I am going to store a large quantity of objects in a std::list.
Since I need those objects to store different types of data, I am storing pointers to a base class which only holds an attribute of an enum type which tells the objects to which of the derived classes it should be casted to. Each derived class has then its own type of data:
struct Base {
enum class Type {
D1,
D2,
...
} type;
Base(Type new_type):
type(new_type) {}
};
struct D1: public Base {
std::string data;
D1(std::string new_data):
Base(Base::Type::D1), data(new_data) {}
};
struct D2: public Base {
double data;
D2(double new_data):
Base(Base::Type::D2), data(new_data) {}
};
To hold pointers to these objects, I am using smart pointers:
std::list<std::unique_ptr<Base>> list;
list.push_back(std::unique_ptr<Base>(new D1("Somestring")));
list.push_back(std::unique_ptr<Base>(new D2(3.14)));
However, while each Base object knows what type it should be casted to to get deleted properly, the smart pointer only knows that it must call Base's destructor. Which would leave undeleted the memory each subclass allocated.
How would I pass a custom deleter to the smart pointers so that they know how to properly cast and free each object's memory? What should I implement that custom deleter?
Just mark the destructor of Base as virtual. Then the default deleter will invoke delete pointer_to_raw_object;, which will call the right destructor based on the dynamic type of object.
Example:
#include <iostream>
#include <memory>
#include <list>
struct Base
{
virtual ~Base(){std::cout << __PRETTY_FUNCTION__ << std::endl;}
};
struct Derived : Base
{
~Derived() override {std::cout << __PRETTY_FUNCTION__ << std::endl;}
};
int main()
{
std::list<std::unique_ptr<Base>> l;
l.emplace_back(new Base);
l.emplace_back(new Derived);
}
Live on Coliru
PS: consider using std::list::emplace_back for cleaner (and more efficient) code.
Related
I would like to store instances of several classes in a vector. Since all classes inherit from the same base class this should be possible.
Imagine this program:
#include <iostream>
#include <vector>
using namespace std;
class Base
{
public:
virtual void identify ()
{
cout << "BASE" << endl;
}
};
class Derived: public Base
{
public:
virtual void identify ()
{
cout << "DERIVED" << endl;
}
};
int main ()
{
Derived derived;
vector<Base> vect;
vect.push_back(derived);
vect[0].identify();
return 0;
}
I expected it to print "DERIVED", because the identify() method is virtual. Instead vect[0] seems to be a Base instance and it prints "BASE".
I guess I could write my own container (probably derived from vector) somehow that is capable of doing this (maybe holding only pointers...).
I just wanted to ask if there is a more C++'ish way to do this. AND I would like to be completely vector-compatible (just for convenience if other users should ever use my code).
What you are seeing is Object Slicing.
You are storing object of Derived class in an vector which is supposed to store objects of Base class, this leads to Object slicing and the derived class specific members of the object being stored get sliced off, thus the object stored in the vector just acts as object of Base class.
Solution:
You should store pointer to object of Base class in the vector:
vector<Base*>
By storing a pointer to Base class there would be no slicing and you can achieve the desired polymorphic behavior as well.
Since you ask for a C++ish way of doing this, the right approach is to use a suitable Smart pointer instead of storing a raw pointer in the vector. That will ensure you do not have to manually manage the memory, RAII will do that for you automatically.
You're experiencing slicing. The vector copies the derived object, a new one of type Base is inserted.
TL;DR: You should not inherit from a publicly copyable/movable class.
It is actually possible to prevent object slicing, at compilation time: the base object should not be copyable in this context.
Case 1: an abstract base
If the base is abstract, then it cannot be instantiated and thus you cannot experience slicing.
Case 2: a concrete base
If the base is not abstract, then it can be copied (by default). You have two choices:
prevent copy altogether
allow copy only for children
Note: in C++11, the move operations cause the same issue.
// C++ 03, prevent copy
class Base {
public:
private:
Base(Base const&);
void operator=(Base const&);
};
// C++ 03, allow copy only for children
class Base {
public:
protected:
Base(Base const& other) { ... }
Base& operator=(Base const& other) { ...; return *this; }
};
// C++ 11, prevent copy & move
class Base {
public:
Base(Base&&) = delete;
Base(Base const&) = delete;
Base& operator=(Base) = delete;
};
// C++ 11, allow copy & move only for children
class Base {
public:
protected:
Base(Base&&) = default;
Base(Base const&) = default;
Base& operator=(Base) = default;
};
I'd use vector<Base*> to store them. If you say vector<Base>, slicing will occur.
