How to convert std::string to lower case?


Question

I want to convert a std::string to lowercase. I am aware of the function tolower(), however in the past I have had issues with this function and it is hardly ideal anyway as use with a std::string would require iterating over each character.

Is there an alternative which works 100% of the time?

1
706
6/14/2018 8:15:17 AM

Accepted Answer

Adapted from Not So Frequently Asked Questions:

#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 really not going to get away without iterating through each character. There's no way to know whether the character is lowercase or uppercase otherwise.

If you really hate tolower(), here's a specialized ASCII-only alternative that I don't recommend you use:

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

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

Be aware that tolower() can only do a per-single-byte-character substitution, which is ill-fitting for many scripts, especially if using a multi-byte-encoding like UTF-8.

849
7/6/2019 12:24:38 AM

Boost provides a string algorithm for this:

#include <boost/algorithm/string.hpp>

std::string str = "HELLO, WORLD!";
boost::algorithm::to_lower(str); // modifies str

Or, for non-in-place:

#include <boost/algorithm/string.hpp>

const std::string str = "HELLO, WORLD!";
const std::string lower_str = boost::algorithm::to_lower_copy(str);

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