Chapter 17 : Series On Design Patterns – Strategy Pattern

Key Words: Strategy design pattern

Topics at a glance:

  • Let us strategize before action
  • Document Editor App is now version 4.0!

Strategy design pattern

In this chapter we will see another behavioral pattern called the strategy pattern. Strategy helps you to encapsulate algorithms inside object instances of a base-class-derived-class system. It exposes only a minimal interface to configure/rather select the algorithm required. This portion is called selecting the strategy. Once the user selects the strategy, the algorithm applicable to that context will run and gives the result. User will code to a base class instance without even knowing which derived class instance is implementing the algorithm. Varying implementations of a family of algorithms will be encapsulated within each of the derived classes. Base class delegates specific implementation of algorithms to it’s derived classes.

For explaining strategy, I am taking our Document Application as an example. Till version 3.0, our Document application is just an application framework with basic elements for user interaction such as menus and menu items. It does not actually read or write any files. So, let us make this document editor a real document editor which can open real files for reading or writing.

Let us see the role of strategy design pattern in this context. When User selects an option to edit the document, he will be prompted with the basic formatting options such as ‘width’ of the document and ‘alignment‘ type. User can select align to ‘left’ or ‘right’. Now, the Document class can use these user inputs to select the formatting strategy and delegates this to an object instance of a specific formatter sub-class. Strategy pattern is used to design, the formatter class. Formatter is our base class. Formatter can read the lines from user/console. For aligning purposes, it has to seek help of its derived class instances. i.e. Formatter class only declares a pure virtual align() member function. Derived classes ‘align_left_formatter’ and ‘align_right_formatter’ implements the actual algorithm (strategy) for left and right alignment, respectively.

That is the philosophy of strategy pattern. i.e. to encapsulate a family of (related) algorithm implementations in derived classes.

The client will program to an abstract base class and select the algorithm or the way the client is expecting the result. Here, client program is the Document class itself which handles the files.

The client does the following:

1. It opens the file through a fstream object based on the user inputs.

2. It selects a strategy based on user inputs.

3. It creates an object instance of a specific formatter sub-class based upon the selected strategy.

4. It then uses this object instance to make the necessary formatting on the open file.

That said, formatter does not take care of opening and closing files. It just expects the client to give a valid file handle for a file stream opened in the appropriate mode (read, write, append, truncate etc.).

NOTE : Use manipulators defined in ‘iomanip‘ header files for opening the file stream in the required mode.

Let us now see our Strategy Pattern i.e. formatter class in action:

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#include <iostream>
#include <string>
#include <memory>
#include <vector>
#include <fstream>
#include <iomanip>

using namespace std;

class formatter
{
public:
    virtual ~formatter(){}
    
    void format(fstream &output_file)
    {
        string line;
        
        cout << "Start entering your text" << endl;
        cout << "Enter 'END' for ending the session" << endl;
        
        while(output_file)
        {
            getline(cin, line);
            
            if(line.compare("END") == 0)
            {
                break;
            }
            
            align(line);
            
            output_file << line << endl;
            line[0] = '\0';
        }
    }
private:
    virtual void align(string &) = 0;
};

class align_right_formatter : public formatter
{
private:
    int width_;
public:
    align_right_formatter(int width = 20) : width_{width}
    {
        
    }
private:
    void align(string & line) override
    {
        int len = line.length();
        const string space = " ";
        const string new_line = "\n";
        
        vector <string> line_lets;
        
        int rem_len = len;
        int pos = 0;
        int count = 1;
        int offset = 0;
        int org = 0;
        
        if(rem_len <= width_)
        {
            line_lets.push_back(line);
        }
        
        while(rem_len > width_)
        {
            offset = width_ * count;
            while(true)
            {
                if(line[offset] == ' ')
                {
                    if(offset - org <= width_)
                    {
                        break;
                    }
                    else if (offset - 1 < 0) 
                    {
                        break;
                    }
                    
