Member Avatar for clutchkiller

Is there a way to use toUpper with a string, or do you have to loop it through an array of chars? Ive been googling everywhere, but havent really found any examples on how to properly use it. Thanks

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  • Latest Post Latest Post by Narue

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As asked: there isn’t a single built‑in std::string::toUpper member. As already showed, the usual approach is to convert each character. Two important practical points: always include the right headers (<algorithm>, <cctype>, <locale> where used), and never pass a possibly‑negative char directly to std::toupper — convert it to unsigned char first to avoid undefined behavior.

A compact, safe pattern that matches the “algorithm” style is to wrap std::toupper in a small lambda and use std::transform so you can do the change in place:

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

std::string s = "Hello, world!";
std::transform(s.begin(), s.end(), s.begin(),
               [](unsigned char c){ return static_cast<char>(std::toupper(c)); });

If you need locale‑aware behavior (so case mapping follows the current locale), use the std::ctype<char> facet on a locale and apply its toupper range function. That works for single‑byte encodings and locale rules, but it still operates on bytes, not Unicode code points:

#include <locale>
#include <string>

std::locale loc(""); // user locale
std::string out(s.size(), '\0');
std::use_facet<std::ctype<char>>(loc).toupper(&s[0], &s[0] + s.size(), &out[0]);

For true UTF‑8 / full Unicode case folding (Turkish dotted/dotless i, multichar mappings, etc.) use a Unicode library such as ICU or Boost.Locale; std::toupper/std::ctype won’t do full Unicode case conversions reliably. Common troubleshooting: check headers, avoid passing signed char into toupper, and prefer the lambda/facet approaches above over using std::toupper directly as a function pointer.

Member Avatar for Narue

>Ive been googling everywhere, but havent really
>found any examples on how to properly use it.
I suspect what you found is what most of us would tell you:

for (string::size_type i = 0; i < s.size(); i++)
  s[i] = std::toupper((unsigned char)s[i]);

You'll probably also find the broken:

std::transform(s.begin(), s.end(), s.begin(), std::toupper);

And variations of the corrected version:

template <typename CharT>
struct Upper {
  CharT operator()(CharT c)
  {
    return (CharT)std::toupper((unsigned char)c);
  }
};

std::transform(s.begin(), s.end(), s.begin(), Upper<char>());
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