Source code from the "Insane Shadow Data Trick in C" YouTube video https://www.youtube.com/watch?v=gtk3RZHwJUA

arr.c

// cc -o arr arr.c && ./arr
#include <assert.h>
#include <stdio.h>
#include <stdlib.h>

typedef struct {
    size_t count;
    size_t capacity;
} Header;

#define ARR_INIT_CAPACITY 1

#define arr_push(arr, x)                                                              \
    do {                                                                              \
        if (arr == NULL) {                                                            \
            Header *header = malloc(sizeof(*arr)*ARR_INIT_CAPACITY + sizeof(Header)); \
            header->count = 0;                                                        \
            header->capacity = ARR_INIT_CAPACITY;                                     \
            arr = (void*)(header + 1);                                                \
        }                                                                             \
        Header *header = (Header*)(arr) - 1;                                          \
        if (header->count >= header->capacity) {                                     \
            header->capacity *= 2; \
            header = realloc(header, sizeof(*arr)*header->capacity + sizeof(Header)); \
            arr = (void*)(header + 1);                                                \
        } \
        (arr)[header->count++] = (x);                                                 \
    } while(0)

#define arr_len(arr) ((Header*)(arr) - 1)->count

#define arr_free(arr) free((Header*)(arr) - 1)

int main()
{
    float *numbers = NULL;
    arr_push(numbers, 69);
    arr_push(numbers, 420);
    arr_push(numbers, 1337);
    arr_push(numbers, 80085);
    if (1) arr_push(numbers, 111); else arr_push(numbers, 222);
    for (size_t i = 0; i < arr_len(numbers); ++i) {
        printf("%f\n", numbers[i]);
    }
    arr_free(numbers);
    return 0;
}

//////////////////////////////

#if 0
#define STB_DS_IMPLEMENTATION
#include "stb_ds.h"

typedef struct {
    const char *key;
    size_t value;
} Item;

Item *table = NULL;

int main2()
{
    shput(table, "foo", 69);
    shput(table, "bar", 420);
    shput(table, "baz", 1337);
    table[shgeti(table, "bar")].value = 80085;
    for (int i = 0; i < shlen(table); ++i) {
        printf("%s => %zu\n", table[i]);
    }
    return 0;
}
#endif

Just curious concerning printf("%s => %zu\n", table[i]); , could this be undefined behavior? i guess due to the how the fields of the Item structures field values are laid out in contiguous memory, and printf being variadic , this will print out the fields as specified by the printf format specifiers and all seems good and it is good =) But if this is the case, I am slightly concerned that your cc compilation did not spit out a warning? Great show!

Had the same doubt.. seems to be something gcc does as msvc doesn't print the values. It also only seems to work if the first element is a char array, because passing a struct with two numbers doesn't work.
You can play here https://godbolt.org/z/d8Eqaecxj

interesting, even stranger if i do something like;
https://godbolt.org/z/34qE15hef
that also works with 2 int's as fields of the struct. i have not looked into the assembler to see what is going on under the covers yet between the difference impl.s etc. cheers.