Let us assume that we have a class called Node which has a member called sequence and id. We want to print the sequence of the Node in many differnt ways. Instead of adding the print functions directly into the Node class, we put them into a seperate class called NodePrinter. Each Node, needs to have a "working" NodePrinter in any case.
Which implies that:
Node has a NodePrinter * printer member
Every constructor of Node needs to create a new NodePrinter
My idea now was to create a BaseNode and move the NodePrinter into that one. It has only one constructor, which takes a Node as an input and assigns it to the NodePrinter:
#include <iostream>
#include <string>
using namespace std;
class NodePrinter;
class Node;
class BaseNode
{
public:
BaseNode() = delete;
BaseNode(Node * node);
~BaseNode();
NodePrinter * printer;
};
class Node: public BaseNode
{
public:
Node(string sequence): BaseNode(this), sequence(sequence){}
Node(int id, string sequence): BaseNode(this), sequence(sequence), id(id){}
int id;
string sequence;
};
class NodePrinter
{
private:
Node * node;
public:
NodePrinter() = delete;
NodePrinter(Node * node): node(node){}
void print_node()
{
std::cout<<node->sequence<<endl;
}
};
BaseNode::BaseNode(Node * node)
{
node->printer = new NodePrinter(node);
}
BaseNode::~BaseNode()
{
delete printer;
printer = nullptr;
}
int main()
{
Node node("abc");
node.printer->print_node();
return 0;
}
Thereby each node is forced to call BaseNode(this) and the resources get allocated.
Is this reasonable, or is this whole approach already twisted from the start? Is there a better way to do this?
One thing that seems odd to me is that the printer depends on an instance of Node, shouldn't it be possible for a single printer to print multiple nodes? And I also wouldn't have Node depend on a NodePrinter either, because then you can't print the same node with multiple printers.
Anyhow, if you really need to keep the 1-to-1 correspondence, the simplest way is to just initialize the NodePrinter directly where the member variable is declared in Node:
#include <iostream>
#include <memory>
#include <string>
class Node;
class NodePrinter
{
private:
Node * node;
public:
NodePrinter() = delete;
NodePrinter(Node * node): node(node){}
void print_node();
};
class Node
{
public:
Node(std::string sequence) : sequence(std::move(sequence)){}
Node(int id, std::string sequence) : id(id), sequence(std::move(sequence)) {}
int id;
std::string sequence;
std::unique_ptr<NodePrinter> printer = std::make_unique<NodePrinter>(this);
};
void NodePrinter::print_node()
{
std::cout<< node->sequence << '\n';
}
int main()
{
Node node("abc");
node.printer->print_node();
return 0;
}
Live demo on wandbox.
Related
So I have a parent Class which is:
class Node
{
public:
Node();
void setParentNode(Node* parent) {this->parentNode = parent;}
Node* getParentNode() {return this->parentNode;}
std::vector<Node> getChildNodes(){return this->childNodes;}
void addChildNode(Node* node);
void removeNode();
private:
std::vector<Node*> childNodes;
Node* parentNode = nullptr;
};
And a child inheriting from that class:
class Cube : public Node
{
public:
Cube();
};
Now I have another file which has a function that uses the child class:
#include "cube.h"
void addCubes(){
Cube mainCube;
for(int i = 0; i < 10; i++){
Cube c;
mainCube.addChildNode(c);
}
}
Problem is that mainCube doesn't see the addChildNode function which the parent has. What is the point of inheriting from another class if the parents functions aren't accessible from another place using the child class?
No, the parent classes public functions are callable from the child object.
However, the prototype of the function is
void Node::addChildNode(Node node);
So it's taking a Node object & not a Cube object.
So your compiler cannot find a function which takes a Cube object & hence the error.
The fix is to use a pointer to Node or reference to Node while declaring/defining the function.
So you function should be
void addChildNode(Node & node);
In which case, the Node object can be passed to the function & the compiler will find it.
Even better would be to have
// if you aren't looking to modify the passed object inside addChildNode
void addChildNode(const Node & node);
The following is fine & hence the function will work
Cube b;
Node &a = b;
or
Node * pn = &b;
The derived classes should be able to see the addChildNode function if you keep the signature of the function aligns. This is not a big issue. However, there are a few more "serious" issues with your code:
You need to make the base class' destructor virtual to avoid some undefined behaviors.
You have to design the ownership of nodes carefully. I guess you want the Node class to own and manage its children nodes. That means function addChildNode actually takes the ownership of the passed in node object, and it should also be deleted during destruction.
In function addCubes(), there is a loop that keeps calling addChildNode function but passes the local variable Cube c; which will be out of scope and destroyed after the loop. Thus, the parent object mainCube will holds pointers to already destroyed objects, and it will cause a crash.