This does mean that you'd have to delete the actual objects yourself after you've removed the pointers from your vector, but otherwise you should be fine.
// Below is the solution by using vector<Based*> vect,
// Base *pBase , and initialized pBase with
// with the address of derived which is
// of type Derived
#include <iostream>
#include <vector>
using namespace std;
class Base
{
public:
virtual void identify ()
{
cout << "BASE" << endl;
}
};
class Derived: public Base
{
public:
virtual void identify ()
{
cout << "DERIVED" << endl;
}
};
int main ()
{
Base *pBase; // The pointer pBase of type " pointer to Base"
Derived derived;
// PBase is initialized with the address of derived which is
// of type Derived
pBase = & derived;
// Store pointer to object of Base class in the vector:
vector<Base*> vect;
// Add an element to vect using pBase which is initialized with the address
// of derived
vect.push_back(pBase);
vect[0]->identify();
return 0;
}
As all others mentioned here, you cannot accomplish to insert an object of derived to a vector of base due to object slicing that would happen on copy construction.
If goal is to avoid memory allocation, you can use an std::variant, but the vector would no longer be of base class
using HierarchyItem = std::variant<Base, Derived>;
int main()
{
vector<HierarchyItem> vect;
vect.push_back(Derived());
std::visit([](auto &&hier_item){ hier_item.identify(); }, vect[0]);
return 0;
}
I would like to store instances of several classes in a vector. Since all classes inherit from the same base class this should be possible.
Imagine this program:
#include <iostream>
#include <vector>
using namespace std;
class Base
{
public:
virtual void identify ()
{
cout << "BASE" << endl;
}
};
class Derived: public Base
{
public:
virtual void identify ()
{
cout << "DERIVED" << endl;
}
};
int main ()
{
Derived derived;
vector<Base> vect;
vect.push_back(derived);
vect[0].identify();
return 0;
}
I expected it to print "DERIVED", because the identify() method is virtual. Instead vect[0] seems to be a Base instance and it prints "BASE".
I guess I could write my own container (probably derived from vector) somehow that is capable of doing this (maybe holding only pointers...).
I just wanted to ask if there is a more C++'ish way to do this. AND I would like to be completely vector-compatible (just for convenience if other users should ever use my code).
What you are seeing is Object Slicing.
You are storing object of Derived class in an vector which is supposed to store objects of Base class, this leads to Object slicing and the derived class specific members of the object being stored get sliced off, thus the object stored in the vector just acts as object of Base class.
Solution:
You should store pointer to object of Base class in the vector:
vector<Base*>
By storing a pointer to Base class there would be no slicing and you can achieve the desired polymorphic behavior as well.
Since you ask for a C++ish way of doing this, the right approach is to use a suitable Smart pointer instead of storing a raw pointer in the vector. That will ensure you do not have to manually manage the memory, RAII will do that for you automatically.
You're experiencing slicing. The vector copies the derived object, a new one of type Base is inserted.
TL;DR: You should not inherit from a publicly copyable/movable class.
It is actually possible to prevent object slicing, at compilation time: the base object should not be copyable in this context.
Case 1: an abstract base
If the base is abstract, then it cannot be instantiated and thus you cannot experience slicing.
Case 2: a concrete base
If the base is not abstract, then it can be copied (by default). You have two choices:
prevent copy altogether
allow copy only for children
Note: in C++11, the move operations cause the same issue.
// C++ 03, prevent copy
class Base {
public:
private:
Base(Base const&);
void operator=(Base const&);
};
// C++ 03, allow copy only for children
class Base {
public:
protected:
Base(Base const& other) { ... }
Base& operator=(Base const& other) { ...; return *this; }
};
// C++ 11, prevent copy & move
class Base {
public:
Base(Base&&) = delete;
Base(Base const&) = delete;
Base& operator=(Base) = delete;
};
// C++ 11, allow copy & move only for children
class Base {
public:
protected:
Base(Base&&) = default;
Base(Base const&) = default;
Base& operator=(Base) = default;
};
I'd use vector<Base*> to store them. If you say vector<Base>, slicing will occur.
This does mean that you'd have to delete the actual objects yourself after you've removed the pointers from your vector, but otherwise you should be fine.
