How can I increment or decrement pointer argument value in function call in C in one line?

Question

I have the following code:

#include <stdio.h>

int foo2(int *px, int *py)
{
    int sum = *px + *py;
    *px = *px + 1;
    *py = *py - 1;
    printf("foo2 : res=%d x=%d y=%d\n", sum, *px, *py);
    return sum;
}

int main() {
    int x = 4, y = 7, res;
    res = foo2(&(x++), &(y--));
    printf("%d", res);

    return 0;
}

I need to increment x , decrement y , then I need to pass them in foo function as arguments.

I have error: lvalue required as unary '&' operand . Also I tried to use x + 1 and y - 1 instead of x++ and y++ .

How can I increment x and y values and pass pointer to them in foo2 function call? Is it possible?

Answer 1

You can use the comma operator:

res = foo2((x++, &x), (y--, &y));

However this is not very readable, so unless you have a really good reason it is better to write it as three separate statements:

x++;
y--;
res = foo2(&x, &y);

Answer 2

I just want to add some points here:

C :

In c both the prefix and postfix increment returns the rvalue . So that's the reason you will get an error when you try to get address of post/pre-increment in c.

C++ :

But in c++, the prefix (++x) returns lvalue and postfix(x++) returns a rvalue . So in c++, &(++x) is correct whereas &(x++) throws the error.

Note:

I encourage you to read about this a bit more. It will help you a lot to understand much better.