The calling of constructors and destructors when objects are created or
destroyed, has a number of consequences which shall be discussed in this
chapter. Many problems in program development in C are caused by
incorrect memory allocation or memory leaks: memory is not allocated, not
freed, not initialized, boundaries are overwritten, etc.. C++ does not
`magically' solve these problems, but it does provide a number of handy tools.
In this chapter the following topics are discussed:
In this section we shall again use the class Person as example:
class Person
{
public:
// constructors and destructor
Person ();
Person (char const *n, char const *a,
char const *p);
~Person ();
// interface functions
void setname (char const *n);
void setaddress (char const *a);
void setphone (char const *p);
char const *getname (void) const;
char const *getaddress (void) const;
char const *getphone (void) const;
private:
// data fields
char *name;
char *address;
char *phone;
};
In this class the destructor is necessary to prevent that memory,
once allocated for the fields name, address and phone, becomes
unreachable when an object ceases to exist. In the following example a
Person object is created, after which the data fields are printed. After
this the main() function stops, which leads to the deallocation of
memory. The destructor of the class is also shown for illustration purposes.
Note that in this example an object of the class Person is also created
and destroyed using a pointer variable; using the operators new and
delete.
Person::~Person ()
{
delete name;
delete address;
delete phone;
}
void main ()
{
Person
kk ("Karel", "Rietveldlaan",
"050-426044"),
*bill = new Person ("Bill Clinton",
"White House",
"09-1-202-142-3045")
printf("%s, %s, %s\n",
kk.getname (), kk.getaddress (), kk.getphone ());
printf("%s, %s, %s\n",
bill->getname (), bill->getaddress (), bill->getphone ());
delete bill;
}
The memory which is occupied by the object kk is released automatically
when main() terminates: the C++ compiler makes sure that the
destructor is called. Note however the object pointed to by bill is
handled differently. The variable bill is a pointer; and a pointer
variable is, even in C++, in itself no Person. Therefore, before
main() terminates, the memory occupied by the object pointed to by
bill must be explicitly released; hence the statement delete
bill. The operator delete will make sure that the destructor is
called, thereby releasing the three strings of the object.
Variables which are structs or classes can be directly assigned in
C++ in the same way that structs can be assigned in C. The
default action of such an assignment is a byte-by-byte copying from one
compound type to the other.
Let us now consider the consequences of this default action in a program
statement as the following:
void printperson (Person const &p)
{
Person
tmp;
tmp = p;
printf ("Name: %s\n"
"Address: %s\n"
"Phone: %s\n",
tmp.getname (), tmp.getaddress (), tmp.getphone ());
}
We shall follow the execution of this function step by step.
- The function
printperson() expects a reference to a
Person as its parameter p. So far, nothing extraordinary is
happening.
- The function defines a local object
tmp. This means that the
default constructor of Person is called, which -if defined properly-
resets the pointer fields name, address and phone of the
tmp object to zero.
- Next, the object referenced by
p is copied to tmp. By
default this means that sizeof(Person) bytes from p are copied
to tmp.
Now a potentially dangerous situation has arisen. Note that the actual
values in p are pointers, pointing to allocated memory.
Following the assignment this memory is addressed by two objects: p
and tmp.
- The potentially dangerous situation develops into an acutely
dangerous situation when the function
printperson() terminates:
the object tmp is destroyed. The destructor of the class Person
releases the memory pointed to by the fields name, address and
phone: unfortunately, this memory is also in use by p!
The incorrect assignment is illustrated in the following figure.
After the execution of printperson() , the object which was
referenced by p may still contain valid pointers to strings, but pointers
which address deallocated memory. This action is undoubtedly not a desired
effect of a function like the above. The deallocated memory will likely become
occupied during subsequent allocations, thereby causing the previously held
strings to become lost.
In general it can be concluded that every class which contains a
constructor and a destructor, and which contains pointer fields to address
allocated memory, is a potential candidate for trouble. There is of course a
possibility to intervene: this possibility will be discussed in the next
section.
Overloading the assignment operator
Obviously, the right way to assign one Person object to another, is
not to copy the contents of the object byte by byte. A better way is to
make an equivalent object; one with its own allocated memory, but which
contains the same strings.
The `right' way to dupliate a Person object is illustrated in the
following figure.
