I'm trying to make a chess program, but I want to be able to implement different AIs in it. Thus I made a abstract AIgeneric class and the derived class AIrandom off of AIgeneric. Then in my chessAI interface, I create a list of the the AIs, and try to call their getNextMove function and run into a segfault. The code is as below:
class AIgeneric {
public:
virtual int getNextMove(int*, const int &) = 0;
}
class AIrandom : public AIgeneric {
public:
AIrandom();
virtual int getNextMove(int*, const int &);
}
class chessAI {
public:
chessAI();
~chessAI();
void setAI();
int getNextMove(int*, const int &);
private:
vector<AIgeneric*> AIlist;
vector<string> names;
int selectedAI;
};
chessAI::chessAI () {
AIrandom randomAI;
AIlist.push_back(&randomAI);
names.push_back("Random AI");
selectedAI = -1;
}
int chessAI::getNextMove(int * board, const int & color) {
return AIlist[selectedAI]->getNextMove(board, color); //segfault on this line
}
It'd be great if anyone could help me on this problem!
Edit: I do set selectedAI to 0 before calling getNextMove.
In this code:
chessAI::chessAI () {
AIrandom randomAI;
AIlist.push_back(&randomAI);
names.push_back("Random AI");
selectedAI = -1;
}
You store a pointer to a local variable into your vector. After the constructor returns that pointer is no longer valid.
Remember that all local variables are stored on the stack, and the stack is reused in other functions. So when you use the pointer in the vector, it now points to some other functions memory and not the one object you declared.
This can be solved in three ways:
Allocate the object on the heap:
AIlist.push_back(new AIRandom);
Not using pointers at all.
Use smart pointers, such as std::unique_ptr.
You call selectedAI = -1; and then AIlist[selectedAI]->.... What do you expect AIlist[-1] to be, other than undefined behavior?
I expect this is because AIlist[selectedAI] is out of bounds. You can confirm this by replacing it with AIlist.at(selectedAI). Keep in mind that this index is -1 immediately after the constructor...
Related
I am working on a c++ object orientation project. I defined a vector as a private variable in my ListOfShapes class:
vector<Shape*> list_of_shapes = {};
Shape is another class, and below is the header file of it:
class Shape{
private:
string color;//private variable
public:
Shape(string c);//non-default constructor
string getColor();//returns color of the object
virtual void print() = 0;//pure virtual "print" function
};
Shape is the parent class of a class called Rectangle:
class Rectangle:public Shape{
public:
Rectangle(const string &c,double length,double breadth);
void print();
private:
double length, breadth;
};
In the main function I create a ListOfShapes object which contains a function that pushes a pointer (pointing to a Shape object) into the vector I mentioned above. Here is the implementation of the called function:
void ListOfShapes::addShape(Shape* s) {
list_of_shapes.push_back(s);
}
And finally here is the action of calling in the main function:
int main() {
ListOfShapes* list;
Shape* r;
r = new Rectangle( "red",1,1);
list->addShape( r);
return 0;
}
My code compiles fine and even runs if I try to create the same vector (that stores pointers to Shape) inside the main function. The issue happens inside the addShape method, when the push_back method inside the vector class is called, it is using a null pointer for some reason. I've spent hours trying to debbug this issue but I cant seem to find the cause of it.
The error I am getting is:
You are accessing an uninitialized pointer list:
int main() {
ListOfShapes* list;
Shape* r;
r = new Rectangle( "red",1,1);
list->addShape( r); // <-- list is not initialized, thus undefined behavior
return 0;
}
If you look at your debugger output, you see that this has a strange value in the addShape function. This is an indication that the object itself is not valid, and anything done within an invalid object is also invalid.
Simply create a list local object. There is no need for a pointer:
int main() {
ListOfShapes list;
Shape* r;
r = new Rectangle( "red",1,1);
list.addShape( r);
return 0;
}
Also, your Shape class should have a virtual destructor. If you attempted to delete the objects stored in list by iterating through list, the program will invoke undefined behavior due to Shape not having a virtual destructor.
I have 1 question because I am pretty curious how to handle with such problem.
