A Deeper Look at the std::string Class

Converting String Case in C++

What's the best way to convert all characters in a std::string to uppercase or lowercase?

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Converting the case of characters in a std::string is a common operation in text processing. C++ provides functions in the <algorithm> and <cctype> headers to accomplish this task efficiently.

Here's how you can convert a string to uppercase or lowercase:

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

int main() {
  std::string text{"Hello, World!"};

  // Convert to uppercase
  std::string uppercase{text};
    uppercase, uppercase.begin(),
    [](unsigned char c) {
      return std::toupper(c);

  // Convert to lowercase
  std::string lowercase{text};
    lowercase, lowercase.begin(),
    [](unsigned char c) {
      return std::tolower(c);

  std::cout << "Original: " << text << '\n';
  std::cout << "Uppercase: " << uppercase << '\n';
  std::cout << "Lowercase: " << lowercase << '\n';
Original: Hello, World!
Uppercase: HELLO, WORLD!
Lowercase: hello, world!

Let's break down the approach:

  1. We use std::ranges::transform() (C++20) to apply a transformation to each character.
  2. The lambda function [](unsigned char c){ return std::toupper(c); } is used to convert each character to uppercase. We use unsigned char to avoid undefined behavior with negative char values.
  3. std::toupper() and std::tolower() from <cctype> are used for the actual character conversion.

If you're using a pre-C++20 compiler, you can use std::transform() instead:

  text.begin(), text.end(), text.begin(),
  [](unsigned char c) {
    return std::toupper(c);

Note that this method works well for ASCII characters, but it may not handle Unicode characters correctly. For proper Unicode support, you might need to use a library like ICU (International Components for Unicode).

Also, be aware that these functions modify the string in-place. If you need to preserve the original string, make a copy before transforming, as shown in the example.

Remember, when working with std::string, it's often more efficient to modify the string in-place rather than creating a new string for each operation. This approach minimizes memory allocations and copies, leading to better performance, especially for longer strings or frequent operations.

Answers to questions are automatically generated and may not have been reviewed.

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