                    // The string is still lengthy
                    // can't break here 
                    
                }
                --offset;
            }
            
            line.insert(offset, new_line);
            len = line.length();
            rem_len = (len - offset) + 1;
            org = len - rem_len;
            count++;
        }
        
        stringstream ss(line);
        string tmp;
        
        while(getline(ss, tmp, '\n'))
        {
            line_lets.push_back(tmp);
        }
        
        line.clear();
        
        for(auto str : line_lets)
        {
            int limit = width_ - str.length() - 1;
            
            for(int i = 0; i < limit; ++i)
            {
                str.insert(i, space);
            }
            
            line.append(str);
            line.append(new_line);
        }
        
        return;
    }

};

class align_left_formatter : public formatter
{
private:
    int width_;
public:
    align_left_formatter(int width = 20) : width_{width}
    {
        // do nothing
    }
private:
    void align(string & line) override
    {
        int count = 1;
        int offset = 0;
        int len = line.length();
        const string new_line{"\n"};
        int rem_len = len;
        int org = 0;
        
        while(rem_len >= width_)
        {
            offset = width_ * count;
            
            while(true)
            {
                if(line[offset] == ' ')
                {
                    if(offset - org <= width_)
                    {
                        break;
                    }
                    else if (offset - 1 < 0) 
                    {
                        break;
                    }
                    
                    // The string is still lengthy
                    // can't break here 
                    
                }
                --offset;
            }
            
            line.insert(offset + 1, new_line);
            len = line.length();
            rem_len = (len - offset) + 1;
            org = len - rem_len;
            count++;
        }
        
        return;
    }

};

class Command
{
public:
    virtual ~Command(){}
    virtual void execute() = 0;
};

class Selectable
{
public:
    virtual ~Selectable(){}
    virtual string get_name() = 0;
    virtual void select() = 0;    
};

class Menu_Item : public Selectable
{
private:
    string name_;
    unique_ptr<Command> command_object_;
public:
    Menu_Item(const char* const name) : 
        name_{name},
        command_object_{nullptr}
    {
    }
    
    ~Menu_Item()
    {
        cout << "Menu_Item : " << name_ << " destroyed" << endl;
        command_object_.reset();        
    }
    
    void add_command_object(Command *command_object)
    {
        command_object_.reset(command_object);
    }
    
    void select() override
    {
        click();
    }
    
    int click()
    {
        int status = -1;
        if(command_object_ != nullptr)
        {
            command_object_->execute();
            status = 0;
        }
        
        return status;
    }
    
    string get_name()
    {
        return name_;
    }
};

class Menu : public Selectable
{
private:
    string name_;
    vector<unique_ptr<Selectable>> menu_items;
public:
    Menu(const char* const name): 
        name_{name}
    {
        
    }
    
    ~Menu()
    {
        cout << "Menu " << name_ << " destroyed" << endl;
        for (auto &item : menu_items)
        {
            item.reset();
        }
    }
    
    void add_menu_item(unique_ptr<Selectable> menu_item)
    {
        menu_items.push_back(std::move(menu_item));
    }
    
    void select() override 
    {
        hover();
    }
    
    void hover()
    {
        int choice;
        int count = 0;
        
        for (auto &item : menu_items)
        {
            cout << item->get_name() << " : " << count++ << endl;
        }
        
        cout << "Enter your choice : ";
        cin >> choice;
        
        if(   (choice >= 0) 
            &&(choice <= menu_items.size()) )
        {
            menu_items[choice]->select();
            return;
        }
        
    }
    
    string get_name()
    {
        return name_;
    }
    
};

class Subject;

enum class doc_states{created = 0, opened, closed, changed, moved};

class Observer
{
public:
    ~Observer(){}
    virtual void update(doc_states current_state) = 0;
};

class Subject
{
public:
    ~Subject(){}
    virtual void notify() = 0;
    virtual void attach(Observer *this_observer) = 0;
    virtual void dettach(Observer *this_observer) = 0;
};

class Document : public Subject 
{
private:
    string name_;
    doc_states current_state;
    vector<Observer*> observers; 
    fstream file;
public:
    typedef void (Document::*doc_function)();
    Document(): current_state{doc_states::created}
    { 
        /* do nothing */ 
    }
    
    ~Document()
    {
        if(   (current_state != doc_states::closed)
            &&(current_state != doc_states::created) )
        {
            // i.e. a document is open. So close it.
            close();
        }
    }
    
    void open_read()
    {
        cout << "Enter the file name to open for reading" << endl;
        cin >> name_;
        
        
        file.open(name_, ios::in); // read only mode 
        
        if(file)
        {
            current_state = doc_states::opened;
            cout << "Document : " << name_ << " is open for reading!\n" << endl;
            string line;
            
            while(file >> line)
            {
                cout << line << endl;            
            }
            
            cout << "\nEnd of File!" << endl;
            
