Okay, I'm writing a game that has a vector of a pairent class (enemy) that s going to be filled with children classes (goomba, koopa, boss1) and I need to make it so when I call update it calls the childclasses respective update. I have managed to create a example of my problem.
#include <stdio.h>
class A{
public:
virtual void print(){printf("Hello from A");}
};
class B : public A{
public:
void print(){printf("Hello from B");}
};
int main(){
A ab = B();
ab.print();
while(true){}
}
Output wanted: "Hello from B"
Output got: "Hello from A"
How do I get it to call B's print function?
Polymorphism only works on pointers and references. If you assign a B to an A, it becomes an A and you lose all B-specific information, including method overrides. This is called "slicing"; the B parts are "sliced" off the object when it is assigned to an object of a parent class.
On the other hand, if you assign a B* to an A*, it looks like an A*, but is still really pointing to a B, and so the B-specific information remains, and B's virtual overrides will be used.
Try:
int main(){
A* ab = new B();
ab->print();
delete ab;
while(true){}
}
The same also applies to assigning a B to an A& (reference-to-A), e.g.
int main(){
B b;
A& ab = b;
ab.print();
while(true){}
}
Your virtual keyword is correctly placed, you need to use pointers or references though.
You need to call the parent's update method before any processing by the descendant classes:
struct Base_Class
{
virtual void update(void)
{
cout << "Updating Base_Class.\n";
}
};
struct Goomba : public Base_Class
{
void update(void)
{
// Invoke the parent method first.
Base_Class::update();
// Perform Descendant operations
cout << "Updating Goomba\n";
}
};
Here is the implementation:
#include <iostream>
using std::cout;
void Update_Player(Base_Class& b)
{
b.update();
return;
}
int main(void)
{
Goomba g;
g.update();
Goomba g2;
std::vector<Base_Class *> container;
container.push_back(&g);
container.push_back(&g2);
std::vector<Goomba>::iterator iter;
for (iter = container.begin();
iter != container.end();
++iter)
{
Update_Player(*(*iter));
}
return 0;
}
Related
I want to call the child's function from parent's function in reinterpreted class, like below.
Example
#include <iostream>
class A {
public:
void func1() {
// some code
func2();
// some code
}
protected:
virtual void func2() {
printf("class A\n");
}
};
class B : public A {
protected:
virtual void func2() {
printf("class B\n");
}
};
int main() {
A* ab = new A();
ab->func1(); // this print "class A"
B* bab = reinterpret_cast<B*>(ab);
bab->func1(); // this also print "class A"
// I want to print "class B" when I use bab->func1()
}
In this situation, Is there any way to print class B using the reinterpreted class bab without redefining func1?
For C++ polymorphism to kick in, you must create an instance of the derived class somewhere, but you can store a pointer to the base class. Using the base-class pointer will dispatch to the overridden functions of the derived class. So your definition of A and B is fine, your usage in the main function is not. reinterpret_cast is not intended for this.
#include <iostream>
#include <memory>
class A {
public:
void func1() {
// some code
func2();
// some code
}
protected:
virtual void func2() {
printf("class A\n");
}
};
class B : public A {
protected:
virtual void func2() {
printf("class B\n");
}
};
int main() {
{
// This works, but don't do this. Naked "new" is not modern C++ and dangerous.
A* ab = new B();
ab->func1();
delete ab;
}
{
// VARIANT 2: use smart pointers
std::unique_ptr<A> ab = std::make_unique<B>();
ab->func1();
}
{
// VARIANT 3: A non-smart pointer is okay, as long as it
// 1. references an existing allocated object
// 2. the pointer does not outlive the object it points to
B b;
A* ab = &b;
ab->func1();
}
{
// VARIANT 4: Like Variant 3, but with reference instead of pointer
B b;
A& ab = b;
ab.func1();
}
}
Output
class B
class B
class B
class B
https://godbolt.org/z/8e5E85nx5
EDIT: Try to avoid allocation with new. Any memory allocated in this fashion must be freed by you using delete and it is very easy to forget (I did when I first wrote this answer, kinda proving my point). Even if you do delete the memory at the end of your function, there is the possibility that your code never reaches this statement, e.g. if exceptions are thrown somewhere between new and delete.Here is some further reading:
Why is it a bad idea to use 'new'?
