Pointer to incomplete class type is not allowed

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i am trying to do this function but i have the error

pointer to incomplete class type is not allowed

when i use variable of Map !!!

What I have tried:

C++

typedef struct Map_t* Map;

struct map_t
{
    char key;
    char value;
    int iterator;
    struct map_t* next;
};

Map mapCreate()
{
    Map new_map = (Map)malloc(sizeof(struct map_t));
    if (new_map == NULL)
    {
        return  NULL;
    }

    new_map->key = NULL;
    new_map->value = NULL;
    new_map->next = NULL;
    new_map->iterator = 0;
    return new_map;
}

Posted 13-Apr-20 5:12am

Karam-sa

Updated 25-Jun-22 13:08pm

Richard MacCutchan

v3

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steveb · Answer 1 · 2020-04-13T13:52:00

Solution 2

You using struct map_t as a member of the struct. This requires the "forward declaration"

C++

/*Forward declaration*/
struct map_t;

struct map_t
{
    char key;
    char value;
    int iterator;
    struct map_t* next;/*at this point this is still an incomplete type thus the forward declaration above*/
};

Posted 13-Apr-20 13:52pm

steveb

Updated 13-Apr-20 13:54pm

v2

Comments

Rick York 14-Apr-20 4:17am

No, a forward declaration is not required. The code will compile without it.

-edit- that is, VS2017 will compile it.

steveb 14-Apr-20 13:42pm

Yes, you right my bad. I think some older compilers required forward decl

merano99 · Answer 2 · 2022-06-25T13:08:00

The reason for the error is obviously that map_t capitalized does not correspond to any declaration.

C

// typedef struct Map_t* Map;
typedef struct map_t* Map;

Additionally warnings are generated, because NULL corresponds to a null pointer, but key and value are defined as char.

I also wonder why the source code was tagged as C++, although pure C is used here.
With C++ I would suggest a class instead of the structure and the function and new() instead of the function malloc().