将 void* 转换为结构

Question

I know that somehow similar question was asked but I couldn't figure out the answer from it. Now I have some specific sturcts and one generic struct with '''void*''' the problem is depending on given parameter in a function I should cast this generic pointer to pointer to struct, when I try to access members in struct compiler doesn't accept this

I want to return struct gen cointaing the data of struct a or struct b

example code

typedef enum
{
    chooseA = 0,
    chooseB = 1,

} chooseVal;

typedef struct
{
    uint16_t x1;
    uint8_t  x2;
    uint8_t* x3;
} a;

typedef struct
{
    uint16_t y1;
    uint8_t  y2;
    uint8_t* y33;
} b;

typedef struct
{
    chooseVal z;
    void* object;
} gen;

void readStruct(gen* out, uint16_t val, uint16_t X1)
{
    out->z==val;
    if(out->z==chooseA)
    {
        a* A;
        A=(a*)out->object;
        out->object->x1 = X1;/*This is the not working line */
    }
}

Answer 1

The specific problem is caused by you still accessing the void pointer instead of the struct pointer A . The whole reason why you created A must be for this purpose, so simply do A->x1 = X1; .

However, this is needlessly complicated. Since both structs have indentical members and just different variable names, this is a perfect case for a union , which means that the struct members could be allocated together with the enum instead of somewhere else. With standard C you can do this:

typedef enum { A, B } choose_val_t;

typedef union
{
  struct
  {
    uint16_t x1;
    uint8_t  x2;
    uint8_t* x3;
  };
  struct
  {
    uint16_t y1;
    uint8_t  y2;
    uint8_t* y3;
  };
} ab_t;

typedef struct
{
  choose_val_t val;
  ab_t ab;
} gen_t;

void readStruct(gen_t* out, choose_val_t val, uint16_t X1)
{
  out->val = val;
  switch(val)
  {
    case A: out->ab.x1 = X1; return ;
    case B: /* something */  return ;
  }
}