C++ 状态模式实现:指向状态机的指针机制变得无效

在尝试实现"Head First Design Patterns"一书中的简单状态模式示例时，我遇到了一种让我感到奇特的情况。请注意，这个问题不是关于正确实现模式，而是关于理解导致观察到的行为的潜在机制。

机器"Gumball_machine"应该有几种可能的状态(No_quarter_state、Has_quarter_state、Sold_out_state等(，行为在运行时通过虚函数调用委托给它们。这些状态公开继承自抽象基类State。 Gumball_machine有一个std::unique_ptr<State>，State类本身就是一个指向Gumball_machine的原始指针(因为没有所有权被假定(。

当满足某些条件时，就会发生状态转换，它们通过分配新的具体状态类并将所有权转移到Gumball_machine发生。

(我将在本文末尾发布一些代码示例，因为我想先"进入正题"。

有一种情况，在切换状态后的同一函数中，调用另一个函数：

void Has_quarter_state::turn_crank()
{         
     std::cout << "You turned...n";
     machine_->state_ = std::make_unique<Sold_state>(machine_);
     machine_->dispense();                    // Invalid read!
     // This works however (don't forget to comment out the above reallocation):
//     Gumball_machine* ptr{machine_};
//     machine_->state_ = std::make_unique<Sold_state>(machine_);
//     ptr->dispense();
}

machine_是指向Gumball_machine的指针，state_是指向具体状态的std::unique_ptr<State>，Has_quarter_state。

如果我声明临时指针ptr并调用Gumball_machine::dispense()，则没有问题。但是，如果我简单地调用machine_->dispense()，valgrind将显示无效的读取(错误消息将显示在下面(。

这个我真的不明白。 ptr 和 machine_ 应引用同一个Gumball_machine实例，在程序结束之前不应销毁该实例。 Has_quarter_state(或者更确切地说是父类"State"(只有一个没有所有权的原始指针。

现在我想起来了，可能是因为unique_ptr重置会导致释放Has_quarter_state实例占用的内存。这可能意味着任何后续操作，即对Gumball_machine::dispense()的函数调用，都会导致未定义的行为。这个假设正确吗？如果内存地址(&memory_ == &ptr(没有改变，为什么我打电话给ptr->dispense()或machine_->dispense()会有所不同？

我觉得内存管理有一些我仍然不了解的复杂性。希望你能帮我把事情搞清楚。

下面我将发布代码以重现此内容("不正确"版本(和 valgrind 给我的错误消息(使用 --leak-check=full ， --leak-kinds=all (。

代码使用 clang 3.6.0 通过clang++ -std=c++14 -stdlib=libc++编译

现在对于实际代码(大大减少为一个最小示例(：

Gumball_machine.hpp：

#ifndef CLASS_GUMBALL_MACHINE_HPP_
#define CLASS_GUMBALL_MACHINE_HPP_
#include <memory>
class State;
class Gumball_machine
{
     friend class Has_quarter_state;
     friend class Sold_state;
     public:
          Gumball_machine();
          ~Gumball_machine();
          void turn_crank();
     private:
          void dispense();
     private:
          std::unique_ptr<State> state_;
};
#endif

Gumball_machine.cpp：

#include "Gumball_machine.hpp"
#include "Has_quarter_state.hpp"
Gumball_machine::Gumball_machine() : state_{std::make_unique<Has_quarter_state>(this)} {}
Gumball_machine::~Gumball_machine() {}
void Gumball_machine::turn_crank() { state_->turn_crank(); }
void Gumball_machine::dispense() { state_->dispense(); }

状态.hpp：

#ifndef CLASS_STATE_HPP_
#define CLASS_STATE_HPP_
class Gumball_machine;
class State
{
     public:
          explicit State(Gumball_machine* m); 
          virtual ~State();
          virtual void turn_crank() = 0;
          virtual void dispense() = 0;
     protected:
          Gumball_machine* machine_ = nullptr;
};
#endif

州.cpp：

#include "State.hpp"
State::State(Gumball_machine* m) : machine_{m} {}
State::~State() {}

Has_quarter_state.hpp：

#ifndef ClASS_HAS_QUARTER_STATE_HPP_
#define ClASS_HAS_QUARTER_STATE_HPP_
#include "State.hpp"
class Gumball_machine;
class Has_quarter_state : public State
{
     public:
          explicit Has_quarter_state(Gumball_machine*);
          ~Has_quarter_state() override;
          void turn_crank() override;
          void dispense() override;
};
#endif

