Boosting String Performance: Alternatives to std::basic_string::resize_and_overwrite

Resizing and Overwriting

• Then, it calls the provided op function on the string's character buffer. This function object (op) is responsible for modifying the content of the string based on the new size.

• The function first resizes the string to the new_size. If the new size is smaller than the current size, characters are erased from the end. If it's larger, new characters are inserted (default-initialized).

• It takes two arguments:

  • new_size: This specifies the desired new size for the string.
  • op: This is a function object that operates on the string's internal character buffer. It determines how the content is modified.

Key Points

• The op function object should not access the string itself through iterators or references during the modification process.
• It's important to note that this function invalidates all iterators, pointers, and references to the string since the underlying storage might be reallocated.
• resize_and_overwrite is efficient because it modifies the string in-place, avoiding unnecessary copying of data.

std::string str;
str.resize_and_overwrite(20, fill_with_pattern);

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Appending a String (Simple Case)

This code snippet demonstrates appending a single string ("apple") to an existing string:

#include <iostream>
#include <string>

int main() {
  std::string str{"Hello, "};
  str.resize_and_overwrite(str.size() + 5, [&str](char* buf, std::size_t n) noexcept {
    std::memcpy(buf + str.size(), "apple", 5);
    return str.size() + 5;
  });
  std::cout << str << std::endl; // Output: Hello, apple
  return 0;
}

• The lambda function returns the updated size of the string.
• std::memcpy copies the characters from "apple" to the string buffer.
• It calculates the new size by adding the original size and the length of the string to be appended ("apple").
• A lambda function is used as the op argument.

Shrinking and Modifying the Content

This example shows how to shrink the string and modify its content based on a delimiter:

#include <iostream>
#include <string>

int main() {
  std::string str{"This is a sentence: विभाग"}; // विभाग is "department" in Hindi
  str.resize_and_overwrite(str.find(':') + 1, [](char* buf, std::size_t n) noexcept {
    return std::find(buf, buf + n, ' ') - buf;
  });
  std::cout << str << std::endl; // Output: This is a
  return 0;
}

• The lambda function returns the new size based on the found delimiter position.
• This effectively shrinks the string to include only the part before the space.
• The op function finds the position of the first space character after the colon (':').

Filling with a Pattern (Using std::fill)

This code demonstrates using std::fill within the op function to fill the string with a specific character:

#include <algorithm>
#include <iostream>
#include <string>

int main() {
  std::string str;
  str.resize_and_overwrite(10, [](char* buf, std::size_t n) noexcept {
    std::fill(buf, buf + n, '*');
    return n;
  });
  std::cout << str << std::endl; // Output: **********
  return 0;
}

• The op function simply returns the new size, which is the same as the requested size (10).
• std::fill is used within the lambda function to fill the entire character buffer with asterisks (*).

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Manual Resizing and Manipulation

• It can be less efficient for larger strings or frequent modifications compared to resize_and_overwrite.
• This involves using std::string::resize to adjust the size and then modifying the content using iterators or loops.

std::string str = "Hello";
str.resize(10, ' '); // Resize with padding
for (int i = 5; i < 10; ++i) {
  str[i] = '*'; // Modify specific characters
}

std::string::assign with Iterators

• This is useful when you already have the modified content prepared elsewhere.
• You can use std::string::assign along with iterators to specify a new range of characters for the string.

std::string str = "World";
std::string modified = "New World";
str.assign(modified.begin(), modified.begin() + 5); // Assign first 5 characters

String Streams (std::stringstream)

• It allows you to create a stream object and manipulate the string content through insertion and extraction operations.
• If you need to perform complex string manipulations that involve building a new string, consider using std::stringstream.

std::stringstream ss;
ss << "Original ";
ss << "Content";
std::string new_str = ss.str();

Choosing the Right Alternative

The best alternative depends on your specific use case. Here are some general guidelines:

• For complex string manipulation scenarios, std::stringstream can be a versatile tool.
• If you need more control over the modification process or have the new content readily available, consider manual resizing or std::string::assign.
• Use std::basic_string::resize_and_overwrite when you want to modify the string in-place for efficiency, especially for larger strings.