类方法读取同一变量 C++ 的不同值 [英] Class methods read different values of the same variable C++

问题描述

我的班级有一个用户定义类型的变量.我在类的一个方法中设置了这个变量.当我尝试从同一个类的另一个方法中获取该变量时，该值始终为 0，尽管该变量在其他任何地方都没有更改.

My class has a variable of a user-defined type. I set this variable in one method of the class. When I try to get this variable from another method of the same class, the value is always 0, although the variable has not been changed anywhere else.

我真的不明白为什么会这样！以下是我的代码示例:

I really don't understand why this happens! Follwoing is a sample of my code:

myclass.h

typedef enum {
    ACTIVE   =  0,  
    SLEEP    =  1,  
    FINISHED =  2,  
    WAITING  =  3,
    KILLED   =  4
} Mode;


class MyClass
{
    public:
      void statusReceive(void);
      Mode getCurrentMode(void);

    private:
      Mode currentMode;
};

myclass.cpp

myclass.cpp

#include "myclass.h"

void MyClass::statusReceive(void)
{
    currentMode = (Mode)interpretMsg(&msg);
    printf("current Mode = %d\n", this->currentMode); // prints 4
}

Mode MyClass::getCurrentMode(void)
{
    printf("current Mode = %d\n", this->currentMode);   // prints 0
    return this->currentMode;
}

main.cpp

#include "myclass.h"

MyClass myclass;

void timerStart(std::function<void(void)> func, unsigned int interval)
{
    std::thread([func, interval]()
            { 
            while (true)
            { 
            auto x = std::chrono::steady_clock::now() + std::chrono::milliseconds(interval);
            func();
            std::this_thread::sleep_until(x);
            }
            }).detach();
}

int main(void)
{
    timerStart(std::bind(&MyClass::statusReceive, myclass), 10);
    Mode x = myclass.getCurrentMode();
    printf("Current Mode = %d\n", x); // prints 0
}

推荐答案

我发现这段代码有两个问题.

I see two problems with this code.

首先，std::bind 复制(或移动)它的所有参数.因此，从 bind 返回的函子值包含另一个从 myclass 对象复制构造的 MyClass 对象，并且对该函子的调用只会改变内部 MyClass，而不是 myclass.

First, std::bind does a copy (or move) of all its arguments. So the functor value returned from bind contains another MyClass object which was copy-constructed from the myclass object, and calls to that functor will only change the internal MyClass, not myclass.

您可以使用引用包装器来指定使用相同的 MyClass 对象:

You could use a reference-wrapper to specify using the same MyClass object:

timerStart(std::bind(&MyClass::statusReceive, std::ref(myclass)), 10);

或者直接切换到 lambda 来调用 myclass 上的成员函数:

or just switch to a lambda to call the member function on myclass directly:

timerStart([](){ myclass.statusReceive(); }, 10);

其次，您正在一个线程中修改 myclass.currentMode 对象并在另一个线程中读取它，而这两个操作之间没有任何同步.这是一种数据竞争和未定义的行为.

Second, you are modifying the myclass.currentMode object in one thread and reading it in another without any synchronization between the two operations. This is a data race and undefined behavior.

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