// Below is the solution by using vector<Based*> vect,
// Base *pBase , and initialized pBase with
// with the address of derived which is
// of type Derived
#include <iostream>
#include <vector>
using namespace std;
class Base
{
public:
virtual void identify ()
{
cout << "BASE" << endl;
}
};
class Derived: public Base
{
public:
virtual void identify ()
{
cout << "DERIVED" << endl;
}
};
int main ()
{
Base *pBase; // The pointer pBase of type " pointer to Base"
Derived derived;
// PBase is initialized with the address of derived which is
// of type Derived
pBase = & derived;
// Store pointer to object of Base class in the vector:
vector<Base*> vect;
// Add an element to vect using pBase which is initialized with the address
// of derived
vect.push_back(pBase);
vect[0]->identify();
return 0;
}
As all others mentioned here, you cannot accomplish to insert an object of derived to a vector of base due to object slicing that would happen on copy construction.
If goal is to avoid memory allocation, you can use an std::variant, but the vector would no longer be of base class
using HierarchyItem = std::variant<Base, Derived>;
int main()
{
vector<HierarchyItem> vect;
vect.push_back(Derived());
std::visit([](auto &&hier_item){ hier_item.identify(); }, vect[0]);
return 0;
}
I would like to store instances of several classes in a vector. Since all classes inherit from the same base class this should be possible.
Imagine this program:
#include <iostream>
#include <vector>
using namespace std;
class Base
{
public:
virtual void identify ()
{
cout << "BASE" << endl;
}
};
class Derived: public Base
{
public:
virtual void identify ()
{
cout << "DERIVED" << endl;
}
};
int main ()
{
Derived derived;
vector<Base> vect;
vect.push_back(derived);
vect[0].identify();
return 0;
}
I expected it to print "DERIVED", because the identify() method is virtual. Instead vect[0] seems to be a Base instance and it prints "BASE".
I guess I could write my own container (probably derived from vector) somehow that is capable of doing this (maybe holding only pointers...).
I just wanted to ask if there is a more C++'ish way to do this. AND I would like to be completely vector-compatible (just for convenience if other users should ever use my code).
What you are seeing is Object Slicing.
You are storing object of Derived class in an vector which is supposed to store objects of Base class, this leads to Object slicing and the derived class specific members of the object being stored get sliced off, thus the object stored in the vector just acts as object of Base class.
Solution:
You should store pointer to object of Base class in the vector:
vector<Base*>
By storing a pointer to Base class there would be no slicing and you can achieve the desired polymorphic behavior as well.
Since you ask for a C++ish way of doing this, the right approach is to use a suitable Smart pointer instead of storing a raw pointer in the vector. That will ensure you do not have to manually manage the memory, RAII will do that for you automatically.
You're experiencing slicing. The vector copies the derived object, a new one of type Base is inserted.
TL;DR: You should not inherit from a publicly copyable/movable class.
It is actually possible to prevent object slicing, at compilation time: the base object should not be copyable in this context.
Case 1: an abstract base
If the base is abstract, then it cannot be instantiated and thus you cannot experience slicing.
Case 2: a concrete base
If the base is not abstract, then it can be copied (by default). You have two choices:
prevent copy altogether
allow copy only for children
Note: in C++11, the move operations cause the same issue.
// C++ 03, prevent copy
class Base {
public:
private:
Base(Base const&);
void operator=(Base const&);
};
// C++ 03, allow copy only for children
class Base {
public:
protected:
Base(Base const& other) { ... }
Base& operator=(Base const& other) { ...; return *this; }
};
// C++ 11, prevent copy & move
class Base {
public:
Base(Base&&) = delete;
Base(Base const&) = delete;
Base& operator=(Base) = delete;
};
// C++ 11, allow copy & move only for children
class Base {
public:
protected:
Base(Base&&) = default;
Base(Base const&) = default;
Base& operator=(Base) = default;
};
I'd use vector<Base*> to store them. If you say vector<Base>, slicing will occur.
This does mean that you'd have to delete the actual objects yourself after you've removed the pointers from your vector, but otherwise you should be fine.
// Below is the solution by using vector<Based*> vect,
// Base *pBase , and initialized pBase with
// with the address of derived which is
// of type Derived
#include <iostream>
#include <vector>
using namespace std;
class Base
{
public:
virtual void identify ()
{
cout << "BASE" << endl;
}
};
class Derived: public Base
{
public:
virtual void identify ()
{
cout << "DERIVED" << endl;
}
};
int main ()
{
Base *pBase; // The pointer pBase of type " pointer to Base"
Derived derived;
// PBase is initialized with the address of derived which is
// of type Derived
pBase = & derived;
// Store pointer to object of Base class in the vector:
vector<Base*> vect;
// Add an element to vect using pBase which is initialized with the address
// of derived
vect.push_back(pBase);
vect[0]->identify();
return 0;
}
As all others mentioned here, you cannot accomplish to insert an object of derived to a vector of base due to object slicing that would happen on copy construction.
