Design Pattern Adapter in C

Definition:

Adapter is a structural design pattern that allows objects with incompatible interfaces to collaborate.

I have to use incompatible callback function pointers with a difference in the first parameter.

We know objects have key-value pairs, and arrays have index-value pairs. char * vs int.

void object_callback(char *key, void *value) {
    printf("%d: %s\n", index, value);
}

void array_callback(int index, void *value) {
    printf("%d: %s\n", index, value);
}

Use Case

void process_object(void (*callback)(char *key, void *value)) {
    // callback if key-value-based properties
    callback(key, value);

    // callback for array-based properties
    callback((int)key, value); // trying to implement adapter pattern for this
}

I could have used void * but looking for an elegant solution based on Adapter Design Pattern.

My Attempt

#include <stdio.h>

typedef struct Adapter {
    void (*obj)(char *key, void *value);
    void (*arr)(int index, void *value);
} Adapter;

void property_callback(char *key, void *value) {
    printf("%s: %s\n", key, value);
}

void array_callback(int index, void *value) {
    printf("%d: %s\n", index, value);
}

Solution

You need to create an adapter function that will bridge the gap between the two different callback interfaces. Here is a simple example of using adapter functions with different callbacks.

#include <stdio.h>

// Define the original callback functions
void objectCallback(char *key, void *value) {
    printf("%s: %s\n", key, (char*)value);
}

void arrayCallback(int index, void *value) {
    printf("%d: %s\n", index, (char*)value);
}

// Adapter structure to encapsulate the original callbacks
typedef struct {
    void (*callback)(void* keyOrIndex, void *value);
} Adapter;

// Adapter function for objectCallback
void objectAdapter(void* key, void *value) {
    objectCallback((char*)key, value);
}

// Adapter function for arrayCallback
void arrayAdapter(void* key, void *value) {
    int idx = (int)(long)key; // Cast void pointer to int
    arrayCallback(idx, value);
}

int main() {
    // Create an adapter for objectCallback
    Adapter objectAdapterInstance = {objectAdapter};

    // Create an adapter for arrayCallback
    Adapter arrayAdapterInstance = {arrayAdapter};

    // Demonstrate the use of the adapters
    char* k = "property_key";
    char* v = "property_value";

    // Use the object adapter
    objectAdapterInstance.callback(k, v);

    // Use the array adapter
    int i = 123;
    arrayAdapterInstance.callback((void*)(long)i, v);

    return 0;
}