        }
    }
    
    void open_write()
    {
        cout << "Enter the file name to open for writing" << endl;
        cin >> name_;
        
        file.open(name_, ios::app); // write in append mode  
        formatter *frmtr = nullptr;
        
        if(file)
        {
            file << "\n" << endl;
            current_state = doc_states::opened;
            cout << "Document : " << name_ << " is open for writing" << endl;
            // start writing
            cout << "Select alignment : Left(0), Right(1)" << endl;
            
            int alignment_selected = 0;
            int width = 0;
            
            cin >> alignment_selected;
            
            cout << "Select width : " << endl;
            cin >> width;
            if( (width < 0) || (width > 30) )
            {
                width = 20;
            }
                
            if(alignment_selected == 1)
            {
                // right 
                frmtr = new align_right_formatter(width);
            }
            else
            {
                // left 
                frmtr = new align_left_formatter(width);
            }
            
            if(frmtr != nullptr)
            {
                frmtr->format(file);
                delete frmtr;
            }
            
        }
    }
    
    void close()
    {
       if(   (current_state != doc_states::closed)
            &&(current_state != doc_states::created) )
        {
            file.close();
            current_state = doc_states::closed;
            cout << "Document : " << name_ << " is closed" << endl;
            name_.clear();
            // notify the subscribed observers 
            notify();
        }
        else
        {
            cout << "No documents are open" << endl;
        }
    }
    
    //void copy(int start, int end, string &copied_text)
    void copy()
    {
        if(current_state != doc_states::closed)
        {
            cout << "copying..." << endl;
            // copy from start to end to copied_text
            cout << "copy complete" << endl;
        }
        else
        {
            cout << "No documents are open" << endl;
        }
    }
    
    //void paste(int position, string& insert_text)
    void paste()
    {
        if(current_state != doc_states::closed)
        {
            cout << "pasting..." << endl;
            cout << "pasting complete" << endl;
        }
        else
        {
            cout << "No documents are open" << endl;
        }
    }
    
    //void cut(int start, int end, string &extracted_text)
    void cut()
    {
        if(current_state != doc_states::closed)
        {
            cout << "cutting..." << endl;
            // copy from start to end to copied_text
            cout << "cut complete" << endl;
        }
        else
        {
            cout << "No documents are open" << endl;
        }
    }
    
    // support for Observer pattern
    void notify() override
    {
        for( auto &observer : observers )
        {
            observer->update(current_state);
        }
    }
    
    void attach(Observer *this_observer) override
    {
        observers.push_back(this_observer);
    }
    
    void dettach(Observer *this_observer) override
    {
        int position = 0;
        auto begin = observers.begin();
        auto end = observers.end();
        
        for( auto itr = begin; itr < end; ++itr )
        {
            auto obs = *itr;
            if(this_observer == obs)
            {
                observers.erase(itr);
            }
        }
    }
    
};

class Open_Document_Command : public Command
{
private:
    shared_ptr<Document> document_;
    Document::doc_function function;

public:
    Open_Document_Command(shared_ptr<Document> document, Document::doc_function call_back):
        document_{document}, function{call_back}
    {
        
    }
    void execute() override
    {
        (document_.get()->*function)();
    }
};


class Close_Document_Command : public Command
{
private:
    shared_ptr<Document> document_;
    Document::doc_function function;

public:
    Close_Document_Command(shared_ptr<Document> document, Document::doc_function call_back):
        document_{document}, function{call_back}
    {
        
    }
    void execute() override
    {
        (document_.get()->*function)();
    }
};


class Copy_Document_Command : public Command
{
private:
    shared_ptr<Document> document_;
    Document::doc_function function;

public:
    Copy_Document_Command(shared_ptr<Document> document, Document::doc_function call_back):
        document_{document}, function{call_back}
    {
        
    }
    void execute() override
    {
        (document_.get()->*function)();
    }
};

class Paste_Document_Command : public Command
{
private:
    shared_ptr<Document> document_;
    Document::doc_function function;

public:
    Paste_Document_Command(shared_ptr<Document> document, Document::doc_function call_back):
        document_{document}, function{call_back}
    {
        
    }
    void execute() override
    {
        (document_.get()->*function)();
    }
};

class Cut_Document_Command : public Command
{
private:
    shared_ptr<Document> document_;
    Document::doc_function function;

public:
    Cut_Document_Command(shared_ptr<Document> document, Document::doc_function call_back):
        document_{document}, function{call_back}
    {
        
    }
    
    void execute() override
    {
        (document_.get()->*function)();
    }
};


class Application : public Observer
{
private:
    vector<unique_ptr<Menu>> menus;
    shared_ptr<Document> current_doc;
    enum class signal{app_launched = 0, app_exit, user_interrupt, other, no_trigger};
    signal trigger;
    
    void prompt_to_open_new_document()
    {
        // prompt the user 
        cout << "Open a document for editing" << endl;        
        return;
    }
    
public:
    Application()
    {
        string doc_name;
        trigger = signal::app_launched;
        
        cout << "Document Editor App, Version : 4.0" << endl;
        
        shared_ptr<Document> document = make_shared<Document>();
        current_doc = document;

