I want to build an array of dervied class objects. I have this base class:
class CandyBox {
protected:
string flavor;
string origin;
public:
inline CandyBox();
inline CandyBox(string s1, string s2);
virtual float getVolume() = 0;
virtual void toString();
CandyBox& operator=(const CandyBox& obj);
virtual ~CandyBox() {}
};
And 2 derived class named class Lindt and class ChocAmor with constructors and methods as well. I'm trying to build now a scratchy list like this:
CandyBox** vec = new CandyBox*[n];
for (int i = 0; i < n; i++) {
cin >> type;
if (strcmp(type, "ChocAmor") == 0) {
vec[i] = new ChocAmor(1, "s", "ro");
}
else vec[i] = new Lindt(1, 2, 3, "f", "it");
}
My question is: What if I want to make another class named CandyBag that contains an attribute like this one CandyBox** vec = new CandyBox*[n]; and I need this method:
CandyBag& operator=(const CandyBag& candy) {
ChocAmor::operator=(candy);
Lindt::operator=(candy);
dim_max = candy.dim_max;
current_dim = candy.current_dim;
vec = new CandyBox*[candy.dim_max];
for (int i = 0; i <= current_dim; i++) {
vec[i] = new ; //HERE I'M STUCK
// because I can't just simply write vec[i] = candy.vec[i], right?
// I need to allocate memory for that vec[i] first
}
return *this;
}
I'm not sure how can I allocate memory for that vec[i] if I don't know the type (if it's a ChocAmor object or a Lindt type in that candy.vec[i]). Should I get an auxiliar array where I should store the types of that array?
All pointers to class-type are the same size and representation. That is essential for opaque pointers to work at all.
If you want to enable cloning the pointed-to objects, add a virtual .clone() to the interface, and/or write your own copying smart-pointer.
Lugging around a pointer to a clone-function or maintaining a mapping from typeid to clone-function would be far more cumbersome, though can be the right solution in other circumstances.
Otherwise, I have a suggestion:
Use smart-pointers, preferably std::unique_ptr, and standard containers, preferably std::vector, to avoid manual memory-management and gain all the associated benefits of using standard types.
Related
I want to define an array of structs, but this is not working because it has a const field without default constructor.
Struct is part of an SDK and looks like following:
struct SParametricParamDef
{
const TXString fUniversalName;
SResString fLocalizedName;
const TXString fDefaultImperial;
const TXString fDefaultMetric;
EFieldStyle fFieldStyle;
short fChoicesChcID;
};
TXString does not have a default constructor. So following is failing:
SParametricParamDef *arrParams = new SParametricParamDef[size]; // <= throws compile time exception
for (int i = 0; i < size; i++)
{
arrParams[i] = params[i].def; // <= also throws an exception, because operator = is not available
}
Is there some way to solve this? I need an SParametricParamDef* as a result, because this array is used in the SDK again...
Info
In an old SDK version, const TXSTring was const char* and back then I did not have problems... Now I need to adjust my code to work with the new structures...
The error you get is not primarily about operator = but about the fact that you default-constructed an object with const members. This will render them immutable and any attempt to modify them, as you are trying in the loop, must fail.
Fortunately, you can use emplace_back to initialize the SParametricParamDef objects right inside the vector without taking the indirection of default-construction and assignment:
std::vector<SParametricParamDef> arrParams;
for(std::size_t n = 0; n < size; ++n) {
arrParams.emplace_back(params[n].def);
}
This should minimize the amount of copying and comes without the need to modify the struct definition.
The compiler is telling you that you are asking for a TXString to be created without directing how it can be initialised. It is difficult to know how to address the problem of creating a TXString object since you haven't given a list of the constructors for the class, but as it stands a change would need to be made to the code you've given. Some ways of solving this are as follows:
The most obvious is to add a default constructor for SParametricParamDef which initialises the TXString objects:
struct SParametricParamDef
{
SParametricParamDef() : fUniversalName(...), ... {}
...