There is a number of solutions for the above wish. One solution consists of
the definition of a special function to handle assignments of objects of the
class Person. The purpose of this function would be to create a copy of
an object, but one with its own name, address and phone
strings. Such a member function might be:
void Person::assign (Person const &other)
{
// delete our own previously used memory
delete name;
delete address;
delete phone;
// now copy the other's data
name = strdup (other.name);
address = strdup (other.address);
phone = strdup (other.phone);
}
Using this tool we could rewrite the offending function func():
void printperson (Person const &p)
{
Person
tmp;
// make tmp a copy of p, but with its own allocated
// strings
tmp.assign (p);
printf ("Name: %s\n"
"Address: %s\n"
"Phone: %s\n",
tmp.getname (), tmp.getaddress (), tmp.getphone ());
// now it doesn't matter that tmp gets destroyed..
}
In itself this solution is valid, although it is purely symptomatic. This
solution requires that the programmer uses a specific member function instead
of the operator =; the problem however remains if this rule is not
strictly adhered to. Our experience shows that errare humanum est; a
solution which doesn't enforce exceptions is therefore preferable.
The problem of the assignment operator is solved by using operator
overloading: the syntactic possibility of C++ to redefine the actions of
an operator in a given context. Operator overloading was discussed earlier,
when the operators << and >> were redefined for the
usage with streams as cin, cout and cerr (see section
CoutCinCerr
).
Overloading the assignment operator is probably the most common form of
operator overloading. However, a word of warning is appropriate: the fact that
C++ allows operator overloading does not mean that this feature should be
used at all times. A few rules are:
- Operator overloading should be used in situations where an operator
has a defined action, but when this action is not desired as it has
negative side effects. A typical example is the above assignment operator
in the context of the class
Person.
- Operator overloading can be used in situations where the usage of
the operator is common and when no ambiguity in the meaning of the
operator is introduced by redefining it. An example may be the
redefinition of the operator
+ for a class which represents a complex
number. The meaning of a + between two complex numbers is quite
clear and unambiguous.
- In all other cases it is preferable to define a member function,
instead of redefining an operator.
Using these rules, operator overloading is minimized which helps keep source
files readable. An operator simply does what it is designed to do. Therefore,
in our vision, the operators << and >> in the context of
streams are misleading: the stream operations do not have anything in common
with the bitwise shift operations.
The function operator=()
To achieve operator overloading in a context of a class, the class is simply
expanded with a public function which states the operator. A
corresponding function is then defined.
For example, to overload the addition operator + a function
operator+() would be defined. The function name consists of the keyword
operator and the operator itself.
In our case we define a new function operator=() to redefine the actions
of the assignment operator. A possible extension to the class Person
could therefore be:
// new declaration of the class
class Person
{
public:
.
.
void operator= (Person const &other);
.
.
private:
.
.
};
// definition of the function
void Person::operator= (Person const &other)
{
// deallocate old data
delete name;
delete address;
delete phone;
// make duplicates of other's data
name = strdup (other.name);
address = strdup (other.address);
phone = strdup (other.phone);
}
The function operator=() which is presented above is the first version of
the overloaded assignment. We shall present better and less bug-prone versions
later.
The actions of this member function are similar to those of the previously
proposed function assign(), but the name makes sure that this function is
also activated when the assignment operator = is used. In fact there are
two ways to call this function, which are illustrated below:
Person
pers ("Frank", "Oostumerweg 23", "2223"),
copy;
// first possibility
copy = pers;
// second possibility
copy.operator= (pers);
It is obvious that the second possibility, in which operator=() is
explicitly stated, is not used often. The code fragment however illustrates
the similarity of the two methods of calling the function.
As we have seen, a member function of a given class is always called in the
context of some object of the class; there is always an implicit `substrate'
for the function to act on. C++ defines a keyword, this, to address
this substrate
(this is not available in the not yet discussed
static member functions)
. The this keyword is a pointer
variable, which always contains the address of the object in question. The
this pointer is implicitly declared in each member function (whether
public or private); therefore, it is as if in each member function of
the class Person would contain the following declaration:
extern Person *this;
A member function like setname(), which sets a name field of a
Person to a given string, could therefore be implemented in two ways:
with or without the this pointer:
// alternative 1: implicit usage of this
void Person::setname (char const *n)
{
delete name;
name = strdup (n);
}
// alternative 2: explicit usage of this
void Person::setname (char const *n)
{
delete this->name;
this->name = strdup (n);
}
Explicit usage of the this pointer is not very frequent. There is however
a number of situations where the this pointer is needed.
