使用标签的 C 结构初始化.它有效，但如何? [英] C struct initialization using labels. It works, but how?

问题描述

我昨天发现了一些结构体初始化代码，这让我陷入了循环.举个例子:

I found some struct initialization code yesterday that threw me for a loop. Here's an example:

typedef struct { int first; int second; } TEST_STRUCT;
void testFunc() {
    TEST_STRUCT test = {
        second: 2,
        first:  1
    };
    printf("test.first=%d test.second=%d
", test.first, test.second);
}

令人惊讶的是(对我而言)，这是输出:

Surprisingly (to me), here's the output:

-> testFunc
test.first=1 test.second=2

如您所见，该结构已正确初始化.我不知道可以这样使用带标签的语句.我已经看到了其他几种进行结构体初始化的方法，但是我没有在任何在线 C 常见问题解答中找到这种结构体初始化的任何示例.有人知道这是如何/为什么工作的吗?

As you can see, the struct gets initialized properly. I wasn't aware labeled statements could be used like that. I've seen several other ways of doing struct initialization, but I didn't find any examples of this sort of struct initialization on any of the online C FAQs. Is anybody aware of how/why this works?

推荐答案

这里是 gcc 手册的一节，解释了结构和数组的指定初始值设定项的语法:

Here is the section of the gcc manual which explains the syntax of designated initializers for both structs and arrays:

在结构初始化器中，指定要初始化的字段的名称'.fieldname =' 在元素值之前.例如，给定以下结构，

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; };

下面的初始化

 struct point p = { .y = yvalue, .x = xvalue }; 

相当于

 struct point p = { xvalue, yvalue }; 

另一种具有相同含义的语法(自 GCC 2.5 起已过时)是fieldname:"，如下所示:

Another syntax which has the same meaning, obsolete since GCC 2.5, is 'fieldname:', as shown here:

 struct point p = { y: yvalue, x: xvalue };

相关页面可以找到这里.

你的编译器应该有类似的文档.

Your compiler should have similar documentation.

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