C++ - 不允许抽象类类型的对象

Question

目前正在研究由可调整大小的数组控制的堆栈的实现。尝试实例化 ResizableArrayStack 的新对象会出错。

不允许抽象类类型“csc232::ResizableArrayStack”的 E0322 对象：ResizableArrayStack.cpp 107 函数“csc232::ResizableArrayStack::isEmpty [with ItemType=int]”是一个纯虚函数 函数“csc232::ResizableArrayStack::push [with ItemType=int]”是一个纯虚函数 函数“csc232::ResizableArrayStack::pop [with ItemType=int]”是一个纯虚函数 函数“csc232::ResizableArrayStack::peek [with ItemType=int]”是一个纯虚函数

堆栈接口.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;

}

Answer 1

在您的 ResizableArrayStack 类定义中，您的虚函数覆盖仍然是“纯”的：

    bool isEmpty() const = 0;

    bool push(const ItemType &newEntry) = 0;

    bool pop() = 0;

    ItemType peek() const = 0;

删除 = 0 并告诉编译器这些是覆盖：

    bool isEmpty() const override;

    bool push(const ItemType &newEntry) override;

    bool pop() override;

    ItemType peek() const override;

Answer 2

如果任何虚函数的最终覆盖器是纯虚的，则类是抽象的。您对模板类ResizableArrayStack 的定义在几个虚函数覆盖的末尾有= 0;。这意味着它们被认为是纯虚拟的，即使它们也有定义。 （定义纯虚函数是有效的 C++，可以使用限定名称调用定义，但仍必须被其他东西覆盖才能获得非抽象类。）

只需从ResizableArrayStack 中取出= 0 部分。

但也请注意，您通常不应将带有模板参数的定义放在 *.cpp 文件中：请参阅 Q&A "Why can templates only be implemented in the header file?"