        // Attach this 'Application' as an Observer of document 
        (document.get())->attach(this);
        
        // add Menus "File" and "Edit"
        unique_ptr<Menu> file_menu = make_unique<Menu>("File");
        unique_ptr<Menu> open_sub_menu = make_unique<Menu>("Open");
        unique_ptr<Menu_Item> open_menu_item_1 = make_unique<Menu_Item>("Open For Read");
        unique_ptr<Menu_Item> open_menu_item_2 = make_unique<Menu_Item>("Open For Write");
        
        
        Open_Document_Command *open_read_cmd_object = new Open_Document_Command(document, &Document::open_read);
        Open_Document_Command *open_write_cmd_object = new Open_Document_Command(document, &Document::open_write);
        open_menu_item_1->add_command_object(open_read_cmd_object);
        open_menu_item_2->add_command_object(open_write_cmd_object);
        
        open_sub_menu->add_menu_item(std::move(open_menu_item_1));
        open_sub_menu->add_menu_item(std::move(open_menu_item_2));
        
        file_menu->add_menu_item(std::move(open_sub_menu));
        
        unique_ptr<Menu_Item> close_item = make_unique<Menu_Item> ("Close");
        Close_Document_Command *close_cmd_object = new Close_Document_Command(document, &Document::close);
        close_item->add_command_object(close_cmd_object);
        
        file_menu->add_menu_item(std::move(close_item));
        
        unique_ptr<Menu> edit_menu = make_unique<Menu>("Edit");
        unique_ptr<Menu_Item> copy_item = make_unique<Menu_Item>("Copy");
        Copy_Document_Command *copy_cmd_object = new Copy_Document_Command(document, &Document::copy);
        copy_item->add_command_object(copy_cmd_object);
        
        edit_menu->add_menu_item(std::move(copy_item));
        
        unique_ptr<Menu_Item> paste_item = make_unique<Menu_Item>("Paste");
        Paste_Document_Command *paste_cmd_object = new Paste_Document_Command(document, &Document::paste);
        paste_item->add_command_object(paste_cmd_object);
        
        edit_menu->add_menu_item(std::move(paste_item));
        
        unique_ptr<Menu_Item> cut_item = make_unique<Menu_Item>("Cut");
        Cut_Document_Command *cut_cmd_object = new Cut_Document_Command(document, &Document::cut);
        cut_item->add_command_object(cut_cmd_object);
        
        edit_menu->add_menu_item(std::move(cut_item));
        
        menus.push_back(std::move(file_menu));
        menus.push_back(std::move(edit_menu));
    }
    
    int select_menu()
    {
        int choice = 0;
        int end_app = 0;
        cout << "Enter your choice : " << endl;
        for(auto &menu : menus)
        {
            cout << menu->get_name() << " : " << choice++ << endl;
        }
        end_app = choice;
        cout << "Exit : " << end_app << endl;
        
        cin >> choice;
        
        if(  (choice >= 0)
           &&(choice < menus.size()))
        {
            menus[choice]->select();
        }
        else if(choice == end_app)
        {
            // set the trigger 
            trigger = signal::app_exit;
            end_application();
            return 1;
        }
        
        return 0;
    }
    // To support Observer pattern
    void update(doc_states current_state)
    {
        cout << "Application : document close() detected" << endl;
        switch(current_state)
        {
            case (doc_states::closed):
            {
                // open new document for editing by prompting the user
                if(trigger != signal::app_exit)
                {
                    prompt_to_open_new_document();
                }
                break;
            }
            default:
            {
                // do nothing
                break;
            }
        }
        return;
    }
    
    void end_application()
    {
        // do clean up
        // release back the memory resources to free store 
        for(auto &menu : menus)
        {
            menu.reset();
        }
        // close the current_doc
        current_doc.reset();
        
        cout << "Application exiting..." << endl;
        return;
    }
    
    ~Application()
    {
        cout << "Inside Application destructor" << endl;
    }
    
};

int main()
{
    Application new_app;
    int end_app = 0;
    
    while(end_app == 0)
    {
        end_app = new_app.select_menu();
    }
    
    cout << "Main Ends" << endl; 
}

The algorithms implemented for Left and right alignment make sure of two things:

1. The text aligns properly to either left or right based upon the selected strategy.

2. Without breaking in between a word (i.e. it breaks the text into next line only at spaces), it makes sure the portion of text is written to the file within the stipulated line width.

