C++ Setting member object which has const member

Question

I am using OIS for handling my input with Ogre and currently, on KeyPress/Release a Message object like the following will be constructed and distributed among subscribers.

class Message
{
public:
    Message();
    ~Message();

    inline void SetKeyEvent(const OIS::KeyEvent& keyEvent) { _keyEvent = keyEvent; }

    const OIS::KeyEvent& GetKeyEvent() const { return _keyEvent; }

private:

    OIS::KeyEvent _keyEvent;

};

Since this object will be constructed/destroyed whenever input is received via keyboard, I am trying to store a pre-constructed Message object and then simply update the _keyEvent field with the new data, before distributing.

The problem is that the OIS::KeyEvent object has a const member which is preventing me from using the assignment operator. The SetKeyEvent method gives me the following syntax error:

function "OIS::KeyEvent::operator=(const OIS::KeyEvent &)" (declared implicitly) cannot be referenced -- it is a deleted function

I was wondering what the best way to achieve this functionality would be?

Thanks in advance

EDIT: Just to clarify, I already use initializer lists when possible. My intention is to have the Message object pre-constructed and then update the _keyEvent field with the new event data from the KeyPress event which OIS fires, using the SetKeyEvent method. I would like to know if this is possible and if so, what the best way to do it would be.

It's not about a const member. It means `OIS::KeyEvent` has no assignment operator. — Hayt, Sep 07 '16 at 08:29
Possible duplicate of [Why should I prefer to use member initialization list?](http://stackoverflow.com/questions/926752/why-should-i-prefer-to-use-member-initialization-list) — LogicStuff, Sep 07 '16 at 08:33
@Hayt it has no default assignment operator *because* there's a const member in it. — n. m. could be an AI, Sep 07 '16 at 08:38
you're right. I totally did not make that connection somehow. — Hayt, Sep 07 '16 at 08:44
Do you really need to store a pre-constructed message? Does it have other members? — n. m. could be an AI, Sep 07 '16 at 09:35
@n.m. I could allocate new Message objects, i was wanting to have them pre-constructed to avoid allocating a new message object every time a key is pressed/released — cmpbedes, Sep 07 '16 at 09:44
You mean, you have to allocate one from the free store? Why? Can you use an automatic variable, or maybe pass messages by value? — n. m. could be an AI, Sep 07 '16 at 09:50

pospich0815 · Accepted Answer · 2016-09-07T09:20:30.980

1

The copy operator is deleted, so you must work with pointers.

class Message
{
public:
    Message();
    ~Message();

    inline void SetKeyEvent(OIS::KeyEvent* keyEvent) { _keyEvent = keyEvent; }

    const OIS::KeyEvent& GetKeyEvent() const { return _keyEvent; }

private:

    OIS::KeyEvent* _keyEvent;

};

And now it's better to check if the argument in the setter isn't nullptr.

inline void SetKeyEvent(OIS::KeyEvent* keyEvent) 
{
   assert(keyEvent != nullptr);
   _keyEvent = keyEvent; 
}

assert() needs #include <assert.h>

EDIT:

Sorry, forgot the getter method. You must use pointers, too.

const OIS::KeyEvent* keyEvent = &Message.GetKeyEvent();

Where Message is your class instance.

edited Sep 07 '16 at 09:20

answered Sep 07 '16 at 09:07

pospich0815

378
1
3
13

Don't do this. Use the constructor [correctly](http://stackoverflow.com/questions/926752/why-should-i-prefer-to-use-member-initialization-list), that's all. – n. m. could be an AI Sep 07 '16 at 09:14
1

The error message throws when the getter was called and there is no constructor call. – pospich0815 Sep 07 '16 at 09:25
What error message? If you have a question about the getter, ask it. – n. m. could be an AI Sep 07 '16 at 09:29
1

There is a error message in the question: function "OIS::KeyEvent::operator=(const OIS::KeyEvent &)" (declared implicitly) cannot be referenced -- it is a deleted function – pospich0815 Sep 07 '16 at 09:30
You fix it by not using assignment. Note the getter returns by reference. – n. m. could be an AI Sep 07 '16 at 09:32
1

Yes, because the assignment operator in OIS::KeyEvent is deleted and he can't use it. – pospich0815 Sep 07 '16 at 09:36
Marked as answer - using the pointer route, i adjusted my implementation which allowed me to pre-construct my objects and set the new `OIS::KeyEvent` data via pointer – cmpbedes Sep 07 '16 at 11:50

score -1 · Answer 2 · answered Sep 07 '16 at 09:45

It is possible by using placement new and an explicit destructor call, things you normally should never do:

 inline void SetKeyEvent(const OIS::KeyEvent& keyEvent) 
 { 
     _keyEvent.~KeyEvent();
     new (&_keyEvent) OIS::KeyEvent(keyEvent);
 }

This is bad ugly horrible code, use at your own risk.

C++ Setting member object which has const member

2 Answers2