What is a designated initializer in C?

The Designed Initializer came up since the ISO C99 and is a different and more dynamic way to initialize in C when initializing struct, union or an array.

The biggest difference to standard initialization is that you don't have to declare the elements in a fixed order and you can also omit element.

From The GNU Guide:

Standard C90 requires the elements of an initializer to appear in a fixed order, the same as the order of the elements in the array or structure being initialized.

In ISO C99 you can give the elements in random order, specifying the array indices or structure field names they apply to, and GNU C allows this as an extension in C90 mode as well

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Examples

1. Array Index

Standard Initialization

  int a[6] = { 0, 0, 15, 0, 29, 0 };

Designed Initialization

  int a[6] = {[4] = 29, [2] = 15 }; // or
  int a[6] = {[4]29 , [2]15 }; // or
  int widths[] = { [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 };

2. Struct or union:

Standard Initialization

struct point { int x, y; };

Designed Initialization

 struct point p = { .y = 2, .x = 3 }; or
 struct point p = { y: 2, x: 3 };

3. Combine naming elements with ordinary C initialization of successive elements:

Standard Initialization

int a[6] = { 0, v1, v2, 0, v4, 0 };

Designed Initialization

int a[6] = { [1] = v1, v2, [4] = v4 };

4. Others:

Labeling the elements of an array initializer

int whitespace[256] = { [' '] = 1, ['\t'] = 1, ['\h'] = 1,
                        ['\f'] = 1, ['\n'] = 1, ['\r'] = 1 };

write a series of ‘.fieldname’ and ‘[index]’ designators before an ‘=’ to specify a nested subobject to initialize

struct point ptarray[10] = { [2].y = yv2, [2].x = xv2, [0].x = xv0 };

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Guides

• designated-initializers-c | geeksforgeeks.org
• using-designated-initializers
• tutorialspoint.com | designated-initializers-in-c

Designated initialisers come in two flavours:

1) It provides a quick way of initialising specific elements in an array:

int foo[10] = { [3] = 1, [5] = 2 };

will set all elements to foo to 0, other than index 3 which will be set to 1 and index 5 which will be set to 2.

2) It provides a way of explicitly initialising struct members. For example, for

struct Foo { int a, b; };

you can write

struct Foo foo { .a = 1, .b = 2 };

Note that in this case, members that are not explicitly initialised are initialised as if the instance had static duration.

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Both are standard C, but note that C++ does not support either (as constructors can do the job in that language.)