struct intitialization notation不能与堆分配存储一起使用 [英] struct intitialization notation not working with heap allocated storage

问题描述

使用gcc（GCC）4.4.6，我试着编译这个程序 -

With gcc (GCC) 4.4.6 , I try to compile this program -

  1 #include <stdio.h>
  2 #include <stdlib.h>
  3 
  4 int main(int argc, char *argv[])
  5 {
  6 
  7     struct B {
  8         int i;
  9         char ch;
 10     };
 11 
 12     struct B *ptr;
 13 
 14     ptr = (struct B*) calloc(1, sizeof(struct B));
 15 
 16     *ptr = {
 17         .i = 10,
 18         .ch = 'c',
 19     };
 20 
 21     printf("%d,%c\n", ptr->i, ptr->ch);
 22 
 23     return 0;
 24 }
 25 

$ make
gcc -g -Wall -o test test.c 
test.c: In function ‘main’:
test.c:16: error: expected expression before ‘{’ token
make: *** [test] Error 1

推荐答案

*ptr = {
   .i = 10,
   .ch = 'c',
};

这个用法称为指定初始值设定项，顾名思义，它只是用于初始化结构或数组，但您正在尝试的是赋值。

This usage is called designated initializer, as the name implies, it's only used to initialize struct or arrays, but what you are trying to do is assigning.

正确使用指定初始值：

The correct usage of designated initializer:

strcut B foo = {.i = 10, .ch = 'c'};

要分配结构，您仍然需要使用：

To assign the struct, you still need to use:

 ptr->i = 10;
 ptr->ch = 'c';

编辑：或者您可以像@Andrey T's答案：

EDIT: Or you can use a compound literal like in @Andrey T's answer:

*ptr = (struct B) {
  .i = 10,
  .ch = 'c',
};

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