Reference to a pointer to a pointer

Exercise 7 from Chapter 11 of Bruce Eckel's "Thinking in C++":

Create a function that takes an argument of a reference to a pointer to a pointer and modifies that argument. In main( ), call the function.

I'm having difficulty figuring out exactly what I can pass as an argument to a function which takes an argument of that kind. For example:

Code:

void f1(int**& x)
{
	x++;
}


int main() 
{
	int x = 0;
	int *px = &x;
	int **ppx = &px;
	f1(ppx); // fine
	//f1(&px) doesn't work

	return 0;
}

I guess what I'm having trouble understanding is why I can pass ppx but not &px.

Gah! It didn't take long after I'd hit "submit" that I realized exactly why I can't pass it.

Move along folks, nothing to see here.....

Experiment:
What if you do:

Code:

void f1(const int** & x)
{
	//x++;
}


int main() 
{
	int x = 0;
	int * px = &x;
	f1(&px);

	return 0;
}

I'll get exactly the same error as I did as before when I tried to compile that same function call, i.e:

Code:

error C2664: 'f1' : cannot convert parameter 1 from 'int **' to 'const int **&'

Originally Posted by Sharke

Gah! It didn't take long after I'd hit "submit" that I realized exactly why I can't pass it.

Move along folks, nothing to see here.....

This is a prime example of the 3rd rule of programming:

"Right after you ask for help and have shown your code to the world, you will realize your stupid mistake and be humiliated"

I live by this rule :-)

Originally Posted by Sharke

I'll get exactly the same error as I did as before when I tried to compile that same function call, i.e:

Code:

error C2664: 'f1' : cannot convert parameter 1 from 'int **' to 'const int **&'

Ah yes, the evil placement of const.
This one should work:

Code:

void f1(int** const & x)

Pointer to a pointer to a reference to const?

More like a reference to a constant pointer to int**.

*head swims*

I'm sure it's a little difficult to get your head wrapped around, but there are two different types of pointers, basically.
const T* -> A pointer to type const T.
T* const -> A constant pointer to type T.

When I placed the const all to the left in the beginning, it became part of the type the pointer pointed to!
But what we actually need a constant pointer.
We can pass temporaries by const reference, and const reference means a reference to a constant type (not a pointer TO a constant type--that's something entirely different!).

Meh. This may be a little difficult to wrap your head around, but you aren't required to learn it.

Originally Posted by Elysia

This may be a little difficult to wrap your head around, but you aren't required to learn it.

Maybe not, but the less dangling threads I leave when I'm learning something, the better! I had just about wrapped my head around const placement in C, I guess it was the addition of references that tipped me over the edge.

Everything to the left of the & of a reference is the type the reference points to.
Similarly for pointers, everything left of the * is the type it points to.
But remember that a pointer is a variable itself, so it too, can be const. You just have to place it to the right of the *.

Got it. But perhaps I'm not understanding the whole passing temporaries thing. Suppose the following:

Code:

class X
{
public:
    X() {}
};

X f1()
{
    X x;
    return x;
}

void f2(X& x) {}

int main() 
{
    f2(f1());
}

I'm confused that the compiler sees nothing wrong with me passing the result of f1() into f2() even though f2() doesn't ask for a const reference.

Originally Posted by Sharke

Got it. But perhaps I'm not understanding the whole passing temporaries thing. Suppose the following:

Code:

class X
{
public:
    X() {}
};

X f1()
{
    X x;
    return x;
}

void f2(X& x) {}

int main() 
{
    f2(f1());
}

I'm confused that the compiler sees nothing wrong with me passing the result of f1() into f2() even though f2() doesn't ask for a const reference.

I'm surprised you call

Code:

temp.cpp:17: error: invalid initialization of non-const reference of type ‘X&’ from a temporary of type ‘X’
temp.cpp:13: error: in passing argument 1 of ‘void f2(X&)’

"nothing wrong".

This is what Visual C++ Express gives me with the exact same code:

Code:

0 error(s), 0 warning(s)
========== Build: 1 succeeded, 0 failed, 0 up-to-date, 0 skipped ==========

EDIT: The compiler's warning level was set to level 3, the default. This is what I get after jacking it up to level 4:

Code:

warning C4239: nonstandard extension used : 'argument' : conversion from 'X' to 'X &'
1>        A non-const reference may only be bound to an lvalue

So, not an error, only a warning - and only after the warning level is raised higher than the default.