I have base class called "Pracownik" (Worker) and 2 subclasses which are made from public Pracownik;
- Informatyk (Informatic)
- Księgowy (Accountant)
Writing classes is easy. Made them pretty fast but I have small problem with main because I am helping friend with program but I was not using C++ for a while. So:
This is my header file "funkcje.h"
#include <iostream>
using namespace std;
class Pracownik
{
private:
string nazwisko;
int pensja;
public:
Pracownik(string="",int=0);
~Pracownik();
string getNazwisko();
int getPensja();
friend double srednia_pensja(int,Pracownik);
};
class Informatyk : public Pracownik
{
private:
string certyfikat_Cisco;
string certyfikat_Microsoft;
public:
Informatyk(string="",int=0, string="", string="");
~Informatyk();
void info();
};
class Ksiegowy : public Pracownik
{
private:
bool audytor;
public:
Ksiegowy(string="",int=0, bool=false);
~Ksiegowy();
void info();
};
double srednia_pensja(int,Pracownik);
These are definitions of my functions "funkcje.cpp"
#include "funkcje.h"
Pracownik::Pracownik(string a,int b)
{
nazwisko=a;
pensja=b;
}
Pracownik::~Pracownik()
{
}
string Pracownik::getNazwisko()
{
return nazwisko;
}
int Pracownik::getPensja()
{
return pensja;
}
Informatyk::Informatyk(string a, int b, string c, string d) : Pracownik(a,b)
{
certyfikat_Cisco=c;
certyfikat_Microsoft=d;
}
Informatyk::~Informatyk()
{
}
Ksiegowy::Ksiegowy(string a, int b, bool c) : Pracownik(a,b)
{
audytor=c;
}
Ksiegowy::~Ksiegowy()
{
}
void Informatyk::info()
{
cout<<"Nazwisko pracownika: "<<Pracownik::getNazwisko()<<endl;
cout<<"Pensja pracownika: "<<Pracownik::getPensja()<<endl;
cout<<"Certyfikat Cisco: "<<certyfikat_Cisco<<endl;
cout<<"Certyfikat Microsoft: "<<certyfikat_Microsoft<<endl;
}
void Ksiegowy::info()
{
cout<<"Nazwisko pracownika: "<<Pracownik::getNazwisko()<<endl;
cout<<"Pensja pracownika: "<<Pracownik::getPensja()<<endl;
cout<<"Audytor: ";
if(audytor)
cout<<"Tak"<<endl;
else
cout<<"Nie"<<endl;
}
double srednia_pensja(int a,Pracownik *b)
{
return 0;
}
And finally main!
#include <iostream>
#include "funkcje.h"
using namespace std;
int main()
{
Pracownik lista[10];
Pracownik *lista_wsk = new Pracownik[10];
Informatyk a("Kowalski1",1000,"Cisco1","Microsoft1");
Informatyk b("Kowalski2",2000,"Cisco2","Microsoft2");
Informatyk c("Kowalski3",3000,"Cisco3","Microsoft3");
Ksiegowy d("Kowalski4",4000,1);
Ksiegowy e("Kowalski5",5000,0);
lista[0]=a;
lista[1]=b;
lista[2]=c;
lista[3]=d;
lista[4]=e;
Informatyk *ab = new Informatyk("Kowalski1",1000,"Cisco1","Microsoft1");
Informatyk *ac = new Informatyk("Kowalski2",2000,"Cisco2","Microsoft2");
Informatyk *ad = new Informatyk("Kowalski3",3000,"Cisco3","Microsoft3");
Ksiegowy *ae = new Ksiegowy("Kowalski4",3000,1);
Ksiegowy *af = new Ksiegowy("Kowalski5",3000,0);
lista_wsk[0]=*ab;
lista_wsk[1]=*ac;
lista_wsk[2]=*ad;
lista_wsk[3]=*ae;
lista_wsk[4]=*af;
for(int i;i<5;i++)
{
lista[i].info();
cout<<endl;
}
cout<<endl;
// for(int i;i<5;i++)
// {
// lista_wsk[i].info();
// }
return 0;
}
Ok and here goes my questions:
I had to create array which is filled with base class objects "Pracownik".
Secondary i had to create array which is full of pointers to class "Pracownik" objects.
(Hope those 2 first steps are done correctly)
Next thing I had to write to array 3 objects of class Informatic and 2 of class Accountant.
So I ve created 5 objects manually and added them into the array in such way array[0]=a;. I guess this is still good.
Next thing i had to create and add similar objects to array of pointers using new. So I ve created array with new and pointers to objects with new. (Hope thats correct 2).
