How can I reference a parameter within a designated initializer?

Question

I want to add the value of a parameter to another parameter within a designated initializer, without having to define a separate variable to hold that value. Is this possible?

typedef struct {
  int x;
  int y;
} point;

int main() {
  point p = {
    .x = 1,
    .y = x + 2 // I want to reference .x
  };
}

score 3 · Accepted Answer · edited Oct 25 '20 at 10:19

This is not possible. The evaluation order of expressions in an initializer are not evaluated in any particular order, so there's no guarantee that .x will be set to 1 before .y is set to .x + 2.

This is spelled out in section 6.7.9p23 of the C standard:

The evaluations of the initialization list expressions are indeterminately sequenced with respect to one another and
thus the order in which any side effects occur is unspecified.¹⁵²⁾

Where footnote 152 states:

In particular, the evaluation order need not be the same as the order of subobject initialization.

How can I reference a parameter within a designated initializer?

1 Answers1