Another approach, given that the variables are const might be to make them const static
Say, for simplicity's sake, that the TXString object was as follows:
struct TXString{
TXString(char a) : _a(a) {}
char _a;
};
You could then change your declaration of SParametricParamDef to:
struct SParametricParamDef
{
const static TXString fUniversalName;
...
and then define fUniversalName in your implementation file as follows:
const TXString SParametricParamDef::fUniversalName('D');
Another way might be to wrap a TXString object in another object that does have a default constructor:
struct TXStringWrapper {
TXStringWrapper() : _s(...) {} // [1]
const TXString& get() { return _s; }
private:
TXString _s;
}
At [1], you create the TXString in whatever specific, non-default way that you care.
That looks like an example for using a placement new:
SParametricParamDef *arrParams = (SParametricParamDef *) new char[size * sizeof(*arrParams)];
for (int i = 0; i < size; i++)
{
// constructs an object in a pre-allocated memory
new(arrParams+1) SParametricParamDef(params[i].def);
}
You should explicitely call a destructor if it is not trivial before freeing the array:
for (int i = 0; i < size; i++)
{
~SParametricParamDef(arrParams+1);
}
delete[] ((char *) arrParams);
This is rather old fashioned because it mimics the initialization of structs in C, but it is the only way I know to build an array of objects that only have non trivial constructors (AFAIK, the std::vector way requires copyable or movable objects)
I'm still somewhat new to c++ and I'm unsure about creating different instances within a list. In my program I have multiple classes inheriting from the base class:
class Foo{
}
class Bar : public Foo {
}
class Fin : public Foo {
}
The problem that I am facing is I need to replace one with another in case one is destroyed. For example:
for (int i = 0; i < list/vector/? ; i++){
if (bar_i[i].destroyed()){
Fin fin_i = new Fin(); // in place of Bar
}
}
which would then take the Bar(s) spot. What could I use to create a list, vector, or whatever to create the instances of multiple classes?
You can't store different types in the standard containers (a standard container requires all elements to be of the same type).
However, you can use polymorphism and store pointers to a parent (common) type.
You may want to rethink your design and move common methods and members to a parent type.
Edit 1: Example implementation
std::vector<Foo *> container;
for (unsigned int i = 0; i < 6; ++i)
{
if (i & 1)
{
container.push_back(new Bar);
}
else
{
container.push_back(new Fin);
}
}
for (unsigned int i = 0; i < 6; ++i)
{
container[i]->Common_Operation();
}
I have really been struggling with a piece of code for a couple days. The error message i receive when i run my code is:
error: array initializer must be an initializer list
accountStore (int size = 0) : accts(size) { }
There seem to be others with similar problems here but unfortunately I am unable to apply their solutions (either don't work or not applicable).
What I am simply attempting to do is create a container class (array, can't use vectors) of a class 'prepaidAccount' but I am just unable to get the constructor portion of the container class 'storeAccount' to work. See code snippet below:
class prepaidAccount{
public:
//prepaidAccount ();
prepaidAccount(string newPhoneNum, float newAvailBal) : phoneNumber(newPhoneNum), availableBalance (newAvailBal){} //constructor
double addBalance(double howMuch) {
availableBalance = howMuch + availableBalance;
return availableBalance;
}
double payForCall(int callDuration, double tariff) {
callDuration = callDuration/60; //convert to minutes
double costOfCall = callDuration * tariff;
if (costOfCall > availableBalance) {
return -1;
}
else {
availableBalance = availableBalance - costOfCall;
return costOfCall;
}
}
void setAvailBal(int newAvailBal) {availableBalance = newAvailBal;}
float getAvailBal() {return availableBalance;}
void setPhoneNum(string newPhoneNum) {phoneNumber = newPhoneNum;}
string getPhoneNum() const {return phoneNumber;}
private:
string phoneNumber;
float availableBalance;
};
class accountStore { //made to store 100 prepaid accounts
public:
accountStore (int size = 0) : accts(size) { }
....
private:
prepaidAccount accts[100];
}
In main I simply call accountStore Account;
Any help is absolutely welcome. I very recently started learning c++ and about classes and constructors so please bear with me.
