C中花括号的奇怪用法

Question

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};

Answer 1

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.

Answer 2

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

Answer 3

它是一个结构的初始化。

Answer 4

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.

Answer 5

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};

Answer 6

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];