实现state机器时面临double free

Question

对于一项任务，我计划在 C++ 中实施一台 state 机器。我试图保留一个上下文 object，它提供以下内容：

1. a state_ object 跟踪当前 state
2. 一个 TransitionTo() 方法，以方便转换到新的 state。

当我编写示例实现并对其进行测试时，我遇到了 double free 错误。

我需要社区的帮助来指导我了解可能出现的问题。 非常感谢。

#include <iostream>
#include <string>

class State;

/* Context class and method implementation */
class Context
{
    State* state_;
public:
    explicit Context(State* state);
    void TransitionTo(State* newState);
};

Context::Context (State* state): state_ (nullptr) 
{
    this->TransitionTo(state);
}

void Context::TransitionTo(State* newState)
{
    std::cout <<"Requesting state transition to " << newState->stateName<<"\n";
    std::string previous_state_name = "None";
    if (this->state_ != nullptr)
    {
        previous_state_name = this->state_->stateName;
        delete this->state_;
    }
    this->state_ = newState;
    std::cout << "State changed from "<< previous_state_name << " to "<< this->state_->stateName << "\n";
    this->state_->set_context(this);
}



/* State class and method implementation */
class State
{
    protected:
        Context* context_;
    public:
        std::string stateName;
        void set_context(Context* newContext);
        virtual ~State();
        
};

State::~State()
{
    std::cout << stateName <<" state deleted \n";
    delete context_ ;
}

void State::set_context(Context *newContext) 
{
    this->context_ = newContext;
}


/* Declaring different states which are derived from State */
class HappyState : public State
{
public:
    HappyState();
};

HappyState::HappyState()
{
    stateName = "Happy";
}

class NeutralState : public State
{
public:
    NeutralState();
};

NeutralState::NeutralState()
{
    stateName = "Neutral";
}

class SadState : public State
{
public:
    SadState();
};

SadState::SadState()
{
    stateName = "Sad";
}

/* Test the implementation */
int main()
{
    Context* ctx  = new Context(( new NeutralState()));
    
    ctx->TransitionTo(new HappyState());
    
    ctx->TransitionTo(new SadState());

    return 0;
}

当我运行这段代码时，我得到以下 output: Output 快照

Answer 1

我发现了这个错误，这确实是一个非常愚蠢的错误。

我正在删除 ~State() 中的 context_ ，这删除了我的原始上下文。