Why should C++ programmers minimize use of 'new'?
Suppose there is a class A which has two subclasses, Aa and Ab. I want to make an array that can store pointers to objects of class Aa and Ab. If an array is declared with the type of class A, is this valid? If not, how can I accomplish this? For example:
A *arr;
//space allocated
Ab *obj1;
arr[x] = obj1;
On a related note, I want to write a function that when given a location, will return the object stored at that location in the array. If the above works and I have an array of objects of either Aa or Ab, the function could return an object of either type Aa or Ab. If the return type of the function is specified as A, the superclass, is this valid? If not, I have looked at template functions but cannot find a straight answer about just having the return type be variable, not the parameters. For this example, the function's parameter is always going to be int, but it could return an Aa or an Ab, depending on what is at that location in the array.
Yes that is the way polymorohism is achieved (using pointer to base class) and virtual methods.
Here is an example:
#include <iostream>
using namespace std;
#include <vector>
class A{
public:
virtual void foo()const{
std::cout << "A::foo()" << std::endl;
}
};
class Aa : public A {
public:
virtual void foo()const{
std::cout << "Aa::foo()" << std::endl;
}
};
class Ab : public A {
public:
virtual void foo()const{
std::cout << "Ab::foo()" << std::endl;
}
};
int main(){
A* ptrA[3];
A* a = new A;
Aa* aa = new Aa;
Ab* ab = new Ab;
ptrA[0] = aa;
ptrA[1] = ab;
ptrA[2] = a;
for(int i(0); i != 3; ++i)
ptrA[i]->foo();
delete a;
delete aa;
delete ab;
return 0;
}
Remember that C++ is Invariant not Contravariant which means you cannot assign a derived object a base object:
A* a = new A;
Ab* ab = a; // error
Is valid to have an array of base pointers, also you can use dynamic_cast to know in run time the return type of your array and use the API from the derived class. See and example below.
struct Base { virtual void do_something() {} };
struct Derived1 : Base
{
void first() const { std::cout << "first" << std::endl; }
void do_something() override {}
};
struct Derived2 : Base
{
void second() const { std::cout << "second" << std::endl; }
void do_something() override {}
};
Base& get(int option)
{
static std::vector<Base*> b {new Derived1{}, new Derived2{}};
return !option ? *b[0] : *b[1];
}
int main()
{
const int option {0};
// const int option {1};
if (Derived1* derived {dynamic_cast<Derived1*>(&get(option))})
{
derived->first();
}
else if (Derived2* derived {dynamic_cast<Derived2*>(&get(option))})
{
derived->second();
}
}
For example, I have three classes: A, B::A and C::A, only B and C have virtual method print(), like that:
#include <iostream>
using namespace std;
class A {
public:
virtual void print() {
return;
//do nothing
}
static void Func() {
//how to call all virtual functions print() for one class A?
print(); //doesn't work
}
};
class B : public A {
public:
virtual void print() {
cout << "B" << endl;
}
};
class C : public A {
public:
virtual void print() {
cout << "C" << endl;
}
};
int main() {
B b1;
B b2
C c;
A::Func();
return 0;
}
I wan't use print() for all inherited objects (b1, b2, c) by using just class A. How can I do it?
Declare a static class member of A that's a container of pointers to all instances of A or its subclasses. A std::list will be an excellent choice:
class A {
static std::list<A *> all_instances;
public:
// ...
};
In A's constructor, add its this to the list, saving the new list entry's iterator. In A's destructor, remove its list entry from the list.
