CODEWITHSUNDEEP
objective-ccinitializationstructure
John
John

Reputation: 2141

Odd use of curly braces in C

Sorry for the simple question but I'm on vacation reading a book on core audio, and don't have my C or Objective C books with me...

What are the curly braces doing in this variable definition?

MyRecorder recorder = {0};

Upvotes: 31

Views: 3613

Answers (6)

lumpidu
lumpidu

Reputation: 759

Unlike C++11, in C99 there has to be at least one element in initializer braces.

C++11 struct:

MyRecorder recorder{};

C struct:

MyRecorder recorder = {0};

Upvotes: 0

Shiplu Mokaddim
Shiplu Mokaddim

Reputation: 57690

Its initializing all members of recorder structure to 0 according to C99 standard. It might seem that it initializes every bit of the structure with 0 bits. But thats not true for every compiler.

See this example code,

#include<stdio.h>

struct s {
    int i;
    unsigned long l;
    double d;
};

int main(){
    struct s es = {0};
    printf("%d\n", es.i);
    printf("%lu\n", es.l);
    printf("%f\n", es.d);
    return 0;
}

This is the output.

$ ./a.out 
0
0
0.000000

Upvotes: 9

Niklas B.
Niklas B.

Reputation: 95358

Assuming that MyRecorder is a struct, this sets every member to their respective representation of zero (0 for integers, NULL for pointers etc.).

Actually this also works on all other datatypes like int, double, pointers, arrays, nested structures, ..., everything you can imagine (thanks to pmg for pointing this out!)

UPDATE: A quote extracted from the website linked above, citing the final draft of C99:

[6.7.8.21] If there are fewer initializers in a brace-enclosed list than there are elements or members of an aggregate, [...] the remainder of the aggregate shall be initialized implicitly the same as objects that have static storage duration.

Upvotes: 26

Bruno Soares
Bruno Soares

Reputation: 796

Actually, it don't initliaze all the elements of the structure, just the first one. But, the others are automatically initialized with 0 because this is what the C standard ask to do.

If you put: MyRecorder recorder = {3};

The first element will be 3 and the others weill be 0.

Upvotes: 5

Sangeeth Saravanaraj
Sangeeth Saravanaraj

Reputation: 16627

MyRecorder could be one of the following and you are attempting to initialize all the elements of that with zero

typedef struct _MyRecorder1 {
    int i;
    int j;
    int k;
}MyRecorder1;

typedef int MyRecorder2[3];

Upvotes: 0

Sergey Kalinichenko
Sergey Kalinichenko

Reputation: 727067

It is an initialization of a structure.

Upvotes: 5

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