投射由迭代器指向的对象

Question

I have this multiset container:

multiset<IMidiMsgExt, IMidiMsgExtComp> queuedNotes;

IMidiMsgExt is a struct I've created myself (I need it for one additional property, mTick ) that extend IMidiMsg :

struct IMidiMsgExt : public IMidiMsg
{
    IMidiMsgExt() {
    }

    double mTick = 0.;

    void IMidiMsgExt::MakeNoteOnMsg(int noteNumber, int velocity, int offset, double tick, int channel)
    {
        Clear();
        mStatus = channel | (kNoteOn << 4);
        mData1 = noteNumber;
        mData2 = velocity;
        mOffset = offset;
        mTick = tick;
    }

    void IMidiMsgExt::MakeNoteOffMsg(int noteNumber, int offset, double tick, int channel)
    {
        Clear();
        mStatus = channel | (kNoteOff << 4);
        mData1 = noteNumber;
        mOffset = offset;
        mTick = tick;
    }

    void IMidiMsgExt::Clear()
    {
        mOffset = 0;
        mStatus = mData1 = mData2 = 0;
        mTick = 0.;
    }
};

Next: I store in that queuedNotes multiset some IMidiMsgExt objects, with:

IMidiMsgExt* noteOff = new IMidiMsgExt;
noteOff->MakeNoteOffMsg(57, 0, tickSize * 111, 0);
queuedNotes.insert(*noteOff);

Now, I need to use a function called SendMidiMsg(IMidiMsg* pMsg) (that takes IMidiMsg type as input) sending my object IMidiMsgExt to it.

I extract the first object from my list to an iterator:

auto midiMessage = queuedNotes.begin();

But when I try to cast it and use SendMidiMsg :

SendMidiMsg((IMidiMsgExt*)midiMessage);

it says no suitable conversion function from "std::_Tree_const_iterator<std::_Tree_val<std::_Tree_simple_types<IMidiMsgExt>>>" to "IMidiMsg *" exists

Where am I wrong? Should I use dynamic casting ?

Answer 1

auto midiMessage = queuedNotes.begin();

midiMessage is of type std::multiset::iterator . And it is not convertable to your type IMidiMsgExt . Iterator is an object that behaves similarly to a pointer, so you can use dereference operator ( * ) to get the object that it "points to". You also don't need to cast derived object to its base, that is done implicitly. All you need to do is get the address of where the iterator "points to" to get a pointer to IMidiMsgExt :

SendMidiMsg(&*midiMessage);

A quick break down of &*midiMessage :

  variable    -     type
midiMessage   - std::multiset::iterator
*midiMessage  - IMidiMsgExt
&*midiMessage - IMidiMsgExt*

---

Edit: About your const_iterator error. std::multiset::begin() is supposed to always return const_iterator . Your function SendMidiMsg() wants a non const pointer - it is saying it wants to edit the message. multiset does not allow changing the elements.

You can copy the message and then call SendMidiMsg() . If you don't need the message inside the container anymore, you can also erase it afterwards.

auto msg = *midiMessage;
SendMidiMsg(&msg);
queuedNotes.erase(midiMessage);

Note: It seems like you have a memory leak in the program. You create messages with new and I don't see any calls to delete, to release the memory.

IMidiMsgExt* noteOff = new IMidiMsgExt;
noteOff->MakeNoteOffMsg(57, 0, tickSize * 111, 0);
queuedNotes.insert(*noteOff);