FAQs in section [13]:
[13.1] What's the deal with operator overloading?
It allows you to provide an intuitive interface to users of your class, plus
makes it possible for templates to work equally
well with classes and built-in/intrinsic types.
Operator overloading allows C/C++ operators to have user-defined meanings on
user-defined types (classes). Overloaded operators are syntactic sugar for
function calls:
class Fred {
public:
...
};
#if 0
// Without operator overloading:
Fred add(const Fred& x, const Fred& y);
Fred mul(const Fred& x, const Fred& y);
Fred f(const Fred& a, const Fred& b, const Fred& c)
{
return add(add(mul(a,b), mul(b,c)), mul(c,a)); // Yuk...
}
#else
// With operator overloading:
Fred operator+ (const Fred& x, const Fred& y);
Fred operator* (const Fred& x, const Fred& y);
Fred f(const Fred& a, const Fred& b, const Fred& c)
{
return a*b + b*c + c*a;
}
#endif
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[13.2] What are the benefits of operator overloading?
By overloading standard operators on a class, you can exploit the intuition of
the users of that class. This lets users program in the language of the
problem domain rather than in the language of the machine.
The ultimate goal is to reduce both the learning curve and the defect rate.
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[13.3] What are some examples of operator overloading?
Here are a few of the many examples of operator overloading:
- myString + yourString might concatenate two std::string
objects
- myDate++ might increment a Date object
- a * b might multiply two Number objects
- a[i] might access an element of an Array object
- x = *p might dereference a "smart pointer" that "points" to
a disk record — it could seek to the location on disk where p "points" and
return the appropriate record into x
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[13.4] But operator overloading makes my class look ugly; isn't it supposed to make my code clearer?
Operator overloading makes life easier for the
users of a class, not for the developer of the class!
Consider the following example.
class Array {
public:
int& operator[] (unsigned i); // Some people don't like this syntax
...
};
inline
int& Array::operator[] (unsigned i) // Some people don't like this syntax
{
...
}
Some people don't like the keyword operator or the somewhat funny
syntax that goes with it in the body of the class itself. But the operator
overloading syntax isn't supposed to make life easier for the developer
of a class. It's supposed to make life easier for the users of the
class:
int main()
{
Array a;
a[3] = 4; // User code should be obvious and easy to understand...
...
}
Remember: in a reuse-oriented world, there will usually be many people who use
your class, but there is only one person who builds it (yourself); therefore
you should do things that favor the many rather than the few.
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[13.5] What operators can/cannot be overloaded?
Most can be overloaded. The only C operators that can't be are . and ?:
(and sizeof, which is technically an operator). C++ adds a few of its own
operators, most of which can be overloaded except :: and .*.
Here's an example of the subscript operator (it returns a reference). First
without operator overloading:
class Array {
public:
int& elem(unsigned i) { if (i > 99) error(); return data[i]; }
private:
int data[100];
};
int main()
{
Array a;
a.elem(10) = 42;
a.elem(12) += a.elem(13);
...
}
Now the same logic is presented with operator overloading:
class Array {
public:
int& operator[] (unsigned i) { if (i > 99) error(); return data[i]; }
private:
int data[100];
};
int main()
{
Array a;
a[10] = 42;
a[12] += a[13];
...
}
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[13.6] Can I overload operator== so it lets me compare two char[] using a string comparison?
No: at least one operand of any
overloaded operator must be of some user-defined type (most of the
time that means a class).
But even if C++ allowed you to do this, which it doesn't, you wouldn't want to
do it anyway since you really should be using a std::string-like class rather than an array of char in the first place
since arrays are evil.
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[13.7] Can I create a operator** for "to-the-power-of" operations?
Nope.
The names of, precedence of, associativity of, and arity of operators is fixed
by the language. There is no operator** in C++, so you cannot create one for
a class type.
If you're in doubt, consider that x ** y is the same as x *
(*y) (in other words, the compiler assumes y is a pointer). Besides,
operator overloading is just syntactic sugar for function calls. Although
this particular syntactic sugar can be very sweet, it doesn't add anything
fundamental. I suggest you overload pow(base,exponent) (a double
precision version is in <cmath>).
By the way, operator^ can work for to-the-power-of, except it has the wrong
precedence and associativity.
