Thread: Unable to store child class in parent class declaration

Hello,

Can anyone explain what's wrong with below code:

Code:

#include <iostream>
#include <vector>


using namespace std;


class A
{
public:
    virtual int get() = 0;
};


class B : public A
{
public:
    int get() { return 0; };
};


int main()
{
    std::vector<A> v;


    B b;
    v.push_back(b);
};

And throws this error:

Code:

/usr/bin/g++ -pthread --std=c++17 -fdiagnostics-color=always -g /home/test/*.cpp -o /home/test/main
In file included from /usr/include/x86_64-linux-gnu/c++/9/bits/c++allocator.h:33,
                 from /usr/include/c++/9/bits/allocator.h:46,
                 from /usr/include/c++/9/string:41,
                 from /usr/include/c++/9/bits/locale_classes.h:40,
                 from /usr/include/c++/9/bits/ios_base.h:41,
                 from /usr/include/c++/9/ios:42,
                 from /usr/include/c++/9/ostream:38,
                 from /usr/include/c++/9/iostream:39,
                 from /home/test/main.cpp:1:
/usr/include/c++/9/ext/new_allocator.h: In instantiation of ‘void __gnu_cxx::new_allocator<_Tp>::construct(_Up*, _Args&& ...) [with _Up = A; _Args = {const A&}; _Tp = A]’:
/usr/include/c++/9/bits/alloc_traits.h:482:2:   required from ‘static void std::allocator_traits<std::allocator<_CharT> >::construct(std::allocator_traits<std::allocator<_CharT> >::allocator_type&, _Up*, _Args&& ...) [with _Up = A; _Args = {const A&}; _Tp = A; std::allocator_traits<std::allocator<_CharT> >::allocator_type = std::allocator<A>]’
/usr/include/c++/9/bits/stl_vector.h:1189:30:   required from ‘void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = A; _Alloc = std::allocator<A>; std::vector<_Tp, _Alloc>::value_type = A]’
/home/test/main.cpp:24:18:   required from here
/usr/include/c++/9/ext/new_allocator.h:145:20: error: invalid new-expression of abstract class type ‘A’
  145 |  noexcept(noexcept(::new((void *)__p)
      |                    ^~~~~~~~~~~~~~~~~~
  146 |        _Up(std::forward<_Args>(__args)...)))
      |        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/test/main.cpp:7:7: note:   because the following virtual functions are pure within ‘A’:
    7 | class A
      |       ^
/home/test/main.cpp:10:17: note:   ‘virtual int A::get()’
   10 |     virtual int get() = 0;
      |                 ^~~


Build finished with error(s).

Originally Posted by john.c

You need to use pointers (or references) to get polymorphism.

Code:

#include <iostream>
#include <vector>
using namespace std;
 
class A
{
public:
    virtual ~A() {}
    virtual int get() const = 0;
};
 
class B : public A
{
public:
    int get() const override { return 0; };
};
 
int main()
{
    std::vector<A*> v;
    v.push_back(new B);
 
    cout << v.back()->get() << '\n';
 
    for (auto p: v) delete p;
}

Thank you so much, it worked for me. But is that necessary to add const and override in B class?

BTW. According to C++ guidelines raw pointers should not be used.
C++ Core Guidelines
So a better option is to use either std::unique_ptr or std::shared_ptr.

For a kind of static polymorphism std::variant would be an option.

Originally Posted by john.c

It depends on what you mean by "necessary". As for 'override', maybe you can forget about that for now. It just ensures that the function is overriding something (as opposed to being misspelled, for example).

As for "const", if a method does not modify the state of the object then it should be declared const so that the compiler knows it can use the method in contexts where the object is const.

So in the example below, if you removed 'const' from all of the get() methods (including the base class A) then function f(), which accepts const A&, won't be able to call get() even though get() doesn't change the state of the object.

Code:

#include <iostream>
#include <vector>
using namespace std;
 
class A
{
public:
    virtual ~A() {}
    virtual int get() const = 0;
};
 
class B : public A
{
public:
    int get() const { cout << "B: "; return 0; };
};
 
class C : public A
{
public:
    int get() const { cout << "C: "; return 0; };
};
 
// f() promises not to modify 'a'.
// So it will only be allowed to call 'const' methods on 'a'.
void f(const A& a) { cout << a.get() << '\n'; }
 
int main()
{
    std::vector<A*> v;
    v.push_back(new B);
    v.push_back(new C);
    v.push_back(new C);
    v.push_back(new B);
 
    for (const auto p: v) f(*p);
 
    for (auto p: v) delete p;
    v.clear();
}

Much clear, thanks again.

Originally Posted by john.c

You need to put & into the declarations in Category.

Code:

class Category
{
public:
    virtual int getX(Product &) = 0;
    virtual void printX(Product &) = 0;
};

Thanks for the answer, much appreciated. I have still concern with above example, if I call the getX(...) function in printX(...) then I'm getting error undefined reference:

Code:

// Category.cpp
void CategoryImpl::printX(Product p) const
{
    std::cout << CategoryImpl::getX(p);
}

// main.cpp
int main()
{
    Category *obj = new CategoryImplType<AAA>();
    obj->printX(Product()); // Error below:

    /*/usr/bin/ld: /tmp/ccEjMMob.o: in function `CategoryImpl::printX(Product) const':/home/test/cat.cpp:7: undefined reference to `Category::getX(Product) const'
collect2: error: ld returned 1 exit status
   */
};

Originally Posted by john.c

It should be more like this:

Code:

void CategoryImpl::printX(const Product &p) const
{
    std::cout << getX(p) << '\n';
}

The main problem was CategoryImpl:: before getX. I also changed it to a reference (Product &) and added const. You need to ensure that previous declarations of printX look the same.

That's much clear, thanks for saving my days.