And FINALLY:
I had to use info() on added to array objects.
This is my main question if my array is type "Pracownik" and I want to use function info() from subclasses how should I do that? And how compiler will know if he should use info() from Accountant or Informatic while I am trying to show those information using "for".
In an array of Pracownik, the elements are of type Pracownik. Any information about the objects being of a subclass of Pracownik are lost when you copy the elements into the array.
This is called object slicing and leads to the fact that there is no way to invoke Informatyk::info() on these objects.
If you want to call methods of a subclass, you have to prevent object slicing by storing pointers or references in the array.
As Oswald says in his answer,
Pracownik * lista_wsk = new Pracownik[10];
allocates an array of 10 Pracownik objects. This is probably not what you want. With polymorphism involved, we usually want to deal with pointers or references. Hence, you'd want an array of Pracownik * pointers. Since you already know at compile-time that it will have 10 members, there is no need for a dynamic allocation here. I think you've meant to write
Pracownik * lista_wsk[10];
instead. Now we don't put objects but pointers to objects into the array. For example:
lista_wsk[2] = new Informatyk("Kowalski3", 3000, "Cisco3", "Microsoft3");
And then we can iterate over the items like so:
for (unsigned i = 0; i < 10; ++i)
std::cout << lista_wsk[i]->getNazwisko() << std::endl;
As you have already discovered, it is impossible to call a subclass function member on a superclass object. It would be possible to figure out the actual type at run-time yourslf by means of a cast.
for (unsigned i = 0; i < 10; ++i)
if (Informatyk * info_ptr = dynamic_cast<Informatyk *>(lista_wsk[i]))
info_ptr->info();
dynamic_cast returns a pointer to the target class if this is possible or a nullptr (which evaluates to false, hence the conditional) otherwise. Note however that this is considered very poor style. It is better to use virtual functions. Therefore, add
virtual void
info()
{
// Do what is appropriate to do for a plain Pracownik.
// Maybe leave this function empty.
}
to the superclass and again to the subclass
virtual void
info() // override
{
// Do what is appropriate to do for an Informatyk.
}
The function in the subclass with the same signature is said to override the function inherited from the superclass. Since the function is marked as virtual, the compiler will generate additional code to figure out at run-time what version of the function to call.
If you are coding C++11, you can make the override explicit by placing the keyword override after its type as shown above (uncomment the override). I recommend you use this to avoid bugs that arise from accidental misspelling or other typos.
I am having issues with trying to figure out how I would turn a data member inside a class (which was originally an int) into a pointer to a dynamically allocated piece of memory.
I know I can do int *num = new int under normal circumstances, but how would I initialize it in a class?
My teacher did an amazing job of not explaining this in the crappiest way possible in class -_-.
This is an example of the class and the constructor.
The header
class Double
{
private:
double *val;
The .cpp
Double::Double()
{
this->equals(0.0);
}
Double::Double(const Double &d)
{
this->equals(d.val);
}
Double::Double(const double &d)
{
this->equals(d);
}
Double::Double(const Interger &i)
{
this->equals(i.toInt());
}
//New overloaded constructor using strings
Double::Double(const string &s)
{
this->equals(s);
}
void Double::equals(const double &d)
{
this->val = d;
}
All I know is I have to make the data member a pointer now, but I have no idea how to create the new memory. I tried looking this up but I could not find an example of how to do DAM inside an actual class for its memory and constructor.
EDIT
Solution was a simpler then I thought.
Double::Double() : val(new double)
{
......
}
just have to do that to every constructor, then change any instance of d.val or this->val to *d.val or *this->val.
SOLUTION TO MY PROBLEM (So the problem is solved)
Solution was simpler then I thought.
Double::Double() : val(new double)
{
......
}
just have to do that to every constructor, then change any instance of d.val or this->val to *d.val or *this->val.
Deconstructors will have to be created to clear the memory though.
I have a class:
class SendData
{
public:
SendData(int SendAMsg(int foo, unsigned char *bar, int length), int number)
{
m_nDefinePos = 0;
m_nOtherStuffDefinedAs =0;
}
void somestuffhere();
void ClearDefinition();
private:
int aLotOfVariableshere;
int m_nDefinePos;
};
This is the class itself. Then some stuff is called:
SendData* m_pData;
m_pData->ClearDefinition();
Which now calls this one:
void SendData::ClearDefinition()
{
printf("Welcome to Clear Definition Script\n");
m_nDefinePos = 0;
// Some more stuff here
}
Here the code breaks somehow. I get the "Welcome to Clear Definition Script" message in my console, but that's all.
