Comparison function as argument to use later on

Question

I am working on generic binary search trees and I have troubles to understand how passing function as parameter/argument works in C. I want to tell to my generic BST that it must use the compare_doubles() when a comparison is needed.

How it possible to instantiate a function pointer somewhere in my code when I am creating my empty BST in createBST() ?

Given this BST.h :

#include <stddef.h>
#include <stdbool.h>

typedef struct tree_t BinarySearchTree;

/* ------------------------------------------------------------------------- *
 * Creates an empty BST.
 * ARGUMENT
 * comparison_fn_t      A comparison function
 * BinarySearchTree bst = newBST(&compare_doubles);
 * ------------------------------------------------------------------------- */

BinarySearchTree* createBST(int comparison_fn_t(const void *, const void *));
bool insertInBST(BinarySearchTree* bst, const void* key, const void* value);

BST.c :

#include <stddef.h>
#include <stdlib.h>
#include "BinarySearchTree.h"

struct tree_t {
    const void *key;
    const void *value;
    const void *leftChild;
    const void *rightChild;
    //const void *comparison_fn_t; //try to have it as an argument 1st try
};

//int (*compare) (const void *, const void *); //try to save it for later 2nd try

BinarySearchTree* newBST(int comparison_fn_t(const void*, const void*)) {
    BinarySearchTree *binarySearchTree = malloc(sizeof(BinarySearchTree));
    if (!binarySearchTree)
        return NULL;
    binarySearchTree->value = NULL;
    binarySearchTree->key = NULL;
    binarySearchTree->leftChild = NULL;
    binarySearchTree->rightChild = NULL;
    compare = &comparison_fn_t; //1st try
    //binarySearchTree->comparison_fn_t = comparison_fn_t //2nd try
    return binarySearchTree;
}
bool insertInBST(BinarySearchTree* bst, const void* key, const void* value) {
    //other code
    int val = (*compare) (key, bst->key); //1st try
    int val = bst->comparison_fn_t... //2nd try
}

In my Main.c :

int compare_doubles(const void *a, const void *b) {
    const double *a_ = (const double*) a;
    const double *b_ = (const double*) b;
    return (*a_ > *b_) - (*a_ < *b_);
}

BinarySearchTree *searchTree= createBST(&compare_doubles);
insertInBST(searchTree, key, value);

Answer 1

Just use the same function pointer definition in the struct:

struct tree_t {
    const void *key;
    const void *value;
    const void *leftChild;
    const void *rightChild;
    int (*comparison_fn)(const void*, const void*);
};

BinarySearchTree* newBST(int (*comparison_fn)(const void*, const void*)) {
    
    ...
    // assign the fn pointer here
    binarySearchTree->comparison_fn = comparison_fn;
    return binarySearchTree;
}

To make the syntax simpler, you can use a typedef:

typedef int (*BinaryComparison)(const void*, const void*); 

struct tree_t {
    const void *key;
    const void *value;
    const void *leftChild;
    const void *rightChild;
    BinaryComparison comparison_fn;
};

BinarySearchTree* newBST(BinaryComparison comparison_fn) {
    BinarySearchTree* binarySearchTree = malloc(sizeof *binarySearchTree);
    binarySearchTree->comparison_fn = comparison_fn;
    return binarySearchTree;
}