Designated initializer specifically initialize a particular
member in a struct, union, or a particular element of an array.
Designated initializer is a part of the C99 standards (ISO/IEC 9899:1999:6.7.8).
Designated initializer eliminate the need to initialize the members
of aggregates in order (from the first member onto the last in a
structure and from the element indexed 0 to the last index in an
array).
Examples |
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The following examples detail the usage of designated initializers
in the C program:
Example
1: Initializing elements
int a[100] = {1,2,[50]=3,4,5,[23]=6,7};
In this example, designated initializers [50] and [23] are used to initialize elements in the middle
of an array without initializing all the elements prior to them.
The element a[50] contains the value 3 while a[51] is initialized to 4. Similarly, a[23]=6 and a[24]=7.
Example
2: Initializing selected members of a structure
struct node
{
int a;
int b;
char c[10];
};
struct node st = {.b=1,.c[4]=1,2,3,.a=4};
In this example, .b and .a initialize particular members of the structure st while the designated initializer.c[4] initializes a particular array element (index
4) of the structure member.c[4] with 1, c[5] with 2, and so on.
Example
3: Initializing union
union u
{
int x;
int c;
};
union u u1={.c=10};
In the above example, the order of initializing union members
need not be followed using designated initializers.
Example
4: Initializing member-wise
struct st {
int a,b;
char c;
};
struct st s1={.a=1,.c=’x’,.b=10}; //new way
struct st s2={1, 10, ’x’}; //old way
The above example shows how member-wise initialization can
be done using a structure and how initialization was done earlier.
Example
5: Initializing both array and structure
struct st{int a[5], b;};
struct st w[]={ [3]={1,2}, [1]={.a={[3]=1}, .b=1} };
In this example, a combination of both array and structure
are initialized. Here w[1].a[3] has a value of 1.
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