The Four Polymorphisms in C++ Subtype Polymorphism (Runtime Polymorphism) Polymorphism in C

Transcription

1 The Four Polymorphisms in C++ When people talk about polymorphism in C++ they usually mean the thing of using a derived class through the base class pointer or reference, which is called subtype polymorphism. But they often forget that there are all kinds of other polymorphisms in C++, such as parametric polymorphism, adhoc polymorphismand coercion polymorphism. These polymorphisms also go by different names in C++, Subtype polymorphism is also known as runtime polymorphism. Parametric polymorphism is also known as compile-time polymorphism. Ad-hoc polymorphism is also known as overloading. Coercion is also known as (implicit or explicit) casting. In this article I'll illustrate all the polymorphisms through examples in C++ language and also give insight on why they have various other names. Subtype Polymorphism (Runtime Polymorphism) Subtype polymorphism is what everyone understands when they say "polymorphism" in C++. It's the ability to use derived classes through base class pointers and references. Here is an example. Suppose you have various cats like these felines, Page 1

2 Polymorphic cats on a mat by James Halliday. Since they are all of Felidae biological family, and they all should be able to meow, they can be represented as classes inheriting from Felid base class and overriding the meow pure virtual function, // file cats.h class Felid { public: virtual void meow() = 0; }; class Cat : public Felid { public: void meow() { std::cout << "Meowing like a regular cat! meow!\n"; } }; class Tiger : public Felid { public: void meow() { std::cout << "Meowing like a tiger! MREOWWW!\n"; } }; Page 2

3 class Ocelot : public Felid { public: void meow() { std::cout << "Meowing like an ocelot! mews!\n"; } }; Now the main program can use Cat, Tiger and Ocelot interchangeably through Felid (base class) pointer, #include <iostream> #include "cats.h" void do_meowing(felid *cat) { } cat->meow(); int main() { Cat cat; Tiger tiger; Ocelot ocelot; do_meowing(&cat); do_meowing(&tiger); do_meowing(&ocelot); } Here the main program passes pointers to cat, tiger and ocelot to do_meowing function that expects a pointer to Felid. Since they are all Felids, the program calls the right meow function for each felid and the output is: Meowing like a regular cat! meow! Meowing like a tiger! MREOWWW! Meowing like an ocelot! mews! Subtype polymorphism is also called runtime polymorphism for a good reason. The resolution of polymorphic function calls happens at runtime through an indirection via the virtual table. Another way of explaining this is that compiler does not locate the address of the function to be called at compile-time, Page 3

4 instead when the program is run, the function is called by dereferencing the right pointer in the virtual table. In type theory it's also known as inclusion polymorphism. Parametric Polymorphism (Compile-Time Polymorphism) Parametric polymorphism provides a means to execute the same code for any type. In C++ parametric polymorphism is implemented via templates. One of the simplest examples is a generic max function that finds maximum of two of its arguments, #include <iostream> #include <string> template <class T> T max(t a, T b) { return a > b? a : b; } int main() { std::cout << ::max(9, 5) << std::endl; // 9 } std::string foo("foo"), bar("bar"); std::cout << ::max(foo, bar) << std::endl; // "foo" Here the max function is polymorphic on type T. Note, however, that it doesn't work on pointer types because comparing pointers compares the memory locations and not the contents. To get it working for pointers you'd have to specialize the template for pointer types and that would no longer be parametric polymorphism but would be ad-hoc polymorphism. Since parametric polymorphism happens at compile time, it's also called compile-time polymorphism. Ad-hoc Polymorphism (Overloading) Ad-hoc polymorphism allows functions with the same name act differently for each type. For example, given two ints and the + operator, it adds them together. Given two std::strings it concatenates them together. This is called overloading. Here is a concrete example that implements function add for ints and strings, #include <iostream> #include <string> Page 4

5 int add(int a, int b) { } return a + b; std::string add(const char *a, const char *b) { std::string result(a); result += b; return result; } int main() { std::cout << add(5, 9) << std::endl; std::cout << add("hello ", "world") << std::endl; } Ad-hoc polymorphism also appears in C++ if you specialize templates. Returning to the previous example about max function, here is how you'd write a max for two char *, template <> const char *max(const char *a, const char *b) { return strcmp(a, b) > 0? a : b; } Now you can call ::max("foo", "bar") to find maximum of strings "foo" and "bar". Coercion Polymorphism (Casting) Coercion happens when an object or a primitive is cast into another object type or primitive type. For example, float b = 6; // int gets promoted (cast) to float implicitly int a = 9.99 // float gets demoted to int implicitly Explicit casting happens when you use C's type-casting expressions, such as (unsigned int *) or(int) or C++'s static_cast, const_cast, reinterpret_cast, or dynamic_cast. Coercion also happens if the constructor of a class isn't explicit, for example, Page 5

