C++ - Object of abstract class type is not allowed

Question

Currently working on an implementation of a stack controlled by a resizable array. Trying to instantiate a new object of ResizableArrayStack gives errors.

E0322   object of abstract class type "csc232::ResizableArrayStack" is not allowed: ResizableArrayStack.cpp 107
            function "csc232::ResizableArrayStack::isEmpty [with ItemType=int]" is a pure virtual function
            function "csc232::ResizableArrayStack::push [with ItemType=int]" is a pure virtual function
            function "csc232::ResizableArrayStack::pop [with ItemType=int]" is a pure virtual function
            function "csc232::ResizableArrayStack::peek [with ItemType=int]" is a pure virtual function

StackInterface.h

#include "pch.h"
#pragma once
#ifndef CSC232_HW05_RESIZABLE_ARRAY_STACK_STACK_INTERFACE_H
#define CSC232_HW05_RESIZABLE_ARRAY_STACK_STACK_INTERFACE_H

namespace csc232 {
    template<typename ItemType>
    class StackInterface {
    public:
        /**
         * Sees whether the stack is empty.
         * @return True if the stack is empty, or false if not.
         */
        virtual bool isEmpty() const = 0;

        /**
         * Adds a new entry to the top of this stack.
         * @param  newEntry The object to be added as a new entry.
         * @return True if the addition is successful or false if not.
         * @post   If the operation was successful, newEntry is at the top of the stack.
         */
        virtual bool push(const ItemType &newEntry) = 0;

        /**
         * Removes the top of this stack.
         * @return True if the removal was successful or false if not.
         * @post   If the operation was successful, the top of the stack has been removed.
         */
        virtual bool pop() = 0;

        /**
         * Returns a copy of the top of this stack.
         * @return A copy of the top the stack.
         * @post   A copy of the top of the stack has been returned, and the stack is unchanged.
         */
        virtual ItemType peek() const = 0;

        /**
         * Destroys this stack and frees its assigned memory
         */
        virtual ~StackInterface() = default;
    };
}

#endif //CSC232_HW05_RESIZABLE_ARRAY_STACK_STACK_INTERFACE_H

ResizableArrayStack.h

#include "pch.h"
#pragma once
#ifndef CSC232_HW05_RESIZABLE_ARRAY_STACK_RESIZABLE_ARRAY_STACK_H
#define CSC232_HW05_RESIZABLE_ARRAY_STACK_RESIZABLE_ARRAY_STACK_H

#include "StackInterface.h"

namespace csc232 {
    template <typename ItemType>
    class ResizableArrayStack : public StackInterface<ItemType> {

    private:

        ItemType* items;

        int top, capacity, count;

        static const int DEFAULT_CAPACITY = 10;

    public:

        ResizableArrayStack();

        ResizableArrayStack(int initial_capacity);

        ResizableArrayStack(const ResizableArrayStack &rhs);

        ResizableArrayStack(ResizableArrayStack &&rhs) = delete;

        void operator=(const ResizableArrayStack<ItemType> &rhs);

        void operator=(ResizableArrayStack &&rhs) = delete;

        ~ResizableArrayStack();

        bool isEmpty() const = 0;

        bool push(const ItemType &newEntry) = 0;

        bool pop() = 0;

        ItemType peek() const = 0;

        int getCapacity() const;

    private:

        void init();

        void increase_size();

    };
#endif // CSC232_HW05_RESIZABLE_ARRAY_STACK_RESIZABLE_ARRAY_STACK_H

ResizableArrayStack.cpp

#include "pch.h"
#include <iostream>

#include "ResizableArrayStack.h"


template<typename ItemType>
csc232::ResizableArrayStack<ItemType>::ResizableArrayStack() : count(0), capacity(DEFAULT_CAPACITY) {
    init();
}

template<typename ItemType>
csc232::ResizableArrayStack<ItemType>::ResizableArrayStack(int initial_capacity) : count(0), capacity(initial_capacity) {
    init();
}

template<typename ItemType>
void csc232::ResizableArrayStack<ItemType>::init() {
    items = new ItemType[capacity];
    count = 0;
}

template<typename ItemType>
csc232::ResizableArrayStack<ItemType>::ResizableArrayStack(const ResizableArrayStack &rhs) {
    *this = rhs;
}

template<typename ItemType>
void csc232::ResizableArrayStack<ItemType>::operator=(const ResizableArrayStack<ItemType> &rhs) {
    if (this != rhs)
    {
        delete[] items;
        init();
        for (int i = 0; i < rhs.count; i++)
        {
            this->push(rhs.items[i]);
        }
    }
}
template<typename ItemType>
csc232::ResizableArrayStack<ItemType>::~ResizableArrayStack() {
    delete[] items;
}

template<typename ItemType>
bool csc232::ResizableArrayStack<ItemType>::isEmpty() const {
    return count == 0;
}

template<typename ItemType>
bool csc232::ResizableArrayStack<ItemType>::push(const ItemType &newEntry) {
    if (count == capacity)
        increase_size();
    items[count] = newEntry;
    return false;
}

template<typename ItemType>
bool csc232::ResizableArrayStack<ItemType>::pop() {
    if (count == 0)
        throw std::underflow_error("Underflow exception.");
    count--;
    return false;
}

template<typename ItemType>
ItemType csc232::ResizableArrayStack<ItemType>::peek() const {
    top = capacity - 1;
    return items[top];
}

template<typename ItemType>
int csc232::ResizableArrayStack<ItemType>::getCapacity() const {
    return capacity;
}

template<typename ItemType>
void csc232::ResizableArrayStack<ItemType>::increase_size() {
    capacity = capacity * 2;
    ItemType *temp = new ItemType[capacity];

    for (int i = 0; i < capacity; i++)
        temp[i] = items[i];

    delete[] items;

    items = temp;
}

int main(int argc, char* argv[]) {

    csc232::ResizableArrayStack<int> stack;

}

What language is that exactly? Which compiler are you using? Which settings? — curiousguy, Mar 29 '19 at 22:35

score 5 · Answer 1 · answered Mar 29 '19 at 22:37

In your ResizableArrayStack class definition your virtual function overrides are still "pure":

    bool isEmpty() const = 0;

    bool push(const ItemType &newEntry) = 0;

    bool pop() = 0;

    ItemType peek() const = 0;

remove the = 0 and for good measure, tell the compiler that these are overrides:

    bool isEmpty() const override;

    bool push(const ItemType &newEntry) override;

    bool pop() override;

    ItemType peek() const override;

score 2 · Answer 2 · answered Mar 29 '19 at 22:36

A class is abstract if the final overrider of any virtual function is pure virtual. Your definition of template class ResizableArrayStack has = 0; at the ends of several virtual function overrides. This means they are considered pure virtual even though they also have definitions. (Defining a pure virtual function is valid C++, and the definitions could be called using a qualified name, but still must be overridden by something else to get a non-abstract class.)

Just take the = 0 pieces out of ResizableArrayStack.

But note also, you normally should not put definitions with template parameters in a *.cpp file: See the Q&A "Why can templates only be implemented in the header file?"

C++ - Object of abstract class type is not allowed

2 Answers2