Default Copy Behavior

When you create a new object from an existing one in C++, you usually do not have to write any special instructions telling the language how to do the copying. Objects can be copied by default. This holds for objects of types you define yourself, and it also holds for builtin types such as int and double.

This article answers a precise question: what, exactly, does that default copy do? The short answer is that the default copy works member by member. Each piece of data that makes up an object is transferred from the source to the new object, one piece at a time. Once you see that pattern, the behavior of many everyday C++ statements becomes predictable.

A member is one of the named pieces of data inside an object. A useful way to picture an object is as a small container with labeled compartments. Each compartment holds one value. The copy operation works through the compartments in turn, moving the value in each one into the matching compartment of the new object.

This is the idea behind the name memberwise copy. The copy is organized around members and is applied to each of them. Nothing about this requires the object to be large or unusual. Even an object with only two members is copied by handling each of those two members.

Read the diagram from left to right. The source object on the left still holds its own members. The target object on the right has its own members, and each one is filled from the matching member of the source. The result is two separate objects that hold equal member values. The source does not describe any further link between the two objects after the copy, so you should not assume one.

The following example is an original illustration rather than the source's own code. It uses a small user-defined type called Pair with two int members, so you can follow each member through the copy. Before reading the output, try to predict what each printed line will show.

Notice that the source in this example is never changed by either copy. The line Pair b = a leaves a with 3 and 7, and the line c = a leaves a with 3 and 7 as well. The copy moves values out of the source and into the target, and the source keeps its own member values.

C++ uses two operations that make use of the default copy. The first is initialization, in which a new object is created and given the values of an existing object at the moment it comes into existence. In the example above, b is created this way. The second is assignment, in which an object that already exists is overwritten with the values of another object. The c in the example is updated this way, even though it had values before the assignment.

The two operations differ in when the target comes into existence, but they share the same core action. The source is explicit that both perform a memberwise copy, so they transfer the same member values. This is why the three variables in the example end up holding the same value even though only one of them was set up by initialization.

A natural question is whether the default copy behaves differently for types you write yourself and for types the language provides. The source says it does not. Objects can be copied by default for user-defined types, and this is also true for builtin types. The default meaning of copy is the same memberwise copy in both cases.