6 #include <iostream> class A { int foo; public: A(int ffoo) : foo(ffoo) {} void giggidy() { std::cout << foo << std::endl; } }; void moo(a a) { } a.giggidy(); int main() { } moo(55); // prints 55 If you made the constructor of A explicit, that would no longer be possible. It's always a good idea to make your constructors explicit to avoid accidental conversions. Also if a class defines conversion operator for type T, then it can be used anywhere where type T is expected. For example, class CrazyInt { int v; public: CrazyInt(int i) : v(i) {} operator int() const { return v; } // conversion from CrazyInt to int }; The CrazyInt defines a conversion operator to type int. Now if we had a function, let's say,print_int that took int as an argument, we could also pass it an object of type CrazyInt, #include <iostream> void print_int(int a) { } std::cout << a << std::endl; Page 6

7 int main() { CrazyInt b = 55; print_int(999); // prints 999 print_int(b); // prints 55 } Subtype polymorphism that I discussed earlier is actually also coercion polymorphism because the derived class gets converted into base class type. Page 7

8 Introduction Inheritance, encapsulation and polymorphism are undoubtedly the cornerstones of OOP/OOD in general and C++ in particular. When programming C, it is very easy to remember how things work. You know that when you add an int variable to a structure it mostly grows by four bytes. You know that long is either four or eight bytes long depending on the architecture you re working with. Things are less obvious when moving to C++. OOP brings more abstractions to the program. As a result you are no longer sure if a+b sums two numbers or calls some overloaded operator method that concatenates contents of two files together. In this article, I would like to give you a short insight into what s going on behind the scenes. In particular we ll see how the three whales of OOP work in C++. Things that I am going to show in this article may differ from compiler to compiler. I will talk mostly about g++ (version 4.2.3). Note however, that same ideas apply everywhere. Encapsulation As you know, encapsulation is a principle by which same entity, the object, encapsulates data and methods that manipulate the data. You may be surprised to find out that underneath, class methods are just plain functions. How methods work In C++ there s one fundamental difference between plain functions and class methods. Class methods receive one additional argument and that is the pointer to the object whose data the method is expected to manipulate. I.e. first argument to a method is pointer to this. To speed things up, C++ developers used single CPU register (ECX/RCX on x86/x86_64) to pass pointer to this, instead of passing it via stack as if it was a regular function argument (no longer true in x86_64). Otherwise, objects know nothing about methods that operate on them. Page 8

9 How overloading works Another thing that we have to take care of in C++ is how to distinguish betweensome_function() and some_class::some_function(). Or betweensome_class::some_function( int ) and some_class::some_function() I.e. what s the difference between two methods with the same name that receive different number and type of arguments? What is the difference between method and function that has same name? Obviously, out of linker, compiler and preprocessor, linker is the one that should be aware of the above difference. This is because we may have some_function() in some distant object file. Linker is the component that should find this distant function and interconnect the call to the function and the actual function. Linker uses function name as a unique identifier of the function. To make things work, g++ and any other modern compiler, mangles the name of the method/function and makes sure that: 1. Mangled method name includes name of the class it belongs to (if it belongs to any class). 2. Mangled method name includes number and type of arguments method receives. 3. Mangled method name includes namespace it belongs to. With these three, some_class::some_function() and some_function() will have totally different mangled name. See the following code sample. 01 namespace some_namespace 02 { 03 class some_class 04 { 05 public: 06 some_class() { } 07 void some_method() { } 08 }; Page 9

10 09 }; class some_class 12 { 13 public: 14 some_class() { } 15 void some_method() { } 16 }; void some_method() 19 { 20 int a; 21 } g++ will turn: void some_class::some_method() into _ZN10some_class11some_methodEv void some_namespace::some_class::some_method() into_zn14some_namespace10some_class 11some_methodEv void some_method() into _Z11some_methodv Adding integer argument to void some_method() will turn it from _Z11some_methodv to_z11some_methodi. Page 10