Want to see the result?

Document Editor App, Version : 4.0
Enter your choice :
File : 0
Edit : 1
Exit : 2
0
Open : 0
Close : 1
Enter your choice : 0
Open For Read : 0
Open For Write : 1
Enter your choice : 1
Enter the file name to open for writing
Hai.txt
Document : Hai.txt is open for writing
Select alignment : Left(0), Right(1)
0
Select width :
20
Start entering your text
Enter 'END' for ending the session
Hai
Hello
How are you? Hope
you are fine!
Good seeing you
:)
END
Enter your choice :
File : 0
Edit : 1
Exit : 2
0
Open : 0
Close : 1
Enter your choice : 1
Document : Hai.txt is closed
Application : document close() detected
Open a document for editing
Enter your choice :
File : 0
Edit : 1
Exit : 2
0
Open : 0
Close : 1
Enter your choice : 0
Open For Read : 0
Open For Write : 1
Enter your choice : 0
Enter the file name to open for reading
Hai.txt
Document : Hai.txt is open for reading!
Hai
Hello
How
are
you?
Hope
you
are
fine!
Good
seeing
you
:)
End of File!
Enter your choice :
File : 0
Edit : 1
Exit : 2
0
Open : 0
Close : 1
Enter your choice : 1
Document : Hai.txt is closed
Application : document close() detected
Open a document for editing
Enter your choice :
File : 0
Edit : 1
Exit : 2
0
Open : 0
Close : 1
Enter your choice : 0
Open For Read : 0
Open For Write : 1
Enter your choice : 1
Enter the file name to open for writing
Hello.txt
Document : Hello.txt is open for writing
Select alignment : Left(0), Right(1)
0
Select width :
20
Start entering your text
Enter 'END' for ending the session
Hello How are you ? I am fine, Thanks. How are you? Am fine too. Heard that you are working on a book on the philosophy of C and C++ ? How it's going ? It's going well and keeps me engaged. Okay good to hear that. All the best for your endeavor!!!
END
Enter your choice :
File : 0
Edit : 1
Exit : 2
0
Open : 0
Close : 1
Enter your choice : 1
Document : Hello.txt is closed
Application : document close() detected
Open a document for editing
Enter your choice :
File : 0
Edit : 1
Exit : 2
0
Open : 0
Close : 1
Enter your choice : 0
Open For Read : 0
Open For Write : 1
Enter your choice : 1
Enter the file name to open for writing
Hello.txt
Document : Hello.txt is open for writing
Select alignment : Left(0), Right(1)
1
Select width :
20
Start entering your text
Enter 'END' for ending the session
I have been pondering about starting a blog on my understanding of the popular programming languages C and C++ for quite some time. I am not a seasoned writer. To be honest with you, I am still a little skeptical to foray into this world of blogging! I always try to understand a thing to it's very core by analyzing it, understand how it works and why it works the way it does. And once I understand these, I will articulate or rather try to teach them to my colleagues and peers. I think, I have got some good experience in that regard.
END
Enter your choice :
File : 0
Edit : 1
Exit : 2
0
Open : 0
Close : 1
Enter your choice : 1
Document : Hello.txt is closed
Application : document close() detected
Open a document for editing
Enter your choice :
File : 0
Edit : 1
Exit : 2
2
Menu File destroyed
Menu Open destroyed
Menu_Item : Open For Read destroyed
Menu_Item : Open For Write destroyed
Menu_Item : Close destroyed
Menu Edit destroyed
Menu_Item : Copy destroyed
Menu_Item : Paste destroyed
Menu_Item : Cut destroyed
Application exiting...
Main Ends
Inside Application destructor

Let us see “Hai.txt” ( left aligned and width 20 characters per line )

Now, Let us see the left aligned portion of text saved in “Hello.txt ( width 20 characters per line selected )

Now, see the right aligned portion of text in “Hello.txt( width 20 characters per line selected )

Note: The image above is truncated at line 49 due to space limitations. Please visit my GitHub page to see the full text in ‘Hello.txt’ file

Document class and it’s open_read and open_write member functions is the client here. Through the user inputs it selects the strategy and decides either to create instance of ‘align_left_formatter’ or ‘align_right_formatter’. Client, invokes the format member function and leave the rest to the strategy pattern.

We don’t have to use std::unique_ptr while creating formatter instances as it’s scope is limited to the functions open_read and open_write. Once the format function returns, the instance is deleted, and resources are freed.

Enjoyed the chapter? Let me know in the comments below. Thanks 🙂

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