# C designated initializers

**C designated initializers** (C99) let you initialize struct/array members by name or index instead of in declaration order. Write `struct Foo x = {.field = value}` or `int arr[] = {[5] = 10}` to set specific fields without touching others.

Use designated initializers for clarity, to skip members, or to initialize non-contiguous elements.

## Example

```c
// compile: gcc -std=c99 -o designated designated.c
// run: ./designated
// description: designated initializers for structs and arrays

#include <stdio.h>

struct Config {
    int timeout;
    char *name;
    int debug;
    int retries;
};

int main() {
    // Traditional: must specify all fields or leave zeros
    struct Config c1 = {5, "app", 0, 3};
    
    // Designated: explicit field names, clearer
    struct Config c2 = {
        .name = "myapp",
        .timeout = 10,
        .retries = 5
        // debug left as 0
    };
    
    printf("Config 2: %s, timeout=%d, retries=%d\n",
           c2.name, c2.timeout, c2.retries);
    
    // Array designated initializers
    int arr[10] = {
        [0] = 1,
        [5] = 50,
        [9] = 99
    };
    
    printf("Array: [0]=%d, [5]=%d, [9]=%d\n", arr[0], arr[5], arr[9]);
    
    return 0;
}
```

## Common patterns

**Struct initialization**:
- `.fieldname = value` syntax
- Can be in any order
- Unspecified fields zero-initialized
- Much clearer than positional args

**Array initialization**:
- `[index] = value` syntax
- Large sparse arrays without listing all zeros
- Mixed with positional: `{1, 2, [5]=6}`

**Benefits**:
- Self-documenting (clear which field each value sets)
- Adding fields doesn't break initialization
- Defaults to zero for unspecified fields
- Skip fields without listing zeros

**C99 vs C11/C17**:
- C99: original syntax
- C11: adds anonymous struct/union support
- Modern compilers support fully

**Nested structs**:
- `.outer.inner = value` for nested initialization
- Can mix designated and positional

**Gotchas**:
- Order of fields in {} doesn't matter
- But only one assignment per field allowed
- Can't use with older C standards

**Alternative syntax** (C11+):
- Designated initializers now standard
- Some compilers allow before C99 as extension