After fixing all these issues, your code looks like this:
class Node
{
public:
Node() {};
virtual ~Node() {
for(auto n: childNodes) delete n;
};
void setParentNode(Node* parent) {this->parentNode = parent;}
Node* getParentNode() {return this->parentNode;}
std::vector<Node*> getChildNodes(){return this->childNodes;}
void addChildNode(Node* node) {
childNodes.push_back(node);
};
void removeNode();
private:
std::vector<Node*> childNodes;
Node* parentNode = nullptr;
};
class Cube : public Node
{
public:
Cube() {};
};
void addCubes(){
Cube mainCube;
for(int i = 0; i < 10; i++){
Cube *c = new Cube();
mainCube.addChildNode(c);
}
}
It is preferred to use smart pointers to manage memory, and the code is more elegant and easier to read, and it makes it harder to make mistakes :-).
#include <memory>
class Node
{
public:
Node() {};
virtual ~Node() {};
void setParentNode(Node* parent) {this->parentNode = parent;}
Node* getParentNode() {return this->parentNode;}
std::vector<std::shared_ptr<Node>>& getChildNodes(){return this->childNodes;}
void addChildNode(std::shared_ptr<Node> node) {
childNodes.push_back(std::move(node));
};
void removeNode();
private:
// childNodes own elements in it, they will be deleted automatically.
std::vector<std::shared_ptr<Node>> childNodes;
Node* parentNode = nullptr;
};
class Cube: public Node
{
public:
Cube() {};
};
void addCubes(){
Cube mainCube;
for(int i = 0; i < 10; i++){
auto c = std::make_unique<Cube>();
mainCube.addChildNode(std::move(c));
}
}
Assuming that you have shared the entire implementation of your code. You have not defined the body of the function addChildNode, similar to setParentNode etc. You need to do something like childNodes.push_back(node); inside that function.
Note: It is also necessary that you pass the input to addChildNode as shown in the answer by #user93353. Also, define childNodes as std::vector<Node *> in order to avoid object slicing.
Node Class
As part of the tree implementation you should implement a Node class. Each node should contain a Customer object, pointers to left and right children and (optionally) the parent.
So, currently I have a customer class such that:
class Customer {
public:
Customer(void);
Customer(string,char,int);
};
In my node class how can I create a customer object while linking the two files?
Do I just include the following in my node header file?
#include "Customer.h"
class Node {
public:
//Customer class
class Customer {
public:
Customer(void);
Customer(string,char,int);
}
Node(void); //default constructor
Node(string,char,int); //Node constructor with customer details
Node* left;
Node* right;
Node* parent;
};
In the node.cpp file to pass in the values to the node:
//Constructor
Node::Node(string x, char y, int z) {
lastName = x;
firstInitial = y;
balance = z;
}
How do I pass the values of the customer object to the node construct?
Do I just include the following in my node header file?
No. You just use an object of type Customer in Node.
#include "Customer.h"
class Node {
public:
Node(void); //default constructor
Node(string,char,int); //Node constructor with customer details
Node* left;
Node* right;
Node* parent;
Customer customer;
};
Implement the constructors as:
Node::Node() : left(nullptr),
right(nullptr),
parent(nullptr),
customer() {}
Node::Node(string x, char y, int z) : left(nullptr),
right(nullptr),
parent(nullptr),
customer(x, y, z) {}
You simply include the header as you did and then declare a Customer object in your Node class (private / protected / public, as you wish). When you'll declare a Node object, the first thing that is constructed are the objects inside your class, and only then the class itself. So, if you would have a coutin both constructors with the class name, when you'll be declaring a Node object, you'll see:
Customer's constructor
Node's constructor
Also, if you want to specify how to construct the Customer object inside the Node constructor, you can use the list of initialization
class Node
{
public:
Customer obj;
Node(string,char,int);
}
and then define the Node constructor like this in your .cpp file:
Node :: Node(string x,char y,int z) : obj(x, y, z) {
}
this is just an example. You can use static values when initializing obj or you could also get more parameters for Node constructor and so on.
I have a class which is a node of a tree, called Node. I need to create a DerivedNode class type which has some extra functionality. The problem is that Node has a vector of Node* as a member variable, so when DerivedNode inherits from Node, it inherits this vector. I've created a basic example showing the issue:
#include <iostream>
#include <vector>
class Node {
public:
Node(int value_) : value(value_) {}
int foo() { return value; }
virtual void add(Node* new_node) {
children.push_back(new_node);
}
protected:
std::vector<Node*> children;
int value;
};
class DerivedNode : public Node {
public:
DerivedNode(int value_) : Node(value_) {}
int bar() { return value*2; }
// Ensures we only add children of type DynamicNode*
virtual void add(DerivedNode* new_node) {
children.push_back(new_node);
}
void print() {
for (size_t i = 0; i < children.size(); ++i) {
std::cout << dynamic_cast<DerivedNode*>(children[i])->bar() << std::endl;
}
}
};
int main() {
DerivedNode* child_a = new DerivedNode(5);
DerivedNode* child_b = new DerivedNode(6);
DerivedNode parent(1);
parent.add(child_a);
parent.add(child_b);
parent.print();
delete child_a;
delete child_b;
}
My question is, how can I do this without the dynamic_cast? My actual code is far more complex which means that there are dynamic casts everywhere.