It breaks on m_nDefinePos = 0;. (I did put in another printf command after it, never showed in the console.)
I just don't know why it breaks there and i cant find any error.
SendData* m_pData;
m_pData->ClearDefinition();
This declares a pointer, but doesn't create an object or initialise the pointer to point to anything, so calling a member function via the pointer will go wrong. Perhaps you wanted to create an object:
SendData data(arguments);
data.ClearDefinition();
or perhaps you wanted to initialise the pointer to point an object that already exists:
SendData* m_pData = whatever;
I've been making a game which uses the Box2D physics engine, and I've come across some weirdness with the stack pointer (ESP) and multiple inheritance. I've managed to reproduce it in a minimal amount of code, and it seems that the order in which I declare the classes to be used in multiple inheritance seems to dictate whether the program crashes or not.
#include <iostream>
#include <string.h>
using namespace std;
class IPhysicsObject
{
public:
virtual void Collide(IPhysicsObject *other, float angle, int pos)=0;
};
class IBoardFeature
{
public:
IBoardFeature(){};
~IBoardFeature(){};
virtual bool OnAttach(int x){ return true; }
virtual bool Update(int x, float dt)=0;
};
/*
class CScorezone : public IBoardFeature, public IPhysicsObject // this breaks !!!
class CScorezone : public IPhysicsObject, public IBoardFeature // this works !!!
*/
class CScorezone : public IBoardFeature, public IPhysicsObject
{
public:
CScorezone(){}
~CScorezone(void){}
virtual bool Update(int x, float dt)
{
return true;
}
virtual void Collide(IPhysicsObject *other, float angle, int pos)
{
}
virtual bool OnAttach(int x){ return true; }
};
int main(int argc, char *argv[])
{
CScorezone *scoreZone = new CScorezone();
CScorezone *otherZone = new CScorezone();
void *voidZone = scoreZone;
IPhysicsObject *physZone = static_cast<IPhysicsObject*>(voidZone);
physZone->Collide(otherZone, 10, 1);
delete scoreZone;
delete otherZone;
// wait for user input
int x;
cin >> x;
return 0;
}
Running this in debug mode causes the following error
Run-Time Check Failure #0 - The value
of ESP was not properly saved across a
function call. This is usually a
result of calling a function declared
with one calling convention with a
function pointer declared with a
different calling convention.
When I step in to the following line of code:
physZone->Collide(otherZone, 10, 1);
I notice it's going into CScoreZone::OnAttach, not CScoreZone::Collide. Why is this? WHen I change the order of inheritance for CScoreZone, it works fine
class CScorezone : public IPhysicsObject, public IBoardFeature
I'm running on VS2005 SP2 (8.0.50727.768) on Windows XP. Any ideas?
You don't have to assign CScorezone* to void* and then cast it to IPhysicsObject*. Since CScorezone is-a IPhysicsObject you can simply assign to base pointer:
IPhysicsObject *scoreZone = new CScorezone();
IPhysicsObject *otherZone = new CScorezone();
You're also missing public virtual destructor in IPhysicsObject declaration.
Edit:
I a callback situation as you describe in the comments (going through some C api?) I'd use simple struct with a pointer to polymorphic type to avoid undefined casts, something like:
// one more level of indirection
struct cb_data
{
IPhysicsObject* target;
};
// callback function
int callback( void* data )
{
const cb_data& cbd( *static_cast<cb_data*>( data ));
return cbd.target->Collide( ... );
}
The problem is that you cast the pointer to void* first.
The compiler doesn't know then how to perform static cast for the pointer.
It needs to change the pointer value during the cast if you use multiple inheritance to use second superclass virtual table.
Just cast the pointer back to CScoreZone* before using static_cast.
Well, in your code you seem to be deliberately destroying the integrity of a hierarchical cast by using void * as an intermediate type in the cast. ScoreZone * is cast to void * first and then cast to IPhysicsObject *. What you get as the result is undefined behavior.
Why are you doing this? And what did you expect will happen?
Nikolai told you how to avoid casting in the first place with your example given. However if you do need to do a typecast, when working with objects always use dynamic_cast, which does runtime type checking.