So now you will have a private container that enumerates all instances of A, or any of its subclasses.
Writing a static class method that invokes each one's print() method becomes trivial, at this point.
Of course, a little bit of additional work is necessary to implement thread safety, if it's an issue here.
Writing the code for A's constructor or destructor will be your homework assignment.
I have a base class instance, there is a derived class that inherits from the base class, I want to transform the base instance into derived instance, (if possible without copying anything (maybe sending to the derived class a reference of the base class)) how can I achieve that?
Note: I need this because I'm using factory design pattern which identify the derived class needed to be created using a parameter located in the base instance.
//class A
//class B: public A (pure virtual)
//class C: public B
B BFactory::makeB(A &a) {
int n=a.getN();
if(n==1){
return new C();
}
}
Thanks.
Consider the case of the car.
You can treat a Lamborghini as a car.
You can treat a Yugo as a car.
You can treat a car as a Lamborghini if it is a Lamborghini. In C++ this means a pointer to car that really points to a Lamborghini. In order to get a Lamborghini pointer back out of the car pointer you should use dynamic_cast. If the car does not point to a Lamborghini, dynamic_cast will return NULL. This keeps you from trying to pass off a Yugo as a Lamborghini and blowing the Yugo's engine.
But when the Lamborghini is being treated as a car, it can only do car things. If you copy a Lamborghini into a car, you strip out all Lamborghini-ness forever. It's gone.
Code time!
This, I'm afraid cannot be done:
//class A
//class B: public A (pure virtual)
//class C: public B
B BFactory::makeB(A &a) {
int n=a.getN();
if(n==1){
return new C();
}
}
C is being copied into a B and the B is being returned. B would need a constructor that took a C, but the point is moot. B cannot be instantiated if it's pure virtual. For now we'll ignore the leak that would be new C()
Also can't use a reference for this job, pretty much the same problem, so you're trapped into returning a pointer
B * BFactory::makeB(A &a) {
int n=a.getN();
if(n==1){
return new C();
}
}
Now I'm going to make a suggestion: Build the make function into B and handle the case where A doesn't map to anything recognized by B.
class B: public A
{
public:
virtual ~B(){}
static B * makeB(A & a)
{
switch(a.getN())
{
case 1:
return new C();
}
return NULL;
}
};
But this leads to another recommendation: Why should B know anything? And What is the point of A at this level? Why is A storing build codes for classes two or more steps down the hierarchy? Bad from a maintenance point of view. The point of objects is they know who they are and how to manipulate themselves. Short-circuiting this leads to pain.
class B: public A
{
public:
virtual ~B(){}
virtual B* makeB() = 0;
};
Now B only makes Bs, needs no help from A, and those who extend B are stuck with figuring out how to make themselves--a task they should know better than anyone else. Much safer because there is never any possibility of a code unrecognised by B for a new class.
class C: public B
{
public:
B* makeB()
{
return new C();
}
};
class D: public B
{
public:
B* makeB()
{
return new D();
}
};
Edit: Traditional factory
You're asking for an abstract factory. For that you need nothing. You don't even need a class. You certainly don't need a class A. The goal of this sort of factory is the caller knows nothing about the class. By providing an A, the caller needs to know how to make an A or have another factory that makes an A.
First a bit of set-up in a header file BFactory.h:
#ifndef BFACTORY_H_
#define BFACTORY_H_
#include <exception>
class B
{
public:
virtual ~B(){}
virtual std::string whatAmI() = 0;
protected:
// data members common to all B subclasses
};
enum bType
{
gimmie_a_C,
gimmie_a_D,
gimmie_an_E
};
class BadTypeException: public std::exception
{
public:
const char* what() const noexcept
{
return "Dude! WTF?!?";
}
};
B* BFactory(enum bType type);
#endif /* BFACTORY_H_ */
Here I'm going to deviate from the book way a little. Rather than using an integer to identify the type to be built, I'm going to use an enum. Two reasons: Easier to read and understand gimme_a_C than 1 and generates a compiler error if you try to provide a value that is not enumerated.
