Question

I’m trying to do something fairly simple. Of course it could be done with an extra line of code, but technically speaking, why does this not work?

int foo = 5;
int *bar = &(--foo);

GCC compiler tells me "invalid lvalue in unary &"

Answer 1

Because --foo is not an lvalue (so named since they typicaly appeared on the left of an asignment).

It’s the same problem with:

--foo = 7;

If, for some bizarre reason, you need it on one line, just use:

--foo; int *bar = &foo;

If you think you need it in one instruction, think again. Even if you make such a horrible monstrosity ^(a), it will probably compile down to the same machine language sequence as that code snippet above.

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^(a) int *bar = (--foo, &(foo));