In object-oriented programming, object copying is creating a copy of an existing object, a unit of data in object-oriented programming. The resulting object is called an object copy or simply copy of the original object. Copying is basic but has subtleties and can have significant overhead. There are several ways to copy an object, most commonly by a copy constructor or cloning. Copying is done mostly so the copy can be modified or moved, or the current value preserved. If either of these is unneeded, a reference to the original data is sufficient and more efficient, as no copying occurs.

Objects in general store composite data. While in simple cases copying can be done by allocating a new, uninitialized object and copying all fields (attributes) from the original object, in more complex cases this does not result in desired behavior.

The design goal of most objects is to give the resemblance of being made out of one monolithic block even though most are not. As objects are made up of several different parts, copying becomes nontrivial. Several strategies exist to treat this problem.

Consider an object A, which contains fields x_i (more concretely, consider if A is a string and x_i is an array of its characters). There are different strategies for making a copy of A, referred to as shallow copy and deep copy. Many languages allow generic copying by one or either strategy, defining either one copy operation or separate shallow copy and deep copy operations. Note that even shallower is to use a reference to the existing object A, in which case there is no new object, only a new reference.

The terminology of shallow copy and deep copy dates to Smalltalk-80. The same distinction holds for comparing objects for equality: most basically there is a difference between identity (same object) and equality (same value), corresponding to shallow equality and (1 level) deep equality of two object references, but then further whether equality means comparing only the fields of the object in question or dereferencing some or all fields and comparing their values in turn (e.g., are two linked lists equal if they have the same nodes, or if they have same values?).^{[clarification needed]}

One method of copying an object is the shallow copy. In that case a new object B is created, and the fields values of A are copied over to B. This is also known as a field-by-field copy, field-for-field copy, or field copy. If the field value is a reference to an object (e.g., a memory address) it copies the reference, hence referring to the same object as A does, and if the field value is a primitive type, it copies the value of the primitive type. In languages without primitive types (where everything is an object), all fields of the copy B are references to the same objects as the fields of original A. The referenced objects are thus shared, so if one of these objects is modified (from A or B), the change is visible in the other. Shallow copies are simple and typically cheap, as they can usually be implemented by simply copying the bits exactly.

An alternative is a deep copy, meaning that fields are dereferenced: rather than references to objects being copied, new copy of objects are created for any referenced objects, and references to these are placed in B. Later modifications to the contents remain unique to A or B, as the contents are not shared.

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.