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[13.8] How do I create a subscript operator for a Matrix class?
Use operator() rather than operator[].
When you have multiple subscripts, the cleanest way to do it is with
operator() rather than with operator[]. The reason is that
operator[] always takes exactly one parameter, but operator()
can take any number of parameters (in the case of a rectangular matrix, two
paramters are needed).
For example:
class Matrix {
public:
Matrix(unsigned rows, unsigned cols);
double& operator() (unsigned row, unsigned col);
double operator() (unsigned row, unsigned col) const;
...
~Matrix(); // Destructor
Matrix(const Matrix& m); // Copy constructor
Matrix& operator= (const Matrix& m); // Assignment operator
...
private:
unsigned rows_, cols_;
double* data_;
};
inline
Matrix::Matrix(unsigned rows, unsigned cols)
: rows_ (rows),
cols_ (cols),
data_ (new double[rows * cols])
{
if (rows == 0 || cols == 0)
throw BadIndex("Matrix constructor has 0 size");
}
inline
Matrix::~Matrix()
{
delete[] data_;
}
inline
double& Matrix::operator() (unsigned row, unsigned col)
{
if (row >= rows_ || col >= cols_)
throw BadIndex("Matrix subscript out of bounds");
return data_[cols_*row + col];
}
inline
double Matrix::operator() (unsigned row, unsigned col) const
{
if (row >= rows_ || col >= cols_)
throw BadIndex("const Matrix subscript out of bounds");
return data_[cols_*row + col];
}
Then you can access an element of Matrix m using m(i,j)
rather than m[i][j]:
int main()
{
Matrix m(10,10);
m(5,8) = 106.15;
std::cout << m(5,8);
...
}
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[13.9] Why shouldn't my Matrix class's interface look like an array-of-array?
Here's what this FAQ is really all about: Some people build a Matrix class that
has an operator[] that returns a reference to an Array object,
and that Array object has an operator[] that returns an element
of the Matrix (e.g., a reference to a double). Thus they access
elements of the matrix using syntax like m[i][j] rather than
syntax like m(i,j).
The array-of-array solution obviously works, but it is less flexible than
the operator() approach. Specifically,
there are easy performance tuning tricks that can be done with the
operator() approach that are more difficult in the [][]
approach, and therefore the [][] approach is more likely to lead to bad
performance, at least in some cases.
For example, the easiest way to implement the [][] approach is to use a
physical layout of the matrix as a dense matrix that is stored in row-major
form (or is it column-major; I can't ever remember). In contrast,
the operator() approach totally hides
the physical layout of the matrix, and that can lead to better performance in
some cases.
Put it this way: the operator() approach is never worse than, and
sometimes better than, the [][] approach.
- The operator() approach is never worse because it is easy to
implement the dense, row-major physical layout using the operator()
approach, so when that configuration happens to be the optimal layout from a
performance standpoint, the operator() approach is just as easy as the
[][] approach (perhaps the operator() approach is a tiny bit
easier, but I won't quibble over minor nits).
- The operator() approach is sometimes better because whenever the
optimal layout for a given application happens to be something other than
dense, row-major, the implementation is often significantly easier using the
operator() approach compared to the [][] approach.
As an example of when a physical layout makes a significant difference, a
recent project happened to access the matrix elements in columns (that is, the
algorithm accesses all the elements in one column, then the elements in
another, etc.), and if the physical layout is row-major, the accesses can
"stride the cache". For example, if the rows happen to be almost as big as the
processor's cache size, the machine can end up with a "cache miss" for almost
every element access. In this particular project, we got a 20% improvement in
performance by changing the mapping from the logical layout (row,column) to the
physical layout (column,row).
Of course there are many examples of this sort of thing from numerical methods,
and sparse matrices are a whole other dimension on this issue. Since it is, in
general, easier to implement a sparse matrix or swap row/column ordering using
the operator() approach, the operator() approach loses nothing
and may gain something — it has no down-side and a potential up-side.
Use the operator() approach.
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[13.10] Should I design my classes from the outside (interfaces first) or from the inside (data first)?
From the outside!
A good interface provides a simplified view that
is expressed in the vocabulary of a user. In the case of OO
software, the interface is normally the set of public methods of either a
single class or a tight group of classes.
