如何初始化堆上的结构的const成员

Question

我想在堆上分配一个结构,初始化它并从函数返回一个指向它的指针.我想知道在这种情况下我是否有办法初始化结构的const成员:

#include <stdlib.h>

typedef struct {
  const int x;
  const int y;
} ImmutablePoint;

ImmutablePoint * make_immutable_point(int x, int y)
{
  ImmutablePoint *p = (ImmutablePoint *)malloc(sizeof(ImmutablePoint));
  if (p == NULL) abort();
  // How to initialize members x and y?
  return p;
}

我应该从中得出结论,在包含const成员的堆上分配和初始化结构是不可能的？

Answer 1

像这样:

ImmutablePoint *make_immutable_point(int x, int y)
{
  ImmutablePoint init = { .x = x, .y = y };
  ImmutablePoint *p = malloc(sizeof *p);

  if (p == NULL) abort();
  memcpy(p, &init, sizeof *p);

  return p;
}

(注意,与C++不同,不需要malloc在C中强制转换返回值,并且它通常被认为是错误的形式,因为它可以隐藏其他错误).

Answer 2

如果这是C而不是C++,除了颠覆类型系统之外,我没有看到任何解决方案.

ImmutablePoint * make_immutable_point(int x, int y)
{
  ImmutablePoint *p = malloc(sizeof(ImmutablePoint));
  if (p == NULL) abort();

  // this 
  ImmutablePoint temp = {x, y};
  memcpy(p, &temp, sizeof(temp));

  // or this
  *(int*)&p->x = x;
  *(int*)&p->y = y;

  return p;
}