Composite pattern

Definition

Combine objects into a tree structure to represent “ part - whole ” Hierarchical structure . The composition mode makes the use of single object and composite object consistent .

motivation

When you find that the requirements are part and whole hierarchies , And you want users to ignore the difference between composite objects and individual objects , When using all objects in the composite object structure in a unified way , It's time to think about using a combination pattern .

UML Class diagram

• Component Abstract build roles , Define common methods and attributes that are exclusive to the portfolio , You can define some default behaviors or attributes
• Leaf Leaf structure , There are no branches , Is the smallest traversal unit
• Composite branch structure , It is a tree structure formed by combining branch nodes and leaf nodes

advantage

High level modules are easy to call , All nodes are Component , There is no difference between the part and the whole for the caller , The high level doesn't have to care whether they are dealing with a single object or the entire composite structure .
Nodes are increased freely , Using the composite pattern , If you want to add branch nodes , The nodes of leaves are easy , Just find the parent node , Comply with opening and closing principle .

shortcoming

Definition of branch and leaf nodes when used , The direct use is the implementation class . Against the principle of Dependence Inversion .

Use scenarios

Maintain and display a part - The whole relationship scenario ：eg Tree menu , File and folder management .
Scenarios that can separate some modules and functions from a whole .

matters needing attention

As long as it's a tree structure , Consider the combination pattern , Just reflect this part of your relationship , Just use the composite pattern
Composite One problem in the implementation of pattern is to provide for child nodes （ Leaf） Management strategy , What we use here is STL Medium vector, Other implementations can be provided , Such as arrays 、 Linked list 、 Hash Table, etc .

Example

With a large company as the demand background , Organize our code . A Beijing head office , There are two branches in Zhengzhou and Xi'an . Then every company has its own human resources department, whether it is the head office or the branch office ,IT Department and marketing department .

// component.h
#ifndef COMPONENT_H
#define COMPONENT_H
#include <QList>

class Component
{
    
public:
    Component(QString name);
    virtual ~Component();
    virtual void add(Component* com, int depth = 0) = 0;
    virtual void remove(Component* com) = 0;
    virtual void display() = 0;
    virtual void LineOfDuty() = 0;

    QString name();
    void setDepth(int depth);
    int depth();
private:
    QString         m_Name;
    int                 m_Depth;
};

class ConcreteCompany : public Component
{
    
public:
    ConcreteCompany(QString name);
    virtual ~ConcreteCompany();
    virtual void add(Component* com, int depth = 0);
    virtual void remove(Component* com);
    virtual void display();
    virtual void LineOfDuty();
private:
    QList<Component*> m_ChildCom;
};

class HumanResource : public Component
{
    
public:
    HumanResource(QString name);
    virtual ~HumanResource();
    virtual void add(Component* com, int depth = 0);
    virtual void remove(Component* com);
     virtual void display();
    virtual void LineOfDuty();
};

class IT : public Component
{
    
public:
    IT(QString name);
    virtual ~IT();
    virtual void add(Component* com, int depth = 0);
    virtual void remove(Component* com);
     virtual void display();
    virtual void LineOfDuty();
};

class Marketing : public Component
{
    
public:
    Marketing(QString name);
    virtual ~Marketing();
    virtual void add(Component* com, int depth = 0);
    virtual void remove(Component* com);
     virtual void display();
    virtual void LineOfDuty();
};
#endif // COMPONENT_H

// component.cpp
/************************************ * @brief :  Set the story ： There is a king pesticide National Corporation in Shenzhen , Now I want to open offices all over the country  * 1. Beijing Office -  Recruitment department , R & D department , The Marketing Department  * 2. Zhengzhou office -  Recruitment department , R & D department , The Marketing Department  * 3. Xi'an Office -  Recruitment department , R & D department , The Marketing Department  * and so on... * @author : wzx * @date : 2020-05-11 * @project : Composite *************************************/
#include <QDebug>
#include "component.h"