First think about what the object logically represents, not how you intend to
physically build it. For example, suppose you have a Stack class that will
be built by containing a LinkedList:
class Stack {
public:
...
private:
LinkedList list_;
};
Should the Stack have a get() method that returns the LinkedList? Or a
set() method that takes a LinkedList? Or a constructor that takes a
LinkedList? Obviously the answer is No, since you should design your
interfaces from the outside-in. I.e., users of Stack objects don't care
about LinkedLists; they care about pushing and popping.
Now for another example that is a bit more subtle. Suppose class LinkedList
is built using a linked list of Node objects, where each Node object has a
pointer to the next Node:
class Node { /*...*/ };
class LinkedList {
public:
...
private:
Node* first_;
};
Should the LinkedList class have a get() method that will let users access
the first Node? Should the Node object have a get() method that will let
users follow that Node to the next Node in the chain? In other words, what
should a LinkedList look like from the outside? Is a LinkedList really a
chain of Node objects? Or is that just an implementation detail? And if it
is just an implementation detail, how will the LinkedList let users access
each of the elements in the LinkedList one at a time?
The key insight is the realization that a LinkedList is not a chain
of Nodes. That may be how it is built, but that is not what
it is. What it is is a sequence of elements. Therefore the LinkedList
abstraction should provide a "LinkedListIterator" class as well, and that
"LinkedListIterator" might have an operator++ to go to the next
element, and it might have a get()/set() pair to access its value
stored in the Node (the value in the Node element is solely the
responsibility of the LinkedList user, which is why there is a
get()/set() pair that allows the user to freely manipulate that value).
Starting from the user's perspective, we might want our LinkedList class to
support operations that look similar to accessing an array using pointer
arithmetic:
void userCode(LinkedList& a)
{
for (LinkedListIterator p = a.begin(); p != a.end(); ++p)
std::cout << *p << '\n';
}
To implement this interface, LinkedList will need a begin() method and an
end() method. These return a "LinkedListIterator" object. The
"LinkedListIterator" will need a method to go forward, ++p; a method to
access the current element, *p; and a comparison operator, p !=
a.end().
The code follows. The important thing to notice is that LinkedList does
not have any methods that let users access Nodes. Nodes are an
implementation technique that is completely buried. This makes the
LinkedList class safer (no chance a user will mess up the invariants and
linkages between the various nodes), easier to use (users don't need to expend
extra effort keeping the node-count equal to the actual number of nodes, or
any other infrastructure stuff), and more flexible (by changing a single
typedef, users could change their code from using LinkedList to some
other list-like class and the bulk of their code would compile cleanly and
hopefully with improved performance characteristics).
#include <cassert> // Poor man's exception handling
class LinkedListIterator;
class LinkedList;
class Node {
// No public members; this is a "private class"
friend LinkedListIterator; // A friend class
friend LinkedList;
Node* next_;
int elem_;
};
class LinkedListIterator {
public:
bool operator== (LinkedListIterator i) const;
bool operator!= (LinkedListIterator i) const;
void operator++ (); // Go to the next element
int& operator* (); // Access the current element
private:
LinkedListIterator(Node* p);
Node* p_;
friend LinkedList; // so LinkedList can construct a LinkedListIterator
};
class LinkedList {
public:
void append(int elem); // Adds elem after the end
void prepend(int elem); // Adds elem before the beginning
...
LinkedListIterator begin();
LinkedListIterator end();
...
private:
Node* first_;
};
Here are the methods that are obviously inlinable (probably in the same header
file):
inline bool LinkedListIterator::operator== (LinkedListIterator i) const
{
return p_ == i.p_;
}
inline bool LinkedListIterator::operator!= (LinkedListIterator i) const
{
return p_ != i.p_;
}
inline void LinkedListIterator::operator++()
{
assert(p_ != NULL); // or if (p_==NULL) throw ...
p_ = p_->next_;
}
inline int& LinkedListIterator::operator*()
{
assert(p_ != NULL); // or if (p_==NULL) throw ...
return p_->elem_;
}
inline LinkedListIterator::LinkedListIterator(Node* p)
: p_(p)
{ }
inline LinkedListIterator LinkedList::begin()
{
return first_;
}
inline LinkedListIterator LinkedList::end()
{
return NULL;
}
Conclusion: The linked list had two different kinds of data. The values of the
elements stored in the linked list are the responsibility of the user of the
linked list (and only the user; the linked list itself makes no attempt
to prohibit users from changing the third element to 5), and the linked list's
infrastructure data (next pointers, etc.), whose values are the
responsibility of the linked list (and only the linked list; e.g., the
linked list does not let users change (or even look at!) the various
next pointers).
