Designated Initializers#
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 any 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. This extension is not implemented in GNU C++.
To specify an array index, write
[index] =
before the element value. For example,
int a[6] = { [4] = 29, [2] = 15 };
is equivalent to
int a[6] = { 0, 0, 15, 0, 29, 0 };
The index values must be constant expressions, even if the array being initialized is automatic.
An alternative syntax for this that has been obsolete since GCC 2.5 but
GCC still accepts is to write [index]
before the element
value, with no =
.
To initialize a range of elements to the same value, write
[first ... last] = value
. This is a GNU
extension. For example,
int widths[] = { [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 };
If the value in it has side effects, the side effects happen only once, not for each initialized field by the range initializer.
Note that the length of the array is the highest value specified plus one.
In a structure initializer, specify the name of a field to initialize
with .fieldname =
before the element value. For example,
given the following structure,
struct point { int x, y; };
the following initialization
struct point p = { .y = yvalue, .x = xvalue };
is equivalent to
struct point p = { xvalue, yvalue };
Another syntax that has the same meaning, obsolete since GCC 2.5, is
fieldname:
, as shown here:
struct point p = { y: yvalue, x: xvalue };
Omitted fields are implicitly initialized the same as for objects that have static storage duration.
The [index]
or .fieldname
is known as a
designator. You can also use a designator (or the obsolete colon
syntax) when initializing a union, to specify which element of the union
should be used. For example,
union foo { int i; double d; };
union foo f = { .d = 4 };
converts 4 to a double
to store it in the union using
the second element. By contrast, casting 4 to type union foo
stores it into the union as the integer i
, since it is
an integer. See Cast to a Union Type.
You can combine this technique of naming elements with ordinary C initialization of successive elements. Each initializer element that does not have a designator applies to the next consecutive element of the array or structure. For example,
int a[6] = { [1] = v1, v2, [4] = v4 };
is equivalent to
int a[6] = { 0, v1, v2, 0, v4, 0 };
Labeling the elements of an array initializer is especially useful
when the indices are characters or belong to an enum
type.
For example:
int whitespace[256]
= { [' '] = 1, ['\t'] = 1, ['\h'] = 1,
['\f'] = 1, ['\n'] = 1, ['\r'] = 1 };
You can also write a series of .fieldname
and
[index]
designators before an =
to specify a
nested subobject to initialize; the list is taken relative to the
subobject corresponding to the closest surrounding brace pair. For
example, with the struct point
declaration above:
struct point ptarray[10] = { [2].y = yv2, [2].x = xv2, [0].x = xv0 };
If the same field is initialized multiple times, or overlapping fields of a union are initialized, the value from the last initialization is used. When a field of a union is itself a structure, the entire structure from the last field initialized is used. If any previous initializer has side effect, it is unspecified whether the side effect happens or not. Currently, GCC discards the side-effecting initializer expressions and issues a warning.