How Does the Default Copy Constructor Handle Classes with Objects Lacking Declared Copy Constructors?

Default Copy Constructor for a Class Containing Other Objects

When declaring classes in C , the compiler may automatically provide a default copy constructor if an explicit implementation is not provided. However, the behavior of this constructor becomes more complex when dealing with classes containing objects that do not have their declared copy constructors.

Consider the following code:

class Foo {
  Bar bar;
};

class Bar {
  int i;
  Baz baz;
};

class Baz {
  int j;
};

Foo f1;
Foo f2(f1);

The question arises: what will the default copy constructor do in this scenario?

Compiler-Generated Copy Constructor Behavior

The compiler-generated copy constructor for Foo will behave as follows:

1. Call the copy constructor of its base class (if any).
2. Copy each member variable of Foo. This involves calling the copy constructors for Bar.
3. The copy constructor of Bar will then call the copy constructors for Baz.

Therefore, in the example provided, when constructing f2 using f1, the compiler will:

1. Call the copy constructor of Foo for f2.
2. The copy constructor of Foo will call the copy constructor of bar.
3. The copy constructor of bar will call the copy constructor of baz.

Since Baz does not have a declared copy constructor, the compiler will generate a default copy constructor for it. This default copy constructor will perform shallow copy, copying the value of j but not the object it points to.

Shallow vs. Deep Copy

It's important to note that this shallow copy behavior means that any objects pointed to by the member variables of Foo will not be copied. This can have unintended consequences if the original objects are modified later, as the modified values will not be reflected in the copied objects.

For deeper control over the copying process, it is recommended to explicitly define copy constructors for any classes that require controlled copying of their member variables.

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