Has_quarter_state.cpp：

#include "Has_quarter_state.hpp"
#include <iostream>
#include "Gumball_machine.hpp"
#include "Sold_state.hpp"
Has_quarter_state::Has_quarter_state(Gumball_machine* m) : State{m} {}
Has_quarter_state::~Has_quarter_state() {}
void Has_quarter_state::turn_crank()
{         
     std::cout << "You turned...n";
     machine_->state_ = std::make_unique<Sold_state>(machine_);
     machine_->dispense();                    // Invalid read!
     // This works however (don't forget to comment out the above reallocation):
//     Gumball_machine* ptr{machine_};
//     machine_->state_ = std::make_unique<Sold_state>(machine_);
//     ptr->dispense();
}
void Has_quarter_state::dispense()
{
     std::cout << "No gumball dispensedn";
}

Sold_state.hpp：

#ifndef ClASS_SOLD_STATE_HPP_
#define ClASS_SOLD_STATE_HPP_
#include "State.hpp"
class Gumball_machine;
class Sold_state : public State
{
     public:
          explicit Sold_state(Gumball_machine*);
          ~Sold_state() override;
          void turn_crank() override;
          void dispense() override;
};
#endif

Sold_state.cpp：

#include "Sold_state.hpp"
#include <iostream>
#include "Gumball_machine.hpp"
#include "Has_quarter_state.hpp"
Sold_state::Sold_state(Gumball_machine* m) : State{m} {}
Sold_state::~Sold_state() {}
void Sold_state::turn_crank()
{         
     std::cout << "Turning twice doesn't give you another gumballn";
}
void Sold_state::dispense()
{
     std::cout << "A gumball comes rolling out the slotn";
//          machine_->state_.reset(new No_quarter_state{machine_});
     machine_->state_ = std::make_unique<Has_quarter_state>(machine_);
}

编辑：主要.cpp

     int 
main ()
{
     Gumball_machine machine;
     machine.turn_crank();
     return 0;
}

最后是瓦尔格林德输出：

==12085== Memcheck, a memory error detector
==12085== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
==12085== Using Valgrind-3.10.1 and LibVEX; rerun with -h for copyright info
==12085== Command: ./main
==12085== 
==12085== Invalid read of size 8
==12085==    at 0x401C61: Has_quarter_state::turn_crank() (in /home/mbw/Documents/Programmieren/CPP/Design_Patterns/Head_First_Design_Patterns/Chapter10_State_pattern/Example1_Revised/example_for_stackoverflow/main)
==12085==    by 0x401730: Gumball_machine::turn_crank() (in /home/mbw/Documents/Programmieren/CPP/Design_Patterns/Head_First_Design_Patterns/Chapter10_State_pattern/Example1_Revised/example_for_stackoverflow/main)
==12085==    by 0x402FF7: main (in /home/mbw/Documents/Programmieren/CPP/Design_Patterns/Head_First_Design_Patterns/Chapter10_State_pattern/Example1_Revised/example_for_stackoverflow/main)
==12085==  Address 0x5e47048 is 8 bytes inside a block of size 16 free'd
==12085==    at 0x4C2CE10: free (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==12085==    by 0x4017B4: operator delete(void*, unsigned long) (in /home/mbw/Documents/Programmieren/CPP/Design_Patterns/Head_First_Design_Patterns/Chapter10_State_pattern/Example1_Revised/example_for_stackoverflow/main)
==12085==    by 0x401858: Has_quarter_state::~Has_quarter_state() (in /home/mbw/Documents/Programmieren/CPP/Design_Patterns/Head_First_Design_Patterns/Chapter10_State_pattern/Example1_Revised/example_for_stackoverflow/main)
==12085==    by 0x401B27: Has_quarter_state::turn_crank() (in /home/mbw/Documents/Programmieren/CPP/Design_Patterns/Head_First_Design_Patterns/Chapter10_State_pattern/Example1_Revised/example_for_stackoverflow/main)
==12085==    by 0x401730: Gumball_machine::turn_crank() (in /home/mbw/Documents/Programmieren/CPP/Design_Patterns/Head_First_Design_Patterns/Chapter10_State_pattern/Example1_Revised/example_for_stackoverflow/main)
==12085==    by 0x402FF7: main (in /home/mbw/Documents/Programmieren/CPP/Design_Patterns/Head_First_Design_Patterns/Chapter10_State_pattern/Example1_Revised/example_for_stackoverflow/main)
==12085== 
==12085== 
==12085== HEAP SUMMARY:
==12085==     in use at exit: 0 bytes in 0 blocks
==12085==   total heap usage: 3 allocs, 3 frees, 48 bytes allocated
==12085== 
==12085== All heap blocks were freed -- no leaks are possible
==12085== 
==12085== For counts of detected and suppressed errors, rerun with: -v
==12085== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)

提前感谢您的帮助！