues more readily than shallow operations.
In C#, rather than using the interface
ICloneable, a generic extension method can be used to create a deep copy using reflection. This has two advantages: First, it provides the flexibility to copy every object without having to specify each property and variable to be copied manually. Second, because the type is generic, the compiler ensures that the destination object and the source object have the same type.
In Objective-C, the methods
mutableCopy are inherited by all objects and intended for performing copies; the latter is for creating a mutable type of the original object. These methods in turn call the
mutableCopyWithZone methods, respectively, to perform the copying. An object must implement the corresponding
copyWithZone method to be copyable.
In Python, the library's copy module provides shallow copy and deep copy of objects through the
deepcopy() functions, respectively. Programmers may define special methods
__deepcopy__() in an object to provide custom copying implementation.
In Ruby, all objects inherit two methods for performing shallow copies, clone and dup. The two methods differ in that
clone copies an object's tainted state, frozen state, and any singleton methods it may have, whereas
dup copies only its tainted state. Deep copies may be achieved by dumping and loading an object's byte stream or YAML serialization. Alternatively, you can use the deep_dive gem to do a controlled deep copy of your object graphs. 
In Perl, nested structures are stored by the use of references, thus a developer can either loop over the entire structure and re-reference the data or use the
dclone() function from the module Storable.
In VBA, an assignment of variables of type
Object is a shallow copy, an assignment for all other types (numeric types, String, user defined types, arrays) is a deep copy. So the keyword
Set for an assignment signals a shallow copy and the (optional) keyword
Let signals a deep copy. There is no built-in method for deep copies of Objects in VBA.
- Goldberg & Robson 1983, pp. 97–99. "There are two ways to make copies of an object. The distinction is whether or not the values of the object's variables are copied. If the values are not copied, then they are shared (
shallowCopy); if the values are copied, then they are not shared (
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- Grogono, Peter; Sakkinen, Markku (12 May 2000). "Copying and Comparing: Problems and Solutions" (PDF). In Elisa Bertino (ed.). Lecture Notes in Computer Science. ECOOP 2000 — Object-Oriented Programming. Vol. 1850. Springer Berlin Heidelberg. pp. 226–250. doi:10.1007/3-540-45102-1_11. Retrieved 2015-06-23.