Dynamic String Builder

Part of the Object Oriented Programming section of Coddy's C journey — lesson 59 of 61.

Challenge

Easy

Let's build a StringBuilder — a dynamic string container that grows automatically as you append text. This is a practical utility that combines dynamic memory management with the OOP patterns you've mastered throughout this course.

You'll organize your code across three files:

stringbuilder.h: Declare the StringBuilder struct with three members: a char* buffer, a size_t for the current length, and a size_t for the total capacity. Declare function prototypes for creating a StringBuilder, appending text to it, retrieving the final string, and freeing the memory. Include guards are essential.
stringbuilder.c: Implement your StringBuilder system:
- create_stringbuilder — allocates a StringBuilder on the heap, initializes the buffer with an initial capacity of 16 bytes, sets length to 0, and ensures the buffer starts as an empty string (null-terminated)
- append — takes a StringBuilder pointer and a string to append. Calculate the new required length. If it exceeds capacity, double the capacity (repeatedly if needed) and use realloc to grow the buffer. Then concatenate the new text using strcat
- get_string — returns a pointer to the internal buffer (read-only access)
- free_stringbuilder — frees the buffer first, then the StringBuilder struct itself
main.c: Read an integer indicating how many strings to append. Then read each string using fgets and append it to your StringBuilder. After all strings are added, print the complete assembled string using get_string. Finally, free the StringBuilder.

Two key functions used in this challenge:

realloc(ptr, new_size) — resizes a previously allocated block of memory. It takes a pointer to the existing block and the new desired size in bytes, and returns a pointer to the (possibly moved) resized block. Use it in append when the buffer needs to grow:
sb->buffer = (char*)realloc(sb->buffer, sb->capacity);
fgets(buffer, size, stream) — reads a line of text from an input stream into a character array, stopping at a newline or when size - 1 characters have been read. It keeps the newline character in the buffer, so you'll need to strip it manually. Use it in main.c to read each input string:
fgets(line, sizeof(line), stdin);
To strip the trailing newline: check if the last character is '\n' and replace it with '\0'.

Your program will receive:

The number of strings to append
Each string on a separate line

Example output when inputs are 3, then Hello, , World:

Hello World

Example output when inputs are 4, then C, is, a, great language!:

C is a great language!

Example output when inputs are 1, then SingleString:

SingleString

Remember to update the length field after each append operation. When checking if reallocation is needed, account for the null terminator — the buffer needs space for length + new_text_length + 1 bytes. Use strlen from <string.h> to measure string lengths. Also call getchar() after scanf in main.c to consume the leftover newline before reading strings with fgets.

Try it yourself

#include <stdio.h>
#include <stdlib.h>
#include "stringbuilder.h"

int main() {
    int n;
    scanf("%d", &n);
    getchar(); // consume newline after number
    
    // TODO: Create a StringBuilder using create_stringbuilder()
    
    // TODO: Read n strings and append each to the StringBuilder
    // Hint: Use fgets or similar to read each line
    // Remember to handle the newline character from fgets if you use it
    
    // TODO: Print the complete assembled string using get_string()
    
    // TODO: Free the StringBuilder using free_stringbuilder()
    
    return 0;
}