c - 当我们使用新的结构初始化格式时，未列出的成员是否有未指定的值？

Question

所以我需要准备一个大型结构:

struct Config {
    int a;
    int b;
    int c;
    struct { int x; int y; } d[40];
};

我想这样填写:

Config config = {
    .a = 3;
    .b = 4;
    .d[0] = {10, 12};
    .d[1] = {14, 16};
};

在此之后，config.c 和config.d[2] 的值是否有未指定的值？还是零？

或者，我需要做:

Config config;
memset(&config, 0, sizeof(config));
config.a = 3;
config.b = 4;
config.d[0].x = 10;
config.d[0].y = 12;
...

Answer 1

它们被初始化为零。

来自 C99 标准，§6.7.8，第 19 项:

The initialization shall occur in initializer list order, each initializer provided for a particular subobject overriding anypreviously listed initializer for the same subobject; all subobjects that are not initialized explicitly shall be initialized implicitly the same as objects that have static storage duration.

static 对象的初始化规则在前面的第 10 项中指定:

If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate. If an object that has static storage duration is not initialized explicitly, then:

• if it has pointer type, it is initialized to a null pointer;
• if it has arithmetic type, it is initialized to (positive or unsigned) zero;
• if it is an aggregate, every member is initialized (recursively) according to these rules;
• if it is a union, the first named member is initialized (recursively) according to these rules.