Preventing self-destruction with this
As we have seen, the operator = can be redefined for the class
Person in such a way that two objects of the class can be assigned,
leading to two copies of the same object.
As long as the two variables are different ones, the previously presented
version of the function operator=() will function properly: the memory of
the assigned object is released, after which it is allocated again to hold new
strings. However, when an object is assigned to itself (which is called
auto-assignment), a problem occurs: the allocated strings of the assigned are
first released, but this also leads to the releasing of the strings of the
right-hand side variable! An example of this situation is illustrated below:
void fubar (Person const &p)
{
p = p; // auto-assignment!
}
In this example it is perfectly clear that something unnecessary, possibly
even wrong, is happening. Auto-assignment can however occur in more hidden
forms:
Person
one,
two,
*pp;
pp = &one;
.
.
*pp = two;
.
.
one = *pp;
The problem of the auto-assignment can be solved by using the this
pointer. In the overloaded assignment operator function we simply test whether
the address of the right-hand side object is the same as the address of the
current object: if so, no action needs to be taken. The definition of the
function operator=() then becomes:
void Person::operator= (Person const &other)
{
// only take action if address of current object
// (this) is NOT equal to address of other
// object (&other):
if (this != &other)
{
delete name;
delete address;
delete phone;
name = strdup (other.name);
address = strdup (other.address);
phone = strdup (other.phone);
}
}
This is the second version of the overloaded assignment function. One, yet
better version remains to be discussed.
Note the usage of the address operator in the statement
if (this != &other)
The variable this is a pointer to the `current' object, while other
is a reference; which is an `alias' to an actual Person object. The
address of the other object is therefore &other, while the address of
the current object is this.
Associativity of operators and this
The syntax of C++ states that the associativity of the assignment
operator is to the right-hand side; i.e., in a statement as
a = b = c;
the expression b = c is evaluated first, and the result is assigned to
a.
The implementation of the overloaded assignment operator so far does not
permit such constructions, as an assignment using the member function returns
nothing (void). We can therefore conclude that the previous
implementation does circumvent an allocation problem, but is not quite
syntactically right.
The syntactical problem can be illustrated as follows. When we rewrite the
expression a = b = c to the form which explicitly
mentions member functions, we get:
a.operator= (b.operator= (c));
This is syntactically wrong, since the sub-expression b.operator=(c)
yields void; and the class Person contains no member functions with
the prototype operator=(void).
This problem can also be remedied by using the this pointer. The
overloaded assignment function expects as its argument a reference to a
Person object; in the same way it can return a reference to such an
object. This reference can then be used as an argument for a nested
assignment.
It is customary to let the overloaded assignment return a reference to the
current object (i.e., *this), as a const reference. The (final)
version of the overloaded assignment operator for the class Person thus
becomes:
// declaration in the class
class Person
{
public:
.
.
Person const &operator= (Person const &other)
.
.
};
// definition of the function
Person const &Person::operator= (Person const &other)
{
// only take action when no auto-assignment occurs
if (this != &other)
{
// deallocate own data
delete address;
delete name;
delete phone;
// duplicate other's data
address = strdup (other.address);
name = strdup (other.name);
phone = strdup (other.phone);
}
// return current object, compiler will make sure
// that a const reference is returned
return (*this);
}
In the following sections we shall look closer at another usage of the
operator =. We shall use a class String as an example. This class is
meant to handle allocated strings and is defined as follows:
class String
{
public:
// constructor, destructors
String ();
String (char const *s);
~String ();
// overloaded assignment
String const &operator= (String const &other);
// interface functions
void set (char const *data);
char const *get (void);
private:
// one data field: ptr to allocated string
char *str;
};
Concerning this definition we remark the following:
Now let's consider the following code fragment. The statement references are
discussed below the example:
String
a ("Hello World\n"), // see (1)
b, // see (2)
c = a; // see (3)
int main ()
{
b = c; // see (4)
return (0);
}
- Statement 1 shows an initialization. The object
a is
initialized with a string ``Hello World''. This construction of the object
a therefore uses the constructor which expects one string argument.
