C++ Passing `this` into method by reference

Question

I have a class constructor that expects a reference to another class object to be passed in as an argument. I understand that references are preferable to pointers when no pointer arithmetic will be performed or when a null value will not exist.

This is the header declaration of the constructor:

class MixerLine {

private:
    MIXERLINE _mixerLine;
    
public:

    MixerLine(const MixerDevice& const parentMixer, DWORD destinationIndex); 

    ~MixerLine();
}

This is the code that calls the constructor (MixerDevice.cpp):

void MixerDevice::enumerateLines() {
    
    DWORD numLines = getDestinationCount();
    for(DWORD i=0;i<numLines;i++) {
        
        MixerLine mixerLine( this, i );
        // other code here removed
    }
}

Compilation of MixerDevice.cpp fails with this error:

Error 3 error C2664: 'MixerLine::MixerLine(const MixerDevice &,DWORD)' : cannot convert parameter 1 from 'MixerDevice *const ' to 'const MixerDevice &'

But I thought pointer values could be assigned to references, e.g.

Foo* foo = new Foo();
Foo& bar = foo;

`const MixerDevice& const parentMixer` the second `const` is unnecessary since references (unlike pointers) cannot be reseated to reference another object. — Praetorian, Jun 23 '12 at 14:57

score 21 · Accepted Answer · answered Jun 23 '12 at 14:52

21

this is a pointer, to get a reference you have to dereference (*this) it:

MixerLine mixerLine( *this, i );

answered Jun 23 '12 at 14:52

Yakov Galka

70,775
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score 3 · Answer 2 · answered Jun 23 '12 at 15:11

You should dereference this, because this is a pointer, not a reference. To correct your code you should write

for(DWORD i=0;i<numLines;i++) {

    MixerLine mixerLine( *this, i ); // Ok, this dereferenced
    // other code here removed
}

Note: the second const at the constructor's parameter const MixerDevice& const parentMixer is completely useless.

score 1 · Answer 3 · answered Jun 23 '12 at 15:20

To obtain a reference from a pointer you need to dereference the pointer, as it has already been mentioned. Additionally (maybe due to copying into the question?) the constructor should not compile:

const MixerDevice& const parentMixer

That is not a proper type, references cannot be const qualified, only the referred type can be, so the two (exactly equivalent) options are:

const MixerDevice& parentMixer
MixerDevice const& parentMixer

(Note that the const qualifying of MixerDevice can be done at either way, and it means exactly the same).

Yes, I removed the second const modifier and it works fine now. — Dai, Jun 23 '12 at 15:27

score 0 · Answer 4 · answered Jun 23 '12 at 14:52

0

Pointer values can be assigned to pointers, but not to references!¹

Foo* foo = new Foo();
Foo& bar = *foo;
           ^
           ^

_{1. Well, they can be used to initialise references-to-pointers, but that's not what you have here...}

answered Jun 23 '12 at 14:52

Oliver Charlesworth

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C++ Passing `this` into method by reference

4 Answers4