If goal is to avoid memory allocation, you can use an std::variant, but the vector would no longer be of base class
using HierarchyItem = std::variant<Base, Derived>;
int main()
{
vector<HierarchyItem> vect;
vect.push_back(Derived());
std::visit([](auto &&hier_item){ hier_item.identify(); }, vect[0]);
return 0;
}
I would like to store instances of several classes in a vector. Since all classes inherit from the same base class this should be possible.
Imagine this program:
#include <iostream>
#include <vector>
using namespace std;
class Base
{
public:
virtual void identify ()
{
cout << "BASE" << endl;
}
};
class Derived: public Base
{
public:
virtual void identify ()
{
cout << "DERIVED" << endl;
}
};
int main ()
{
Derived derived;
vector<Base> vect;
vect.push_back(derived);
vect[0].identify();
return 0;
}
I expected it to print "DERIVED", because the identify() method is virtual. Instead vect[0] seems to be a Base instance and it prints "BASE".
I guess I could write my own container (probably derived from vector) somehow that is capable of doing this (maybe holding only pointers...).
I just wanted to ask if there is a more C++'ish way to do this. AND I would like to be completely vector-compatible (just for convenience if other users should ever use my code).
What you are seeing is Object Slicing.
You are storing object of Derived class in an vector which is supposed to store objects of Base class, this leads to Object slicing and the derived class specific members of the object being stored get sliced off, thus the object stored in the vector just acts as object of Base class.
Solution:
You should store pointer to object of Base class in the vector:
vector<Base*>
By storing a pointer to Base class there would be no slicing and you can achieve the desired polymorphic behavior as well.
Since you ask for a C++ish way of doing this, the right approach is to use a suitable Smart pointer instead of storing a raw pointer in the vector. That will ensure you do not have to manually manage the memory, RAII will do that for you automatically.
You're experiencing slicing. The vector copies the derived object, a new one of type Base is inserted.
TL;DR: You should not inherit from a publicly copyable/movable class.
It is actually possible to prevent object slicing, at compilation time: the base object should not be copyable in this context.
Case 1: an abstract base
If the base is abstract, then it cannot be instantiated and thus you cannot experience slicing.
Case 2: a concrete base
If the base is not abstract, then it can be copied (by default). You have two choices:
prevent copy altogether
allow copy only for children
Note: in C++11, the move operations cause the same issue.
// C++ 03, prevent copy
class Base {
public:
private:
Base(Base const&);
void operator=(Base const&);
};
// C++ 03, allow copy only for children
class Base {
public:
protected:
Base(Base const& other) { ... }
Base& operator=(Base const& other) { ...; return *this; }
};
// C++ 11, prevent copy & move
class Base {
public:
Base(Base&&) = delete;
Base(Base const&) = delete;
Base& operator=(Base) = delete;
};
// C++ 11, allow copy & move only for children
class Base {
public:
protected:
Base(Base&&) = default;
Base(Base const&) = default;
Base& operator=(Base) = default;
};
I'd use vector<Base*> to store them. If you say vector<Base>, slicing will occur.
This does mean that you'd have to delete the actual objects yourself after you've removed the pointers from your vector, but otherwise you should be fine.
// Below is the solution by using vector<Based*> vect,
// Base *pBase , and initialized pBase with
// with the address of derived which is
// of type Derived
#include <iostream>
#include <vector>
using namespace std;
class Base
{
public:
virtual void identify ()
{
cout << "BASE" << endl;
}
};
class Derived: public Base
{
public:
virtual void identify ()
{
cout << "DERIVED" << endl;
}
};
int main ()
{
Base *pBase; // The pointer pBase of type " pointer to Base"
Derived derived;
// PBase is initialized with the address of derived which is
// of type Derived
pBase = & derived;
// Store pointer to object of Base class in the vector:
vector<Base*> vect;
// Add an element to vect using pBase which is initialized with the address
// of derived
vect.push_back(pBase);
vect[0]->identify();
return 0;
}
As all others mentioned here, you cannot accomplish to insert an object of derived to a vector of base due to object slicing that would happen on copy construction.
