Modify String In-Place with resize_and_overwrite in C++23

Manipulating string contents often involves resizing and updating characters, which traditionally required multiple steps: resizing the string, then filling or modifying its elements. This approach can be verbose and may create unnecessary overhead when working with large strings or frequent updates.

For example, a common pattern might look like this:

#include <iostream>

int main() {
    std::string letters;
    letters.reserve(5);

    for (char c = 'a'; c <= 'e'; ++c) {
        letters += c;
    }
    std::cout << letters << std::endl; // abcde

    return 0;
}

Here, the string must be manually appended character by character, which works but requires separate operations for resizing and updating. This can be inefficient in situations where the exact size of the final string is known in advance.

Since C++23, we can use the resize_and_overwrite function, allowing the string to be resized and its contents modified in a single, efficient operation.

#include <iostream>

int main() {
    std::string letters;

    letters.resize_and_overwrite(5, [](char *buffer, size_t size) -> size_t {
        char c = 'a';
        for (size_t i = 0; i < size; ++i) {
            buffer[i] = c++;
        }

        return size;
    });
    std::cout << letters << std::endl; // abcde

    return 0;
}

In this example, the lambda receives direct access to the internal buffer of the string. It fills the allocated space, then returns the number of valid characters. This approach avoids extra copies, streamlines the workflow, and improves performance, especially in scenarios involving large or frequently modified strings.