Why the following code run all the time?

Question

1.00/5 (1 vote)

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C#

#include <Windows.h>

class App;

class System
{
public:
    System(App * app) : mApp(app) { }
    bool update();
    App * mApp;
};

class App
{
public:
    App(void) : mExit(false) { }

    void setSystem(System * s) { mSystem = s; }

    void run(void)
    {
        while (!mExit)
        {
            mExit = mExit || !mSystem->update();
            ::Sleep(500);
        }
    }

    System * mSystem;
    bool mExit;
};

bool System::update()
{
    mApp->mExit = true;
    return true;
}

int main()
{
    App a;
    System s(&a);
    a.setSystem(&s);

    a.run();

    return 0;
}

Posted 24-Mar-12 4:37am

komdragon

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YvesDaoust · Accepted Answer · 2012-03-24T05:24:00

Solution 1

Why would it terminate ? The value of mExit starts as false and remains so forever. (update() always returns true).

Posted 24-Mar-12 5:24am

YvesDaoust

Updated 24-Mar-12 5:25am

v2

Comments

Sergey Alexandrovich Kryukov 24-Mar-12 18:26pm

Of course; a 5.
--SA

nv3 · Accepted Answer · 2012-03-24T07:01:00

I see that your intention was to set mExit to true in System::update. In fact mExit gets set to true. But when the update function returns mExit is overwritten once again with the result of the expression

mExit || !mSystem->update()

The clue is that the mExit variable of this expression has already been evaluated before update() gets called. So the result of the expression is false and mExit is set to false again.

I would recommend that you update the mExit variable only in the System::update() function and rewrite the loop to

while (!mExit)
 {
     mSystem->update();
     ::Sleep(500);
 }