Why does line 2 &x not work but line 3 does? Because &x returns a pointer, a number representing a memory address. This is an important distinction. A pointer doesn’t hold a memory address, it holds a number that represents a memory address.
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No, that is not why. Note that the following does work:
int * x = 0;
and the following works, though typically yields a warning:
int * x = 20;
Line 2 fails because & doesn't give back an l-value.
The point I was trying to make is that &x would return a number not an assignable memory location. The reason it isn't an lvalue is because it doesn't return a memory address the way a variable assignment or dereference would.
I was probably a bit confusing in what I was trying to say.
It is quite confusing, because it implies that this is equivalent and equivalently wouldn't work:
int *ptr = &x;
ptr = 20;
The rvalue/lvalue distinction is completely unrelated to pointers, holding memory addresses versus numbers representing memory addresses, etc. The & operator simply doesn't give you an lvalue, like most operators in C, and this has nothing to do with addresses, pointers, or anything of the like
After rereading it a couple times I think you and others were right. I was trying to say something that in the end muddied issue. I have removed the section on r-value and l-values.
It absolutely does return a memory address; it even has the type of a memory address ("int *"). You just can't write to it because it doesn't live in a writable location (an lvalue), for the same reason you can't write:
(x + 1) = 12
because (x+1) doesn't live in a writable location.
2 &x = 20; // this doesn't work
3 * (&x) = 20; // this does work
Why does line 2 &x not work but line 3 does? Because &x returns a pointer, a number representing a memory address. This is an important distinction. A pointer doesn’t hold a memory address, it holds a number that represents a memory address.
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No, that is not why. Note that the following does work:
int * x = 0;
and the following works, though typically yields a warning:
int * x = 20;
Line 2 fails because & doesn't give back an l-value.