First add function in derived class is totally useless, it does not override add function from base class, it overloads it. In such way you still can add Node*'s to the derived class. To prevent this you should override add(Node*) as private.
If you does not like dynamic cast, you may use static cast instead
or
You may have virtual bar in base class that does not do anything
or
you can cast the vector itself (the whole thing) and assign to reference or pointer to std::vector DerivedNode*
You can't have it both ways. You either have IS-A principle reflected in the design of your classess, or you don't. If DerivedNode is Node, than the vector of Nodes should be indistinguishable from vector of DerivedNodes - and no casts are neccessary. If this can not be achieved, that you simply can not use vector of base pointers.
Any dynamic_cast in production code for me is a hard block for any review, as it clearly violates the basic design principles.
For the implementation of linked list which is better
Using structure
#include <iostream>
using namespace std;
struct Node {
int data;
Node* next;
};
Using class
class ListNodeClass
{
private:
ItemType Info;
ListNodeClass * Next;
public:
ListNodeClass(const ItemType & Item, ListNodeClass * NextPtr = NULL):
Info(Item), Next(NextPtr)
{
};
void GetInfo(ItemType & TheInfo) const;
friend class ListClass;
};
typedef ListNodeClass * ListNodePtr;
Or is their any better way for doing linked list in C++ ?
The only one thing which class and struct makes differ in C++ is the default interface. if you write:
struct MyStruct
{
int a;
}
and:
class MyClass
{
int a;
}
the only one difference is a field in both them. In MyStruct field a is public and in MyClass field a is private. Of course you can manipulate them using public and private keywords in structs and classes both.
If you are programming in C++ you should use classes.
A linked list is one thing, its nodes are another thing. The nodes are part of the implementation of the list. They should not be visible in the interface of a list so their form doesn't really matter. I would do this
class List
{
private:
struct Node
{
int data;
Node* next;
};
public:
...
};
I'm interested in doing something like the following to adhere to a Null Object design pattern and to avoid prolific NULL tests:
class Node;
Node* NullNode;
class Node {
public:
Node(Node *l=NullNode, Node *r=NullNode) : left(l), right(r) {};
private:
Node *left, *right;
};
NullNode = new Node();
Of course, as written, NullNode has different memory locations before and after the Node class declaration. You could do this without the forward declaration, if you didn't want to have default arguments (i.e., remove Node *r=NullNode).
Another option would use some inheritence: make a parent class (Node) with two children (NullNode and FullNode). Then the node example above would be the code for FullNode and the NullNode in the code above would be of type NullNode inheriting from Node. I hate solving simple problems by appeals to inheritence.
So, the question is: how do you apply Null Object patterns to recursive data structures (classes) with default arguments (which are instances of that same class!) in C++?
Use extern:
extern Node* NullNode;
...
Node* NullNode = new Node();
Better yet, make it a static member:
class Node {
public:
static Node* Null;
Node(Node *l=Null, Node *r=Null) : left(l), right(r) {};
private:
Node *left, *right;
};
Node* Node::Null = new Node();
That said, in both existing code, and amendments above, you leak an instance of Node. You could use auto_ptr, but that would be dangerous because of uncertain order of destruction of globals and statics (a destructor of some global may need Node::Null, and it may or may not be already gone by then).
I've actually implemented a recursive tree (for JSON, etc.) doing something like this. Basically, your base class becomes the "NULL" implementation, and its interface is the union of all interfaces for the derived. You then have derived classes that implement the pieces- "DataNode" implements data getters and setters, etc.
That way, you can program to the base class interface and save yourself A LOT of pain. You set up the base implementation to do all the boilerplate logic for you, e.g.
class Node {
public:
Node() {}
virtual ~Node() {}
virtual string OutputAsINI() const { return ""; }
};
class DataNode {
private:
string myName;
string myData;
public:
DataNode(const string& name, const string& val);
~DataNode() {}
string OutputAsINI() const { string out = myName + " = " + myData; return out; }
};
This way I don't have to test anything- I just blindly call "OutputAsINI()". Similar logic for your whole interface will make most of the null tests go away.
Invert the hierarchy. Put the null node at the base:
class Node {
public:
Node() {}
virtual void visit() const {}
};
Then specialize as needed:
template<typename T>
class DataNode : public Node {
public:
DataNode(T x, const Node* l=&Null, const Node* r=&Null)
: left(l), right(r), data(x) {}
virtual void visit() const {
left->visit();
std::cout << data << std::endl;
right->visit();
}
private:
const Node *left, *right;
T data;
static const Node Null;
};
template<typename T>
const Node DataNode<T>::Null = Node();
Sample usage:
int main()
{
DataNode<char> a('A', new DataNode<char>('B'),
new DataNode<char>('C'));
a.visit();
return 0;
}
Output:
$ ./node
B
A
C