::TOUPPER

TOUPPER

Converting a string to uppercase with transform() and ::toupper requires a few specific headers. The <algorithm> header provides access to the std::transform() function, which applies an operation across a sequence of elements. The <cctype> header supplies the global ::toupper function used to convert characters to uppercase. Finally, the <string> header is needed when working with std:string objects as the source or destination of the transformation. Together, these headers ensure the code is portable, standards-compliant, and avoids relying on compiler-specific behavior.

std::transform(str.begin(), str.end(), str.begin(), ::toupper)
 
 * str.begin(),  // The beginning of the range (start of the string)
   str.end(),    // The end of the range (end of the string)
   str.begin(),  // The destination where transformed elements are stored (same string)
   ::toupper     // The transformation applied to each element (convert to uppercase) 

transform(
    variableName.begin(),         // 1. Start of the range
    variableName.end(),           // 2. End of the range
    variableName.begin(),         // 3. Destination start (same as source, so changes in place)
    ::toupper                     // 4. Operation to apply (convert each char to uppercase)
);

 * variableName.begin() – Iterator pointing to the first character of the string.
 * variableName.end() – Iterator pointing just past the last character.
 * variableName.begin() (again) – The place where transformed characters will be written.
    - Since this is the same as the input range, it modifies the string in place.
    - If a different destination is specified, then the original isn't modified
 * ::toupper – A function from <cctype> that converts a single char to uppercase.
    - The :: means “use the global scope” to make sure we call the C library toupper instead of something else.

VERSION 1: MODIFICATION IN-PLACE

#include <iostream>     // For input and output
#include <algorithm>    // For transform
#include <cctype>       // supplies the global ::toupper
#include <string>       // required for std::string objects

using namespace std;

int main() 
{
  string str = "coding is fun";                              // Original string
  transform(str.begin(), str.end(), str.begin(), ::toupper); // Convert to uppercase
  cout << "Uppercase String: " << str << endl;               // Display result
  return 0;
}

VERSION 2: ORIGINAL NOT MODIFIED

#include <iostream>
#include <algorithm>
#include <string>
#include <cctype>
using namespace std;

int main() 
{
    string text = "Hello";                                              // original data
    string result(text.size(), '\0');                                   // separate container for results
    
    // Transform into uppercase but write into `result`, not `text`
    transform(text.begin(), text.end(), result.begin(), ::toupper);

    // Print original
    cout << "Original: " << text << endl;                              // Hello

    // Print transformed
    cout << "Transformed: " << result << endl;                         // HELLO
    
    return 0;
}