c string implementation

string_demo.c

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <assert.h>

const size_t INITIAL_SIZE = 16;

typedef struct String
{
    size_t len;
    size_t cap;
    char *chars;

    size_t (*length)(struct String *this);
    void (*append)(struct String *this, char *s);
} String;

size_t length(String *this)
{
    return this->len;
}

void append(String *this, char *s)
{
    size_t s_len = strlen(s);
    if (this->len + s_len >= this->cap)
    {
        size_t new_cap = this->cap << 1;
        // 防止溢出
        assert(new_cap != 0);

        this->chars = realloc(this->chars, new_cap);
        this->cap = new_cap;
    }

    for (size_t i = 0; i < s_len; i++)
    {
        this->chars[this->len + i] = s[i];
    }
    this->chars[this->len + s_len] = '\0';
    this->len = this->len + s_len;
}

String new_string()
{
    String s = {
        .len = 0,
        .cap = INITIAL_SIZE};
    s.chars = malloc(INITIAL_SIZE);
    memset(s.chars, '\0', INITIAL_SIZE);
    s.append = append;
    s.length = length;
    return s;
}

int main(int argc, char const *argv[])
{
    String s = new_string();
    s.append(&s, "abcd");
    s.append(&s, "abcd");
    s.append(&s, "abcd");
    printf("%s, length: %zu, cap: %zu\n", s.chars, s.length(&s), s.cap);
    return 0;
}