If goal is to avoid memory allocation, you can use an std::variant, but the vector would no longer be of base class
using HierarchyItem = std::variant<Base, Derived>;
int main()
{
vector<HierarchyItem> vect;
vect.push_back(Derived());
std::visit([](auto &&hier_item){ hier_item.identify(); }, vect[0]);
return 0;
}
I would like to store instances of several classes in a vector. Since all classes inherit from the same base class this should be possible.
Imagine this program:
#include <iostream>
#include <vector>
using namespace std;
class Base
{
public:
virtual void identify ()
{
cout << "BASE" << endl;
}
};
class Derived: public Base
{
public:
virtual void identify ()
{
cout << "DERIVED" << endl;
}
};
int main ()
{
Derived derived;
vector<Base> vect;
vect.push_back(derived);
vect[0].identify();
return 0;
}
I expected it to print "DERIVED", because the identify() method is virtual. Instead vect[0] seems to be a Base instance and it prints "BASE".
I guess I could write my own container (probably derived from vector) somehow that is capable of doing this (maybe holding only pointers...).
I just wanted to ask if there is a more C++'ish way to do this. AND I would like to be completely vector-compatible (just for convenience if other users should ever use my code).
What you are seeing is Object Slicing.
You are storing object of Derived class in an vector which is supposed to store objects of Base class, this leads to Object slicing and the derived class specific members of the object being stored get sliced off, thus the object stored in the vector just acts as object of Base class.
Solution:
You should store pointer to object of Base class in the vector:
vector<Base*>
By storing a pointer to Base class there would be no slicing and you can achieve the desired polymorphic behavior as well.
Since you ask for a C++ish way of doing this, the right approach is to use a suitable Smart pointer instead of storing a raw pointer in the vector. That will ensure you do not have to manually manage the memory, RAII will do that for you automatically.
You're experiencing slicing. The vector copies the derived object, a new one of type Base is inserted.
TL;DR: You should not inherit from a publicly copyable/movable class.
It is actually possible to prevent object slicing, at compilation time: the base object should not be copyable in this context.
Case 1: an abstract base
If the base is abstract, then it cannot be instantiated and thus you cannot experience slicing.
Case 2: a concrete base
If the base is not abstract, then it can be copied (by default). You have two choices:
prevent copy altogether
allow copy only for children
Note: in C++11, the move operations cause the same issue.
// C++ 03, prevent copy
class Base {
public:
private:
Base(Base const&);
void operator=(Base const&);
};
// C++ 03, allow copy only for children
class Base {
public:
protected:
Base(Base const& other) { ... }
Base& operator=(Base const& other) { ...; return *this; }
};
// C++ 11, prevent copy & move
class Base {
public:
Base(Base&&) = delete;
Base(Base const&) = delete;
Base& operator=(Base) = delete;
};
// C++ 11, allow copy & move only for children
class Base {
public:
protected:
Base(Base&&) = default;
Base(Base const&) = default;
Base& operator=(Base) = default;
};
I'd use vector<Base*> to store them. If you say vector<Base>, slicing will occur.
This does mean that you'd have to delete the actual objects yourself after you've removed the pointers from your vector, but otherwise you should be fine.
// Below is the solution by using vector<Based*> vect,
// Base *pBase , and initialized pBase with
// with the address of derived which is
// of type Derived
#include <iostream>
#include <vector>
using namespace std;
class Base
{
public:
virtual void identify ()
{
cout << "BASE" << endl;
}
};
class Derived: public Base
{
public:
virtual void identify ()
{
cout << "DERIVED" << endl;
}
};
int main ()
{
Base *pBase; // The pointer pBase of type " pointer to Base"
Derived derived;
// PBase is initialized with the address of derived which is
// of type Derived
pBase = & derived;
// Store pointer to object of Base class in the vector:
vector<Base*> vect;
// Add an element to vect using pBase which is initialized with the address
// of derived
vect.push_back(pBase);
vect[0]->identify();
return 0;
}
As all others mentioned here, you cannot accomplish to insert an object of derived to a vector of base due to object slicing that would happen on copy construction.
If goal is to avoid memory allocation, you can use an std::variant, but the vector would no longer be of base class
using HierarchyItem = std::variant<Base, Derived>;
int main()
{
vector<HierarchyItem> vect;
vect.push_back(Derived());
std::visit([](auto &&hier_item){ hier_item.identify(); }, vect[0]);
return 0;
}