Polymorphism and Operator Overloading
Polymorphism means “many shapes.” In C++ it refers to the ability to define functions with the same name but different arguments, or in different classes; the latter case amounts to at minimum a different type for the hidden instance variable among the arguments. There are two types of polymorphism: compile time and runtime.
For a compiler, binding is the process of associating the calls to a function name with the actual definition of the function. Compile-time polymorphism means that this binding takes place at compile time and does not change when the program is run. Hence it is often called static binding or early binding. Runtime binding is also possible; in this case the compiler’s runtime library is responsible for associating the invocation with the function definition. This is called dynamic binding or late binding.
Compile-Time Polymorphism
Overloading and Templating
Compile-time polymorphism is not limited to user-defined classes. Functions may be overloaded by defining different functions with the same name but different return types and/or different argument lists, in number and/or type of arguments. The compiler internally generates a “mangled” function name that creates a unique function for each case.
#include <iostream>
using namespace std;
int sum(int i, int j) {
return i+j;
}
double sum(double x, double y) {
return x+y;
}
int main() {
cout << sum(9,14) << '\n';
cout << sum(2.0,3.5) << '\n';
return 0;
}
Templating is a form of overloading. We can convert our sum function into a template, and it will work for any type for which the + operator is defined.
#include <iostream>
using namespace std;
template <class T>
T sum(T a, T b) {
T result=a+b;
return result;
}
int main() {
cout << sum<int>(9,14) << '\n';
cout << sum<double>(2.0,3.5) << '\n';
return 0;
}
The class variable name (typename may also be used) is arbitrary, but T is customary.
Many built-in C++ libraries use templates for type declarations.
vector<float> v;
Overriding Class Methods
Even more generally, inherited class methods can be overridden by derived classes, modifying them to be more appropriate to the derived class.
#include<iostream>
#include <string>
using namespace std;
class Animal {
public:
string speak() {
return "nothing";
}
};
class Horse: public Animal {
public:
string speak() {
return "whinny";
}
};
class Wolf: public Animal {
public:
string speak() {
return "howl";
}
};
class Lion: public Animal {
public:
string speak() {
return "roar";
}
};
int main(void) {
Horse secretariat;
Lion simba;
Wolf fang;
cout<<"I am a horse and I "<<secretariat.speak()<<".\n";
cout<<"I am a lion and I "<<simba.speak()<<".\n";
cout<<"I am a wolf and I "<<fang.speak()<<".\n";
return 0;
}
Runtime Polymorphism
Runtime polymorphism, or late binding, is achieved in C++ through virtual functions. The virtual function is declared in the base class and is referenced through pointers or references in the derived classes.
#include<iostream>
#include <string>
using namespace std;
class Animal {
public:
virtual string speak() {
return "nothing";
}
};
class Horse: public Animal {
public:
string speak() override {
return "whinny";
}
};
class Wolf: public Animal {
public:
string speak() override {
return "howl";
}
};
class Lion: public Animal {
public:
string speak() override {
return "roar";
}
};
int main(void) {
Horse secretariat;
Lion simba;
Wolf fang;
Horse& s=secretariat;
Lion& l=simba;
Wolf& w=fang;
cout<<"I am a horse and I "<<s.speak()<<".\n";
cout<<"I am a lion and I "<<l.speak()<<".\n";
cout<<"I am a wolf and I "<<w.speak()<<".\n";
return 0;
}
In this example we attach instances of each type to a reference to the appropriate type.
Among other things, virtual functions enable behaviors to occur when an instance of a class is passed (as a pointer or reference) to a function. The function can bind the class method dynamically when it is invoked.
#include<iostream>
#include <string>
using namespace std;
class Animal {
public:
virtual string speak() {
return "nothing";
}
};
class Horse: public Animal {
public:
string speak() {
return "whinny";
}
};
class Wolf: public Animal {
public:
string speak() {
return "howl";
}
};
class Lion: public Animal {
public:
string speak() {
return "roar";
}
};
void printme(Animal* a) {
cout<<"I "<<a->speak()<<"\n";
}
int main(void) {
Horse* secretariat = new Horse;
Lion* simba = new Lion;
Wolf* fang = new Wolf;
printme(secretariat);
printme(simba);
printme(fang);
return 0;
}
Without virtual each derived class would need its own version of printme.
Note that dynamic/late binding can be slow, so use it judiciously for codes where performance is important.
Operator Overloading
Just as named functions can be overloaded, so can operators. This allows the programmer to define operators on user-defined types (structs and classes), which can result in much more compact and intuitive code, as long as the operators chosen are appropriate and make sense in the context in which they are used. For example, a programmer can define
//Partial implementation
class Point {
public:
float x,y;
Point(float x, float y);
Point operator+(const Point &another);
};
Point Point::operator+(const Point &another);
return Point(x+another.x,y+another.y);
}
In this example,
Point A(x1,y1), B(x2,y2);
Point C(0.,0.);
C=A+B;
makes intuitive sense. We would not want to overload multiplication with this definition.
Most of the standard C++ operators, including several we have not discussed, can be overloaded. Exceptions are . (member selection), .* (pointer to member selection), :: (scope resolution), and :? (conditional). No preprocessor arguments (#) may be redefined.
In general, operator redefinitions do not have to be class members, but typically they are when it is possible. If they are not members, they must be global (not recommended) or a friend function (better). Some operators, such as << for printing, cannot be members because they must take the instance as an argument.
Assignment has a few extra requirements. It must be a member function of a struct or class. The argument is the right side of the equality and must be declared const.
class Point {
public:
float x,y;
Point(float x, float y);
Point operator+(const Point another);
Point& operator=(const Point&);
};
Point& Point::operator=(const Point& rhs) {
x=rhs.x;
y=rhs.y;
return *this;
}
There are some additional complications for overloading assignment that we will not consider here; the major subtleties are copying versus assignment. Copying creates a new instance to hold the data, whereas assignment assigns values to an instance that already exists. C++ also allows self-assignment (f1=f1) and our simple-minded assignment operator above may fail in this case if memory must be allocated. We can improve our example by checking for self-assignment by adding at the top of the function the lines
if (this == &str)
return *this;
See here for details.
For a good discussion of operator overloading, see here or here.
Overloaded operators other than assignment (=) can be inherited by derived classes.