C++中的堆栈实现

Question

I'm trying to implement a stack class as a learning exercise. This is my StackNode class,

template <typename T>
class StackNode{
public:
    T data_{};
    StackNode<T> next_;

    StackNode() : data_(nullptr), next_(nullptr){}
    StackNode(const T& data_) : data_(data_),next_(nullptr) {}

};

I use this node to create a Stack, this is the code,

template <typename T>
class Stack{
private:

    StackNode<T> top_;
    std::size_t size_{};

public:
    Stack() : top_(nullptr), size_ (0){}
    void push(const T& item){
        StackNode<T> p{item};
        p.next_ = top_;
        top_ = p;
    }

    T pop(){
       StackNode<T> p = top_;
        top_ = top_.next_;
        return p;
    }

    T peek(){
        return top_.data_;
    }

};

This is the calling Client,

Stack<int> stack{};
stack.push(12);
stack.push(13);
std::cout << stack.peek() << std::endl;
stack.pop();
std::cout << stack.peek() << std::endl;

I get the following compilation errors

In file included from /Users/mstewart/Dropbox/codespace/private/cpp-drive/data-structures/main.cpp:2:
In file included from /Users/mstewart/Dropbox/codespace/private/cpp-drive/data-structures/Stack.hpp:5:
/Users/mstewart/Dropbox/codespace/private/cpp-drive/data-structures/StackNode.h:12:18: error: field has incomplete type 'StackNode<int>'
    StackNode<T> next_;
                 ^
/Users/mstewart/Dropbox/codespace/private/cpp-drive/data-structures/Stack.hpp:13:18: note: in instantiation of template class 'StackNode<int>' requested here
    StackNode<T> top_;
                 ^
/Users/mstewart/Dropbox/codespace/private/cpp-drive/data-structures/main.cpp:6:16: note: in instantiation of template class 'Stack<int>' requested here
    Stack<int> stack{};
               ^
/Users/mstewart/Dropbox/codespace/private/cpp-drive/data-structures/StackNode.h:9:7: note: definition of 'StackNode<int>' is not complete until the closing '}'
class StackNode{
      ^

Can someone help me understand what I'm doing wrong. I'm new to C++

Answer 1

"A class instance cannot contain an instance of itself" - Oliver Charlesworth, The Comment Section.

As mentioned elsewhere , the class is not fully defined at the point of use (see the last line in the compiler error message), and would have problems with memory layout.

You therefore need to use pointers to manage the items in the array. One relatively straightforward adjustment to your code is to use std::unique_ptr instead (avoiding manual memory management with new and delete ).

IDEOne link w/ code

Note the ownership semantics where the stack owns the top node, which owns the node underneath, etc. You could emulate this with manually with raw pointers using new in the push function, and delete in the pop function.