Thanks
You can't initialize an array with int like accountStore (int size = 0) : accts(size) {}.
prepaidAccount doesn't have a default constructor, you have to write member initialization list like,
accountStore (int size = 0) : accts{prepaidAccount(...), prepaidAccount(...), ...} { }
The array has 100 elements, it's not a practical solution here.
As a suggestion, think about std::vector, which has a constructor constructing with the spicified count of elements with specified value. Such as,
class accountStore {
public:
accountStore (int size = 0) : accts(size, prepaidAccount(...)) { }
....
private:
std::vector<prepaidAccount> accts;
};
Given that you have specified that you do not want to use a container such as std::vector but would like to specify the size at runtime, your only option would be to manually implement dynamic allocation yourself. Also given that you are wanting create 100 objects at a time, I would suggest making a function that can construct a temporary object according to your needs and then use this to initialise your dynamically allocated array. Consider the below code as a good starting point. (WARNING untested code.)
class prepaidAccount {
public:
// Constructor
prepaidAccount(string newPhoneNum, float newAvailBal)
: phoneNumber(newPhoneNum), availableBalance(newAvailBal) {}
// Default Constructor needed for dynamic allocation.
prepaidAccount() {}
/* your code*/
};
// Used to construct a tempoary prepaid account for copying to the array.
// Could use whatever constructor you see fit.
prepaidAccount MakePrepaidAccount(/*some parameters*/) {
/* Some code to generate account */
return some_var;
}
class accountStore {
public:
// Explicit constructor to avoid implicit type-casts.
explicit accountStore(const int &size = 0)
: accts(new prepaidAccount[size]) {
for (int i = 0; i < size; i++) {
// Will call defualt assignment function.
prepaidAccount[i] = MakePrepaidAccount(/*some parameters*/);
}
}
// Destructor
~accountStore() {
// Cleans up dynamically allocated memory.
delete[] prepaidAccount;
}
prepaidAccount *accts;
};
Edit: Amongst the c++ community it is often questionable when choosing to use dynamic allocation when there is such an excellent and comprehensive library of smart pointers. For example an std::vector would be perfect in this situation.
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.
How do I initialize an array of objects from a function? I'm aware the code below is impractical; I'm just teaching myself C++.
Here is a structure that contains data.
struct pointStruct {
int numberPoints;
Point2D pointArray;
};
The Point2D class has instance variables x and y. In a separate function, I have:
void setPoints(void) {
pointStruct myPointData;
myPointData.numberPoints = 4;
myPointData.pointArray[4]; // here is the problem
// loop with i
myPointData.pointArray[i].x = ...;
myPointData.pointArray[i].y = ...;
}
I'm trying to initialize the array so that I can loop through it and set the x,y coordinates. I've tried using new and some other methods but I can't work through what I need to do. How can I fix this?
When I try to compile this code, I get the error "no match for 'operator[]' in 'myPointData.pointStruct::pointArray[4]' "
You should probably use std::vector like MadScienceDreams suggests.
However, if you want to learn about such things, you could use a pointer instead. For example:
struct pointStruct {
int numberPoints;
Point2D* pointArray;
};
void setPoints(void) {
pointStruct myPointData;
const int num_points = 4;
myPointData.numberPoints = num_points;
myPointData.pointArray = new Point2D[num_points];
for(int i = 0; i < num_points; ++i) {
myPointData.pointArray[i].x = ...;
myPointData.pointArray[i].y = ...;
}
// Do stuff with myPointData...
// Don't forget to have a "delete" for every "new" when you're done.
delete[] myPointData.pointArray;
}
Point2D pointArray;
pointArray is a single instance to Point2D. It is not an array of instances in which case it's type is Point2D [N].
myPointData.pointArray[4];
The above statement calls operator [] taking a parameter of type int, which is not you actually want. Since there is no such member function in Point2D, compiler complains. If you wish to create array of instances, use std::vector<Point2D>.