It should be noted here that this form is identical to
String
a = "Hello World\n";
Even though this code fragment uses the operator =, this is no
assignment: rather an initialization, and hence, construction.
- In statement 2 a second
String object is created. Again a
constructor is called; but since no special arguments are present, this is
the default constructor.
- In statement 3 a new object
c is created. Again, a constructor
is therefore called. The new object is also initialized with (the data of)
object a.
This form of initializations has not been discussed yet. Since we can
rewrite this statement in the form
String
c (a);
this initialization suggests that a constructor is called, with as
argument a (reference to a) String object. Such constructors are
quite common in C++ and are called copy constructors. More
properties of these constructors are discussed below.
- In statement 4 one object is assigned to another. No object is
created in this statement; hence, this is just an assignment, using
the overloaded assignment operator.
The simple rule which applies here is that whenever an object is created,
a constructor is needed. The form of the constructor is still the following:
- The constructor has no return value.
- The constructor is defined in a function with the same name as the
corresponding class.
- The argument list of the constructor can be deduced from the code;
the argument is either present between parentheses or following a
=.
We conclude therefore that, given the above code statement (3), the class
String must be rewritten to define a copy constructor:
// class definition
class String
{
public:
.
.
String (String const &other);
.
.
};
// constructor definition
String::String (String const &other)
{
str = strdup (other.str);
}
The actions of the copy constructor are similar to those of the overloaded
assignment operator function: an object is duplicated, so that it
contains its own allocated data. The copy constructor function is however
simpler in the following respect:
- A copy constructor doesn't need to deallocate previously allocated
memory: since the object in question has just been created, it cannot
already have its own allocated data.
- A copy constructor never needs to check whether auto-duplication
occurs. No variable can be initialized with itself.
Besides the above mentioned quite obvious usage of the copy constructor, this
constructor has other important tasks. All of these tasks are related to the
fact that the copy constructor is always called when an object is created and
initialized with another object; even when this new object is a hidden or
temporary variable:
- When a function takes an object as argument, instead of e.g. a
pointer or a reference, C++ calls the copy constructor to pass a copy
of an object as the argument. This argument, which usually is passed via
the stack, is therefore a new object; created and initialized with the
data of the passed argument.
This is illustrated in the following code fragment:
void func (String s) // no pointer, no reference
{ // but the String itself
puts (s.get ());
}
int main ()
{
String
hi ("hello world");
func (hi);
return (0);
}
In this code fragment hi itself is not passed as an argument, but
instead a
temporary (stack) variable is created using the copy constructor. This
temporary variable is known within func() as s. Note that by
defining func() with a reference argument, extra stack usage and a
call to the copy constructor would have been avoided.
- The copy constructor is also implicitly called when a function
returns an object.
This situation occurs when, e.g., a function returns keyboard input in a
String format:
String getline ()
{
char
buf [100]; // buffer for kbd input
gets (buf); // read buffer
String
ret = buf; // convert to String
return (ret); // and return it
}
A hidden String object is here initialized with the return value
ret (using the copy constructor) and is returned by the function. The
local variable ret itself ceases to exist when getline()
terminates.
To demonstrate that copy constructors are not called in all situations,
consider the following. We could rewrite the above function getline() to
the following form:
String getline ()
{
char
buf [100]; // buffer for kbd input
gets (buf); // read buffer
return (buf); // and return it
}
This code fragment is quite valid, even though the return value char*
doesn't match the prototype String. In this situation, C++ will try
to convert the char* to a String: this is indeed possible, given a
constructor which expects a char* argument. This means that the copy
constructor is not used in this version of getline(). Instead, the
constructor expecting a char* argument is used.
Similarities between the copy constructor and operator=()
The similarities between on one hand the copy constructor and on the other
hand the overloaded assignment operator are reinvestigated in this section.
We present here two primitive functions which often occur in `our' code, and
which we think are quite useful. We remark that:
- The duplication of (private) data occurs in the copy constructor
and in the overloaded assignment function.
- The deallocation of used memory occurs in the overloaded assignment
function and in the destructor.
The two above actions (duplication and deallocation) can be coded in two
primitive functions, say copy() and destroy(), which are used in the
overloaded assignment operator, the copy constructor, and the destructor. When
we apply this method to the class Person, we can rewrite the code as
follows.
