How to convert an instance of std string to lower case

Question

I'd want to change a std::string to lowercase.

I am aware of the lower function ().

However, I've had problems with this method in the past, and using it with a std::string would require iterating over each character.

Is there an option that works 100% of the time?

Answer 1

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

std::string data = "Abc";
std::transform(data.begin(), data.end(), data.begin(),
    [](unsigned char c){ return std::tolower(c); });

You're not going to get away with not going over each character. 

Otherwise, there is no way to tell whether the character is lowercase or uppercase.

If you despise tolower(), here's a customised ASCII-only solution I don't recommend:

char asciitolower(char in) {
    if (in <= 'Z' && in >= 'A')
        return in - ('Z' - 'z');
    return in;
}

std::transform(data.begin(), data.end(), data.begin(), asciitolower);

Tolower() can only do single-byte character substitutions, which is inconvenient for many scripts, notably those that use a multi-byte encoding like UTF-8.