Monday, 14 December 2015
Copy Constructors:
The copy constructor lets you create a new object from an existing one by initialization.
A copy constructor of a class A is a non-template constructor in which the first parameter is of type A&, const A&, volatile A&, or const volatile A&, and the rest of its parameters (if there are any) have default values.
Used to make deep copy of objects.
There are 3 important places where a copy constructor is called.
- When an object is created from another object of the same type
- When an object is passed by value as a parameter to a function
- When an object is returned from a function
Example:
A variable is declared which is initialized from another object
A value parameter is initialized from its corresponding argument.
An object is returned by a function.
Copy constructor needs to overridden to support deep copy, if copy constructor is required then it requires a destructor and needs to override assignment operator as well.
If the object has no pointers to dynamically allocated memory, a shallow copy is probably sufficient. Therefore the default copy constructor, default assignment operator, and default destructor are ok and you don't need to write your own.
Implementation Example:
The rule of three
Sometimes you need to implement a class that manages a resource. (Never manage multiple resources in a single class, this will only lead to pain.) In that case, remember the rule of three:
If you need to explicitly declare either the destructor, copy constructor or copy assignment operator yourself, you probably need to explicitly declare all three of them.
So when should we declare those special member functions explicitly? When our class manages a resource, that is, when an object of the class is responsible for that resource. That usually means the resource is acquired in the constructor (or passed into the constructor) and released in the destructor.
C++ treats variables of user-defined types with value semantics. This means that objects are implicitly copied in various contexts, and we should understand what "copying an object" actually means.
Let us consider a simple example:
class person
{
std::string name;
int age;
public:
person(const std::string& name, int age) : name(name), age(age)
{
}
};
int main()
{
person a("Bjarne Stroustrup", 60);
person b(a); // What happens here?
b = a; // And here?
}
Special member functions
What does it mean to copy a
person object? The main function shows two distinct copying scenarios. The initialization person b(a); is performed by the copy constructor. Its job is to construct a fresh object based on the state of an existing object. The assignment b = a is performed by the copy assignment operator. Its job is generally a little more complicated, because the target object is already in some valid state that needs to be dealt with.
Since we declared neither the copy constructor nor the assignment operator (nor the destructor) ourselves, these are implicitly defined for us. Quote from the standard:
The [...] copy constructor and copy assignment operator, [...] and destructor are special member functions. [ Note: The implementation will implicitly declare these member functions for some class types when the program does not explicitly declare them. The implementation will implicitly define them if they are used. [...]
By default, copying an object means copying its members:
The implicitly-defined copy constructor for a non-union class X performs a member wise copy of its sub objects.
The implicitly-defined copy assignment operator for a non-union class X performs memberwise copy assignment of its sub objects.
Implicit definitions
The implicitly-defined special member functions for
person look like this:// 1. copy constructor
person(const person& that) : name(that.name), age(that.age)
{
}
// 2. copy assignment operator
person& operator=(const person& that)
{
name = that.name;
age = that.age;
return *this;
}
// 3. destructor
~person()
{
}
Member wise copying is exactly what we want in this case:
name and age are copied, so we get a self-contained, independent person object. The implicitly-defined destructor is always empty. This is also fine in this case since we did not acquire any resources in the constructor. The members' destructors are implicitly called after the person destructor is finished:
After executing the body of the destructor and destroying any automatic objects allocated within the body, a destructor for class X calls the destructors for X's direct [...] members
Managing resources
So when should we declare those special member functions explicitly? When our class manages a resource, that is, when an object of the class is responsible for that resource. That usually means the resource is acquired in the constructor (or passed into the constructor) and released in the destructor.
Let us go back in time to pre-standard C++. There was no such thing as
std::string, and programmers were in love with pointers. The person class might have looked like this:class person
{
char* name;
int age;
public:
// the constructor acquires a resource:
// in this case, dynamic memory obtained via new[]
person(const char* the_name, int the_age)
{
name = new char[strlen(the_name) + 1];
strcpy(name, the_name);
age = the_age;
}
// the destructor must release this resource via delete[]
~person()
{
delete[] name;
}
};
Even today, people still write classes in this style and get into trouble: "I pushed a person into a vector and now I get crazy memory errors!" Remember that by default, copying an object means copying its members, but copying the
name member merely copies a pointer, not the character array it points to! This has several unpleasant effects:- Changes via
acan be observed viab. - Once
bis destroyed,a.nameis a dangling pointer. - If
ais destroyed, deleting the dangling pointer yields undefined behavior. - Since the assignment does not take into account what
namepointed to before the assignment, sooner or later you will get memory leaks all over the place.
Explicit definitions
Since memberwise copying does not have the desired effect, we must define the copy constructor and the copy assignment operator explicitly to make deep copies of the character array:
// 1. copy constructor
person(const person& that)
{
name = new char[strlen(that.name) + 1];
strcpy(name, that.name);
age = that.age;
}
// 2. copy assignment operator
person& operator=(const person& that)
{
if (this != &that)
{
delete[] name;
// This is a dangerous point in the flow of execution!
// We have temporarily invalidated the class invariants,
// and the next statement might throw an exception,
// leaving the object in an invalid state :(
name = new char[strlen(that.name) + 1];
strcpy(name, that.name);
age = that.age;
}
return *this;
}
Note the difference between initialization and assignment: we must tear down the old state before assigning to
name to prevent memory leaks. Also, we have to protect against self-assignment of the form x = x. Without that check, delete[] name would delete the array containing the source string, because when you write x = x, both this->name and that.name contain the same pointer.Exception safety
Unfortunately, this solution will fail if
new char[...] throws an exception due to memory exhaustion. One possible solution is to introduce a local variable and reorder the statements:// 2. copy assignment operator
person& operator=(const person& that)
{
char* local_name = new char[strlen(that.name) + 1];
// If the above statement throws,
// the object is still in the same state as before.
// None of the following statements will throw an exception :)
strcpy(local_name, that.name);
delete[] name;
name = local_name;
age = that.age;
return *this;
}
This also takes care of self-assignment without an explicit check. An even more robust solution to this problem is the copy-and-swap idiom, but I will not go into the details of exception safety here. I only mentioned exceptions to make the following point: Writing classes that manage resources is hard.
Noncopyable resources
Some resources cannot or should not be copied, such as file handles or mutexes. In that case, simply declare the copy constructor and copy assignment operator as
private without giving a definition:private:
person(const person& that);
person& operator=(const person& that);
Alternatively, you can inherit from
boost::noncopyable or declare them as deleted (C++0x):person(const person& that) = delete;
person& operator=(const person& that) = delete;