First, the class definition is expanded with two private functions
copy() and destroy(). The purpose of these functions is to
unconditionally copy the data of another object or to deallocate the
memory of the current object. Hence these functions implement `primitive'
functionality:
// class definition, only relevant functions are shown here
class Person
{
public:
// constructors, destructor
Person (Person const &other);
~Person ();
// overloaded assignment
Person const &operator= (Person const &other);
.
.
private:
// data fields
char *name, *address, *phone;
// the two primitives
void copy (Person const &other);
void destroy (void);
};
Next, we present the implementation of the functions copy() and
destroy():
// copy(): unconditionally copy other object's data
void Person::copy (Person const &other)
{
name = strdup (other.name);
address = strdup (other.address);
phone = strdup (other.phone);
}
// destroy(): unconditionally deallocate data
void Person::destroy ()
{
delete name;
delete address;
delete phone;
}
Finally the three public functions in which other object's memory is
copied or in which memory is deallocated are rewritten:
// copy constructor
Person::Person (Person const &other)
{
// unconditionally copy other's data
copy (other);
}
// destructor
Person::~Person ()
{
// unconditionally deallocate
destroy ();
}
// overloaded assignment
Person const &Person::operator= (Person const &other)
{
// only take action if no auto-assignment
if (this != &other)
{
destroy ();
copy (other);
}
// return (reference to) current object for
// chain-assignments
return (*this);
}
The following sections present more examples of operator overloading.
Overloading operator[]
As one more example of operator overloading, we present here a class which is
meant to represent an array of ints. Indexing the array elements occurs
with the standard array operator [], but additionally the class checks
for boundary overflow.
An example of the usage of the class is given below:
int main ()
{
Intarray
x (20); // 20 ints
for (register int i = 0; i < 20; i++)
x [i] = i * 2; // assign the elements
for (i = 0; i <= 20; i++)
printf ("At index %d: value %d\n",
i, x [i]);
return (0);
}
This example shows how an array is created to hold 20 ints. The elements
of the array can be assigned or retrieved. The above example should produce a
run-time error, which is generated by the class Intarray: the last
for loop causes a boundary overflow, since x[20] is addressed while
legal indexes are range from 0 to 19.
The definition of the class is given below:
class Intarray
{
public:
// constructors, destructor etc.
Intarray (int sz = 1); // default size: 1 int
Intarray (Intarray const &other);
~Intarray ();
Intarray const &operator= (Intarray const &other);
// the interface
int &operator[] (int index);
private:
// data
int *data, size;
};
Concerning this class definition we remark:
- The class has a constructor with a default
int argument,
specifying the array size. This function serves also as the default
constructor, since the compiler will substitute 1 for the argument when
none is given.
- The class uses internally a pointer to address allocated memory.
Hence, the necessary tools are provided: a copy constructor, an overloaded
assignment function and a destructor.
- The interface is defined as a function which returns a reference to
an
int. This allows an expression like x[10] to be used on the
left-hand side and on the right-hand side of an assignment. We can
therefore use the same function to retrieve and to set data of the class.
The member functions of the class are given below.
// constructor
Intarray::Intarray (int sz)
{
// check for legal size specification
if (sz < 1)
{
printf ("Intarray: size of array must be >= 1, not %d!\n", sz);
exit (1);
}
// remember size, create array
size = sz;
data = new int [sz];
}
// copy constructor
Intarray::Intarray (Intarray const &other)
{
// set size
size = other.size;
// create array
data = new int [size];
// copy other's values
for (register int i = 0; i < size; i++)
data [i] = other.data [i];
}
// overloaded assignment
Intarray const &Intarray::operator= (Intarray const &other)
{
// take action only when no auto-assignment
if (this != &other)
{
// set size
size = other.size;
// remove previous memory, create new array
delete [] data;
data = new int [size];
// copy other's data
for (register int i = 0; i < size; i++)
data [i] = other.data [i];
}
return (*this);
}
// here is the interface function
int &Intarray::operator[] (int index)
{
// check for array boundary over/underflow
if (index < 0 || index >= size)
{
printf ("Intarray: boundary overflow or underflow, "
"index=%d, should range from 0 to %d\n",
index, size - 1);
exit (1);
}
// emit the reference
return (data [index]);
}