Syntax question: &(++x)

Hi, just a weird syntax-related question. Why can't I do this:

Code:

---

some_function( &(++x) );

---

where its passing a pointer to an already-incremented variable x? Seems to me it should be legal, but gcc complains about invalid lvalues. Any switches I can pass to make this code work? Cheers,
Yoshi


EDIT: In other words, why isn't ++x an lvalue? Two lines for one thing is so ugly :-)

Honestly, this looks like the address of a pointer. The correct answer depends on what you are doing. If you want to increment the *value at* the pointer and pass it in, you are no longer passing a pointer, but if you had a data structure and need to pass in a certain part of it, you'd just increment *the pointer*.

Code:

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  // my favorite function f...
  void f(int * in) {

  }
...
  int x[] = {42, 1, 0, -42};
  f( (x++) ); // make sure that the pointer is incremented,
              // so we pass x[1]

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Yea, I guess that would work. The reason I had it is this:

Code:

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void skip_space(char** str)
{
        for(; **str != '\0' && isspace(**str); (*str)++);
}

---

which takes a pointer to a pointer to a string and slides it over until it points to the first nonspace. I wanted something like this:

Code:

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char x[50] = "#    Some comment with leading spaces...";
char* y = x;

skip_space( &(++x) );

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to skip the first '#' and then skip spaces, but then I realized there were much better ways. Thanks anywas :-). Cheers,
Yoshi

Quote:

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Originally Posted by yoshiznit123

Yea, I guess that would work. The reason I had it is this:

Code:

---

void skip_space(char** str)
{
        for(; **str != '\0' && isspace(**str); (*str)++);
}

---

which takes a pointer to a pointer to a string and slides it over until it points to the first nonspace. I wanted something like this:

Code:

---

char x[50] = "#    Some comment with leading spaces...";
char* y = x;

skip_space( &(++x) );

---

to skip the first '#' and then skip spaces, but then I realized there were much better ways. Thanks anywas :-). Cheers,
Yoshi

---

Your problem there is that you are trying to increment an array. You cannot do this. What you can do is &(x+1), becouse then x stays the same.

Any form of arithmetic will degrade LValues to RValues. You have to do it on two lines.

Quote:

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Originally Posted by OnionKnight

Any form of arithmetic will degrade LValues to RValues. You have to do it on two lines.

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No you don't. You simmply cannot use an array as an LValue in asignment.

Quote:

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Originally Posted by King Mir

No you don't. You simmply cannot use an array as an LValue in asignment.

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You're on crack, Mir:

Code:

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itsme@itsme:~/C$ cat lvartest.c
#include <stdio.h>

void print_it(char **str)
{
  puts(*str);
}

int main(void)
{
  char str[] = "Hello world!";
  char *pstr = str;

  print_it(&(++pstr));

  return 0;
}

---

Code:

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itsme@itsme:~/C$ gcc -Wall lvartest.c -o lvartest
lvartest.c: In function `main':
lvartest.c:13: error: invalid lvalue in unary `&'
itsme@itsme:~/C$

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OnionKnight is correct.

Take the ++ out of print_it(&(++pstr)); and it compiles and runs just fine. And don't try to tell me it's because pstr points to an array. I'm trying to increment the pointer, not the array.

You also get the same error with print_it(&(pstr + 1));.

I think King Mir's point is that the OP's code will not work, even if split on two lines, as an array is not an l-value and therefore the ++ and other assignment operators can not be used on it.

I don't know why they bother doing anything with x at all, since they already have a pointer to it with y. Increment that. You still have to split it on two lines however.


Quzah.