Thus the only get()/set() methods were to get and set the elements
of the linked list, but not the infrastructure of the linked list. Since the
linked list hides the infrastructure pointers/etc., it is able to make very
strong promises regarding that infrastructure (e.g., if it was a doubly linked
list, it might guarantee that every forward pointer was matched by a backwards
pointer from the next Node).
So, we see here an example of where the values of some of a class's
data is the responsibility of users (in which case the class needs to
have get()/set() methods for that data) but the data that the class wants
to control does not necessarily have get()/set() methods.
Note: the purpose of this example is not to show you how to write a
linked-list class. In fact you should not "roll your own" linked-list
class since you should use one of the "container classes" provided with your
compiler. Ideally you'll use one of the standard container
classes such as the std::list<T> template.
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[13.11] How can I overload the prefix and postfix forms of operators ++ and --?
Via a dummy parameter.
Since the prefix and postfix ++ operators can have two definitions,
the C++ language gives us two different signatures. Both are called
operator++(), but the prefix version takes no parameters and the
postfix version takes a dummy int. (Although this discussion revolves
around the ++ operator, the -- operator is completely
symmetric, and all the rules and guidelines that apply to one also apply to
the other.)
class Number {
public:
Number& operator++ (); // prefix ++
Number operator++ (int); // postfix ++
};
Note the different return types: the prefix version returns by reference, the
postfix version by value. If that's not immediately obvious to you, it should
be after you see the definitions (and after you remember that y = x++
and y = ++x set y to different things).
Number& Number::operator++ ()
{
...
return *this;
}
Number Number::operator++ (int)
{
Number ans = *this;
++(*this); // or just call operator++()
return ans;
}
The other option for the postfix version is to return nothing:
class Number {
public:
Number& operator++ ();
void operator++ (int);
};
Number& Number::operator++ ()
{
...
return *this;
}
void Number::operator++ (int)
{
++(*this); // or just call operator++()
}
However you must *not* make the postfix version return the 'this' object by
reference; you have been warned.
Here's how you use these operators:
Number x = /* ... */;
++x; // calls Number::operator++(), i.e., calls x.operator++()
x++; // calls Number::operator++(int), i.e., calls x.operator++(0)
Assuming the return types are not 'void', you can use them in larger
expressions:
Number x = /* ... */;
Number y = ++x; // y will be the new value of x
Number z = x++; // z will be the old value of x
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[13.12] Which is more efficient: i++ or ++i?
++i is sometimes faster than, and is never slower than, i++.
For intrinsic types like int, it doesn't matter: ++i and
i++ are the same speed. For class types like iterators or the
previous FAQ's Number class, ++i very well might be faster
than i++ since the latter might make a copy of the this
object.
The overhead of i++, if it is there at all, won't probably make any
practical difference unless your app is CPU bound. For example, if your app
spends most of its time waiting for someone to click a mouse, doing disk I/O,
network I/O, or database queries, then it won't hurt your performance to waste
a few CPU cycles. However it's just as easy to type ++i as
i++, so why not use the former unless you actually need the old value
of i.
So if you're writing i++ as a statement rather than as part of a
larger expression, why not just write ++i instead? You never lose
anything, and you sometimes gain something. Old line C programmers are used
to writing i++ instead of ++i. E.g., they'll say, for (i
= 0; i < 10; i++) .... Since this uses i++ as a
statement, not as a part of a larger expression, then you might want to use
++i instead. For symmetry, I personally advocate that style even when
it doesn't improve speed, e.g., for intrinsic types and for class types with
postfix operators that return void.
Obviously when i++ appears as a part of a larger expression, that's
different: it's being used because it's the only logically correct solution,
not because it's an old habit you picked up while programming in C.
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