enum bType
{
gimmie_a_C,
gimmie_a_D,
gimmie_an_E
};
And an exception to flag stupidity if the enum is updated with new types (gimmie_an_E) but the factory is not.
class BadTypeException: public std::exception
{
public:
const char* what() const noexcept
{
return "Dude! WTF?!?";
}
};
This is all the Factory client needs to see. They don't see C. They don't see D. They have no clue that C and D exist in any way other than the names listed in enum bType. All they ever see is pointers to B.
Now for the implementation BFactory.cpp:
#include "BFactory.h"
class C:public B
{
std::string whatAmI()
{
return "C";
}
};
class D:public B
{
std::string whatAmI()
{
return "D";
}
};
B* BFactory(enum bType type)
{
switch(type)
{
case gimmie_a_C:
return new C();
case gimmie_a_D:
return new C();
default:
throw BadTypeException();
}
}
I'll leave it up to the reader to spot the stupid bug in the above code that makes these error prone and why I don't like them.
And usage, main.cpp:
#include "BFactory.h"
int main()
{
B * temp;
temp = BFactory(gimmie_a_C);
std::cout << temp->whatAmI() << std::endl;
delete temp;
temp = BFactory(gimmie_a_D);
std::cout << temp->whatAmI() << std::endl;
delete temp;
//temp = BFactory(1001); // won't compile
try
{
temp = BFactory(gimmie_an_E); // will compile, throws exception
std::cout << temp->whatAmI() << std::endl;
}
catch(BadTypeException& wtf)
{
std::cerr << wtf.what() << std::endl;
}
}
There is still absolutely no use for or involvement of A. A if it exists, should no nothing about B or the children of B.
These days there is a little improvement we can make so that the pointers are a little safer. unique_ptr allows us to maintain the polymporphic advantages of a pointer to B without the memory management woes.
std::unique_ptr<B> BFactory(enum bType type)
{
switch(type)
{
case gimmie_a_C:
return std::unique_ptr<B>(new C());
case gimmie_a_D:
return std::unique_ptr<B>(new D());
default:
throw BadTypeException();
}
}
and the new main:
int main()
{
std::unique_ptr<B> temp;
temp = BFactory(gimmie_a_C);
std::cout << temp->whatAmI() << std::endl;
temp = BFactory(gimmie_a_D);
std::cout << temp->whatAmI() << std::endl;
}
You might want to define a constructor that takes the base class instance as the argument so you can later use static_cast to convert from the base class to the derived class.
class Derived : public Base
{
public:
Derived(const Base& base) : Base{base} {}
};
int main()
{
Base a;
Derived b = static_cast<Derived>(a);
}
If you want to create a derived class instance using the base class instance then there is some conversion rule between the two, which you can specify explicitly using a derived class constructor.
Although it is impossible to alter the type of an object you still can make instances of base and derived classes share the same data:
#include <memory>
#include <iostream>
class Base
{
protected:
struct CommonData
{
int A;
int B;
};
std::shared_ptr<CommonData> m_data;
public:
Base() : m_data(std::make_shared<CommonData>())
{
m_data->A = 0;
m_data->B = 0;
}
void SetData(Base * source)
{
m_data = source->m_data;
}
int A() const { return m_data->A; }
int B() const { return m_data->B; }
void SetA(int value) { m_data->A = value; }
void SetB(int value) { m_data->B = value; }
};
class Derived : public Base
{
public:
int C;
};
using namespace std;
int _tmain(int argc, _TCHAR* argv[])
{
Base base;
base.SetA(12);
base.SetB(46);
Derived derived;
derived.SetData(&base);
derived.C = 555;
cout << derived.A() << endl; // 12
cout << derived.C << endl; // 555;
cin.get();
}
A base class should not "know" about how to make its own derived class instances. That is the point of inheritance.