11 How mangling solves the problem So when you create two methods with same name, but with different arguments, compiler turns them into two functions with different names. Later, when linker links the code together it doesn t know that these are two methods of the same class. From linkers standpoint, these are two different functions. Structure and size of the object You probably already know that C++ class and good old C structures are nearly the same thing. Perhaps the only difference is that all class members are private unless specified otherwise. On the contrary, all structure members are public. When looking at the memory layout of the object, it is very similar to C structure. Differences begin when you add virtual methods. Once you add virtual methods to the class, compiler will create virtual methods table for the class. Then it will place pointer to the table in the beginning of each instance of this class. Page 11

12 So, bear in mind that once your class has virtual methods, each object of this class will be four or eight bytes (depends on whether you have 64-bit support or not) bigger. Actually, pointer to the virtual methods table does not have to be at the beginning of the object. It is just handy to keep it at the beginning, so g++ and most of the modern compilers do it this way. Adding virtual methods to the class will also increase amount of RAM your program consumes and its size on your hard drive. How inheritance and polymorphism work Let s say we have two classes. A and B. Class B inherits from class A. 01 #include <iostream> using namespace std; class A 06 { 07 public: 08 A() { a_member = 0; } 09 int a_member; 10 }; Page 12

13 11 12 class B : public A 13 { 14 public: 15 B() : A() { b_member = 0; }; 16 int b_member; 17 }; int main() 20 { 21 A *a = new B; 22 a->a_member = 10; return 0; 25 } The interesting thing to notice here is that a actually points to instance of class B. When dereferencing a_member, we re actually dereferencing a_member that defined in class A, but belongs to class B (via inheritance). To make this happen, compiler has to make sure that common part of both classes (a_member in our case) located at the same offset in the object. Page 13

14 Now what if we have some virtual methods. How basic polymorphism works Let s change our example a bit and add some virtual methods. 01 #include <iostream> using namespace std; class A 06 { 07 public: 08 A() { a_member = 0; } 09 virtual int reset() { a_member = 0; } 10 void set_a_member( int a ) { a_member = a; } 11 int get_a_member() { return a_member; } Page 14

15 12 protected: 13 int a_member; 14 }; class B : public A 17 { 18 public: 19 B() : A() { b_member = 0; }; 20 virtual int reset() { a_member = b_member = 0; } 21 virtual void some_virtual_method() { } 22 void set_b_member(int b ) { b_member = b; } 23 int get_b_member() { return b_member; } 24 protected: 25 int b_member; 26 }; int main() Page 15

16 29 { 30 B *b = new B; 31 A *a = b; b->set_b_member( 20 ); 34 b->set_a_member( 10 ); a->reset(); cout << b->get_a_member() << " " << b->get_b_member() << 39 endl; return 0; 42 } If you compile and run this program it will obviously print 0 0. But how, you may ask. After all we did a->reset(). Without our understanding of polymorphism we could think that we re calling method that belongs to class A. The reason it works is because when compiler sees code that dereferences pointer to A it expects certain internal object structure and when it dereferences pointer to B it expects different object structure. Let us take a look at both of them. Page 16

17 However even more important here is the structure of the virtual methods tables of both classes. It is because of the virtual methods table structure compilers knows what virtual method to call. When it generates the code that dereferences pointer to A, it expects that first method in the virtual methods table of the object will be pointer to right reset() routine. It doesn t really care if the pointer actually points to B object. It will call first method of the virtual methods table anyway. How multiple inheritance works Multiple inheritance makes things much more complicated. The problem is that when class C inherits from both A and B, we should have both members of class A and class B in the instance of class C. 01 #include <iostream> using namespace std; class A Page 17

18 06 { 07 public: 08 A() { a_member = 0; } 09 protected: 10 int a_member; 11 }; class B 14 { 15 public: 16 B() { b_member = 0; } 17 protected: 18 int b_member; 19 }; class C : public A, public B 22 { Page 18

19 23 public: 24 C() : A(), B() { c_member = 0; } 25 protected: 26 int c_member; 27 }; int main() 30 { 31 C c; A *a1 = &c; 34 B *b1 = &c; A *a2 = reinterpret_cast<a *>( &c ); 37 B *b2 = reinterpret_cast<b *>( &c ); printf( "%p %p %p\n", &c, a1, b1 ); Page 19

