Question

Sorry for bad English.

Suppose the code (C99 or later):

typedef struct {
    int a, b;
} foo_t;

foo_t f = { .a = 1, .b = 2  };

f = (foo_t){ .b = 3 };

What is f.a now? Does C standard say anything about this?

I know that for partial initialization the standard guarantees all uninitialized members will be initialized to “appropriate zero” (0 for integers, 0.0 for floating, NULL for pointers, etc.). But the last statement isn’t initialization (as I understand), because f is already exists. I’m confused.

Answer 1

Point 6 in section 6.5.2.5 Compound literals of the C99 standard states:

The value of the compound literal is that of an unnamed object initialized by the initializer list.

The unnamed foo_t is partially initialized, by the same rules you mention.

The last statement is an assignment, but the compound literal is an unnamed object initialized by the initializer list. Meaning the unnamed.a has value zero and f.a is zero after the assignment.