Pointer to a function with variable arguments number

Question

I have declared:

class aaa {
public:
    static std::queue<QPair<void (*)( ... ), int> > m_commands;
    static int bbb();
    static void ccc(...);
};

and in bbb() method I wrote:

int aaa::bbb() {
    m_commands.push( qMakePair( aaa::ccc, 0 ) );
}

but it complains about:

error C2664: 'void std::queue<_Ty>::push(QPair<T1,T2> &&)' : cannot convert parameter 1 from 'QPair<T1,T2>' to 'QPair<T1,T2> &&'

why? When I had function like that:

void reg( void ( *invoker )( ... ), int args  ) {
    m_commands.push( qMakePair( invoker, args ) );
}

I could easily send to the above function a static function this way:

reg( aaa::ccc, 0 );

I'm pretty sure that passing a no-arg function pointer to that reg function doesn't compile; see ideone.com/gf8J3. — Oliver Charlesworth
– Oliver Charlesworth, Commented Jul 9, 2012 at 19:39

ecatmur · Accepted Answer · 2012-07-09 19:40:32Z

1

qMakePair( aaa::ccc, 0 ) is (presumably) returning a value of type QPair<void (*)(), int>, since it doesn't know that you want a value of type QPair<void (*)( ... ), int>. Invoke it explicitly as qMakePair<void (*)( ... ), int>( aaa::ccc, 0 ) or reinterpret_cast aaa::ccc to the desired function pointer type.

Not to mention that this is (almost certainly) illegal, as aaa::ccc does not have the correct signature and cannot be invoked through the function pointer type you're using.

answered Jul 9, 2012 at 19:40

ecatmur

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7 Comments

Oliver Charlesworth Over a year ago

Whilst the pointer conversion is valid, attempting to dereference that pointer (i.e. call the function) leads to undefined behaviour.

ecatmur Over a year ago

@OliCharlesworth yes; AIUI it's legitimate to cast it back to the correct type and then indirect-call it.

Oliver Charlesworth Over a year ago

Yes, that would be valid (although a little unorthodox!).

tobi Over a year ago

@OliCharlesworth I am sorry guys! Take my apologize. There was a bug in the post, I changed ccc( ... ). Now it compiles ideone.com/0vxpi anyway the explicitly invoking (qMakePair<void (*)( ... ), int>( aaa::ccc, 0 )) worked for me.

tobi Over a year ago

@ecatmur but now there's written ccc( ... ) as the m_commands wants, so why do I have to use qMakePair explicitly with template arguments?

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