#define DELETEOBJECT(x) if(x) {
       delete x; x = nullptr; }

Component::Component(QString name):m_Name(name) {
    }

Component::~Component(){
    }

QString Component::name()
{
    
    return m_Name;
}

void Component::setDepth(int depth)
{
    
    m_Depth = depth;
}

int Component::depth()
{
    
    return m_Depth;
}

ConcreteCompany::ConcreteCompany(QString name)
    : Component(name)
{
    

}

ConcreteCompany::~ConcreteCompany()
{
    
    for(auto com : m_ChildCom)
    {
    
        DELETEOBJECT(com);
    }
    m_ChildCom.clear();
}

void ConcreteCompany::add(Component* com, int depth)
{
    
    com->setDepth(depth);
    m_ChildCom.append(com);
}

void ConcreteCompany:: remove(Component* com)
{
    
    m_ChildCom.removeOne(com);
}

void ConcreteCompany::display()
{
    
    QString str;
    for(int n = 0; n < depth(); ++n)
        str += "--";
    qDebug() << qPrintable(str)  <<  (name());
    for(auto com : m_ChildCom)
    {
    
        com->display();
    }
}

void ConcreteCompany::LineOfDuty()
{
    

}

HumanResource::HumanResource(QString name)
    : Component(name)
{
    

}

HumanResource::~HumanResource()
{
    

}

void HumanResource::add(Component* com, int depth)
{
    
    com->setDepth(depth);
}

void HumanResource::remove(Component* com)
{
    

}

void HumanResource::display()
{
    
    QString str;
    for(int n = 0; n < depth(); ++n)
        str += "--";
    qDebug() << qPrintable(str)  <<  (name());
}

void HumanResource::LineOfDuty()
{
    
    qDebug() << " Human Resources Department , Responsible for recruitment ";
}

IT::IT(QString name)
    : Component(name)
{
    

}

IT::~IT()
{
    

}

void IT::add(Component* com, int depth)
{
    
    com->setDepth(depth);
}

void IT::remove(Component* com)
{
    

}

void IT::display()
{
    
    QString str;
    for(int n = 0; n < depth(); ++n)
        str += "--";
    qDebug() << qPrintable(str)  <<  (name());
}

void IT::LineOfDuty()
{
    
    qDebug() << "IT department , Responsible for writing code ";
}

Marketing::Marketing(QString name)
    : Component(name)
{
    

}

Marketing::~Marketing()
{
    

}

void Marketing::add(Component* com, int depth)
{
    
    com->setDepth(depth);
}

void Marketing::remove(Component* com)
{
    

}

void Marketing::display()
{
    
    QString str;
    for(int n = 0; n < depth(); ++n)
        str += "--";
    qDebug() << qPrintable(str)  <<  name();
}

void Marketing::LineOfDuty()
{
    
    qDebug() << " Marketing Department , Responsible for marketing ";
}

// main.cpp

#include <QCoreApplication>
#include <QDebug>
#include "component.h"

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

    Component* root = new ConcreteCompany(" Beijing headquarters ");
    root->setDepth(0);
    root->add(new HumanResource(" Human Resources Department "), 1);
    root->add(new IT("IT department "), 1);
    root->add(new Marketing(" Marketing Department "), 1);

    Component* zz = new ConcreteCompany(" Zhengzhou office ");
    zz->add(new HumanResource(" Human Resources Department "), 2);
    zz->add(new IT("IT department "), 2);
    zz->add(new Marketing(" Marketing Department "), 2);
    root->add(zz, 1);

    Component* xa = new ConcreteCompany(" Xi'an Office ");
    xa->add(new HumanResource(" Human Resources Department "), 2);
    xa->add(new IT("IT department "), 2);
    xa->add(new Marketing(" Marketing Department "), 2);
    root->add(xa, 1);

    root->display();
    return a.exec();
}

Running results

" Beijing headquarters "

-- " Human Resources Department "

-- "IT department "

-- " Marketing Department "

-- " Zhengzhou office "

---- " Human Resources Department "

---- "IT department "

---- " Marketing Department "

-- " Xi'an Office "

---- " Human Resources Department "

---- "IT department "

---- " Marketing Department "

Example 2

Arithmetic expressions include operands 、 Operator and another operands , among , Another operand can also be an operand 、 Operator and another operands .