20 40 printf( "%p %p %p\n", &c, a2, b2 ); return 0; 43 } Once we cast pointer to class C into class B, we cannot keep the value of the pointer as is because first fields in the object occupied by fields defined in class A (a_member). Therefore, when we do casting we have to do a very special kind of casting the one that changes the actual value of the pointer. If you compile and run above code snippet, you will see that all the values are the same except for b1, which should be 4 bytes bigger than other values. Page 20

21 This is what (C style casting in our case) casting does it increments the value of the pointer to make sure that it points to the beginning of the, inherited from B, part of the object. In case you wonder what other types of casting will do, here is a short description. Difference between different casting types There are five types of casting in C reinterpret_cast<>() 2. static_cast<>() 3. dynamic_cast<>() 4. const_cast<>() 5. C style cast. I guess you know already what const_cast<>() does. Also, it is only a compile time casting. C style cast is same as static_cast<>(). This leaves us with three types of casting. 1. reinterpret_cast<>() 2. static_cast<>() 3. dynamic_cast<>() From the above example we learn that reinterpret_cast<>() does nothing to the pointer value and leaves it as is. static_cast<>() and dynamic_cast<>() both modify value of the pointer. The difference between two is that the later relies on RTTI to see if the casting is legal it looks inside the object to see if it truly belongs to the type we re trying to cast from. static_cast<>() on the other hand, simply increments the value of the pointer. Page 21

22 Polymorphism and multiple inheritance Things getting even more complicated when we have virtual methods in each one of the classes A, B and C that we already met. Let s add following virtual methods to the classes. virtual void set_a( int new_a ) { a_member = new_a; } To class A. virtual void set_b( int new_b ) { b_member = new_b; } To class B and virtual void set_c( int new_c ) { c_member = new_c; } To class C. Page 22

23 You could have assumed that even in this case class C objects will have only one virtual tables methods, but this is not true. When you static_cast class C object into class B object, class B object must have its own virtual tables method. If we want to use same casting method as with regular objects (that is adding few bytes to the pointer to reach right portion of the object), then we have no choice but to place another virtual tables method in the middle of the object. As a result, you can have many different virtual methods tables for the same class. The above diagram shows very simple case of inheritance and the truth is that it does not get more complicated than this. Take a look at the following, more complex, class hierarchy. It may surprise you, but structure of the class X object will be quiet simple. In our previous example inheritance hierarchy had two branches. This one has three: 1. A-C-F-X 2. D-G-X 3. B-E-H-X All end up with X of course. They are a little longer than in our previous example, but there is nothing special about them. The structure of the object will be the following: Page 23

24 As a rule of thumb, g++ (and friends) calculates the branches that lead to the target class, class X in our case. Next it creates a virtual methods table for each branch and places all virtual methods from all classes in the branch into virtual methods table. This includes pointer to virtual methods of the class itself. If we project this rule onto our last example. A-C-F-X branch virtual methods table will include pointers to virtual methods from classes A, C, F and X. Same with other two branches. What if we try something even more complicatedback TO TOC The thing is that you can t. Lets say we try to create even more complicated hierarchy by changing class D from our previous example to inherit from class C. Page 24

25 This will immediately create ambiguous inheritance and the compiler will not hesitate to tell you that this is what happened. This is because now class X will have all members of classes A and C twice. Once it will have it via A-C-F-X branch and once via A-C-D-G-X branch. It will not tell you that there s a problem immediately. Instead, once you will try to reference one of the members of X inherited from either A or C, g++ will tell you that it has two variations of the same member/method and that it does not know which one of them to call. This what would be g++ output if you try to compile this file. main.cc: In function 'int main()': main.cc:110: error: request for member 'set_a' is ambiguous main.cc:29: error: candidates are: virtual void A::set_a(int) main.cc:29: error: virtual void A::set_a(int) All this because I was trying to do x.set_a( 20 ); in line 110. Few words about C++ constructors I guess you know what constructors are good for. In light of what we ve seen, you may ask yourself, who is building all those virtual methods tables and who writes right pointer into the object. Page 25

26 Obviously compiler builds all the virtual methods tables. And constructor is the one who fills in the right virtual methods table. And this is another reason why you cannot call constructor directly you don t want to mess up with virtual methods tables. Page 26