compile error cannot convert 'this' pointer

Hello everyone,


Which C++ rule do I break in the following code? Why there is compile error?

Code:

class Foo{
public:
	Foo()
	{
	}

	int foo1()
	{
		return 100;
	}

	const Foo getCurrent1()
	{
		return *this;
	}

	const Foo& getCurrent2()
	{
		return *this;
	}
};

int main()
{
	Foo f;
	f.getCurrent1().foo1(); // error C2662: 'Foo::foo1' : cannot convert 'this' pointer from 'const Foo' to 'Foo &'
	f.getCurrent2().foo1(); // error C2662: 'Foo::foo1' : cannot convert 'this' pointer from 'const Foo' to 'Foo &'

	return 0;
}

thanks in advance,
George

I got a slightly more helpful warning message from gcc:

error: no matching function for call to `Foo::foo1() const'

foo1 is not labeled as a const function, therefore it cannot take a const "argument".

Thanks tabstop,


Your fix works, cool!

I am confused about what the compile outputs,

cannot convert 'this' pointer from 'const Foo' to 'Foo &'

I think it should change "this" to "*this", right? This pointer is a pointer to class (Foo*) and not class type itself (Foo)?

Should be the following? Any comments?

cannot convert '*this' from 'const Foo' to 'Foo &'

Originally Posted by tabstop

I got a slightly more helpful warning message from gcc:

error: no matching function for call to `Foo::foo1() const'

foo1 is not labeled as a const function, therefore it cannot take a const "argument".

regards,
George

If you think of member functions as normal functions that take the this reference (this is a pointer, but we'll treat it as a reference to match the compiler's message) as the first argument, foo1() becomes

Code:

int Foo::foo1(Foo *this);

and the call becomes (this is invalid C++, you can't bind a temporary to a non-const reference)

Code:

Foo::foo1( Foo::getCurrent1(foo) )

So you're passing a const object to a function that takes a non-const reference. Thus the failure to convert const Foo to Foo&.

getCurrent1 and getCurrent2 both return const objects (temporary or otherwise). Therefore you can't call non-const method on them.

As for the exact error message you get, yes maybe what you suggest is slightly more sensible, but I don't think it really matters.

Thanks CornedBee,


> you can't bind a temporary to a non-const reference

I understand and agree with this rule.

But after reading your reply a couple of times, I am confused. How do you get conclusion from,

Code:

int Foo::foo1(Foo *this);

to

Code:

Foo::foo1( Foo::getCurrent1(foo) )

What is foo? I read the code agian in my original question, there is no code or variable called foo?


regards,
George

Ah, sorry, you called that variable just f.

Thanks CornedBee,


My question is answered.

Originally Posted by CornedBee

Ah, sorry, you called that variable just f.

regards,
George