The "is a" relationship of derived classes means that any subclass instance will pass as a base class instance transparently, and you can treat it as one, and by default base class non-virtual methods are called on a base class reference, even if it a derived class instance. Only virtual methods use the derived class method.
In the case of creating a base class instance from a derived class you want to "slice" the instance data (normally a bad thing and normally a mistake).
class A{ // ... A stuff };
class B : A
{ // ... B stuff
A make_A() { return (A) B(*this); } // copy cast to A
};
Under no circumstances try to do this:
class B;
class A { // ...
B make_B() { return B(*this); }
};
That is inverted OO logic. It requires at least 2 scans of the source code, which C++ does not do. It fails.
I have a particular scenario below. The code below should print 'say()' function of B and C class and print 'B says..' and 'C says...' but it doesn't .Any ideas..
I am learning polymorphism so also have commented few questions related to it on the lines of code below.
class A
{
public:
// A() {}
virtual void say() { std::cout << "Said IT ! " << std::endl; }
virtual ~A(); //why virtual destructor ?
};
void methodCall() // does it matters if the inherited class from A is in this method
{
class B : public A{
public:
// virtual ~B(); //significance of virtual destructor in 'child' class
virtual void say () { // does the overrided method also has to be have the keyword 'virtual'
cout << "B Sayssss.... " << endl;
}
};
class C : public A {
public:
//virtual ~C();
virtual void say () { cout << "C Says " << endl; }
};
list<A> listOfAs;
list<A>::iterator it;
# 1st scenario
B bObj;
C cObj;
A *aB = &bObj;
A *aC = &cObj;
# 2nd scenario
// A aA;
// B *Ba = &aA;
// C *Ca = &aA; // I am declaring the objects as in 1st scenario but how about 2nd scenario, is this suppose to work too?
listOfAs.insert(it,*aB);
listOfAs.insert(it,*aC);
for (it=listOfAs.begin(); it!=listOfAs.end(); it++)
{
cout << *it.say() << endl;
}
}
int main()
{
methodCall();
return 0;
}
Your problem is called slicing and you should check this question: Learning C++: polymorphism and slicing
You should declare this list as a list of pointers to As:
list<A*> listOfAs;
and then insert these aB and aC pointers to it instead of creating copies of objects they are pointing to. The way you insert elements into list is wrong, you should rather use push_back function for inserting:
B bObj;
C cObj;
A *aB = &bObj;
A *aC = &cObj;
listOfAs.push_back(aB);
listOfAs.push_back(aC);
Then your loop could look like this:
list<A*>::iterator it;
for (it = listOfAs.begin(); it != listOfAs.end(); it++)
{
(*it)->say();
}
Output:
B Sayssss....
C Says
Hope this helps.
Polymorphism of virtual class hierarchies only works through references or pointers to a base subobject:
struct Der : Base { /* ... */ };
Der x;
Base & a = x;
a.foo(); // calls Der::foo() from x
The function foo is dispatched polymorphically if it is a virtual function in Base; the polymorphism refers to the fact that while you are calling a member function of an object of type Base, the function that actually gets called may be implemented in the class Der.
Containers can only store elements of a fixed type. In order to store a polymorphic collection, you could instead have a container of pointers to the base class. Since you need to store the actual objects elsewhere, lifetime management is non-trivial and best left to a dedicated wrapper such as unique_ptr:
#include <list>
#include <memory>
int main()
{
std::list<std::unique_ptr<Base>> mylist;
mylist.emplace_back(new Der1);
mylist.emplace_back(new Der2);
// ...
for (p : mylist) { p->foo(); /* dispatched dynamically */ }
}
list::iterator it;
B bObj;
C cObj;
A *aB = &bObj;
A *aC = &cObj;
listOfAs.insert(it,*aB);
Do you not need to initialize "it" ?
I believe you should do it = listOfAs.begin(); before starting to insert.