Output Quoted Strings Finite State Machine (FSM) in C++ - Topic 29 of Pragmatic Programmer

QuotedStringsFSM.cpp

// Based on general design of this FSM: https://www.aleksandrhovhannisyan.com/blog/implementing-a-finite-state-machine-in-cpp/
/*
 * stateDiagram
    direction LR
    LookForString --> LookForString: *
    LookForString --> InString: ch == ", InitResult()

    InString --> InString: ch == anything else, AddToResult()
    InString --> LookForString: ch == ", OutputResult()
    InString --> CopyNextChar: ch == \, AddToResult()

    CopyNextChar --> InString: ch == *, AddToResult()
 */
 
 #include <iostream>


class StringSearchState;
class StringSearchFSM;

// Define an abstract state class
// we need enter actions, exit actions, as well as what to do when we get something that may change
// the state
class StringSearchState
{
public:
    // Its important that you pass in the thing that HAS the states to each
    // of these functions. These classes are extremely tightly interwoven, so they can affect
    // each other's private states.
    virtual void enter(StringSearchFSM *s) = 0;
    virtual void processChar(StringSearchFSM *s, char inputChar) = 0;
    virtual void exit(StringSearchFSM *s) = 0;
    virtual ~StringSearchState() = default;
};

class InStringState : public StringSearchState
{
public:
    static StringSearchState& GetInstance();

    void enter(StringSearchFSM *s) override;

    void processChar(StringSearchFSM *s, char inputChar) override;

    void exit(StringSearchFSM *s) override;
};

class CopyNextCharState : public StringSearchState
{
public:
    static StringSearchState& GetInstance();

    void enter(StringSearchFSM *s) override;

    void processChar(StringSearchFSM *s, char inputChar) override;

    void exit(StringSearchFSM *s) override;
};

class LookForStringState : public StringSearchState
{
public:
    static StringSearchState& GetInstance();

    void enter(StringSearchFSM *s) override;

    void processChar(StringSearchFSM *s, char inputChar) override;

    void exit(StringSearchFSM *s) override;
};

class StringSearchFSM
{
public:
    void SetState(StringSearchState* newState)
    {
        mState->exit(this);
        mState = newState;
        mState->enter(this);
    };

    [[ nodiscard ]] const StringSearchState& GetCurrentState()
    {
        return *mState;
    }


    void processChar(char c)
    {
        mState->processChar(this, c);
    }

    friend InStringState;
    friend LookForStringState;
    friend CopyNextCharState;
private:

    void InitializeResult()
    {
        mResult.clear();
    };

    void AddToResult(char c)
    {
        mResult += c;
    }

    void OutputResult() const
    {
        std::cout << mResult << "\n";
    }

    StringSearchState* mState { &LookForStringState::GetInstance() };
    std::string mResult {};
};



StringSearchState &CopyNextCharState :: GetInstance()
{
    static CopyNextCharState singleton;
    return singleton;
}

void CopyNextCharState::enter(StringSearchFSM *s)
{

}

void CopyNextCharState::processChar(StringSearchFSM *s, char inputChar)
{
    s->AddToResult(inputChar);
    s->SetState(&InStringState::GetInstance());
}

void CopyNextCharState::exit(StringSearchFSM *s)
{

}
StringSearchState &InStringState :: GetInstance()
{
    static InStringState singleton;
    return singleton;
}

void InStringState::enter(StringSearchFSM *s)
{

}

void InStringState::processChar(StringSearchFSM *s, char inputChar)
{

    if (inputChar == '"')
    {
        s->OutputResult();
        s->SetState(&LookForStringState::GetInstance());
    }
    else if (inputChar == '\\')
    {
        s->AddToResult(inputChar);
        s->SetState(&CopyNextCharState::GetInstance());
    }
    else
    {
        s->AddToResult(inputChar);
    }
}

void InStringState::exit(StringSearchFSM *s)
{

}


StringSearchState &LookForStringState :: GetInstance()
{
    static LookForStringState singleton;
    return singleton;
}

void LookForStringState::enter(StringSearchFSM *s)
{

}

void LookForStringState::processChar(StringSearchFSM *s, char inputChar)
{
    if (inputChar == '"')
    {
        s->InitializeResult();
        s->SetState(&InStringState::GetInstance());
    }
}

void LookForStringState::exit(StringSearchFSM *s)
{
}


int main()
{
    StringSearchFSM machine;
    machine.SetState(&LookForStringState::GetInstance());

    for (auto& ch : R"(Hi there "test string" aloha "second test")")
    {
        machine.processChar(ch);
    }
}