# std::tolower

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< cpp‎ | string‎ | byte

Defined in header `<cctype>` |  |
int tolower( int ch ); |  |

Converts the given character to lowercase according to the character conversion rules defined by the currently installed C locale.

In the default "C" locale, the following uppercase letters `ABCDEFGHIJKLMNOPQRSTUVWXYZ` are replaced with respective lowercase letters `abcdefghijklmnopqrstuvwxyz`.

### Parameters

ch | - | character to be converted. If the value of `ch` is not representable as unsigned char and does not equal EOF, the behavior is undefined

### Return value

Lowercase version of `ch` or unmodified `ch` if no lowercase version is listed in the current C locale.

### Notes

Like all other functions from `<cctype>`, the behavior of `std::tolower` is undefined if the argument's value is neither representable as unsigned char nor equal to `EOF`. To use these functions safely with plain chars (or signed chars), the argument should first be converted to unsigned char:

    char my_tolower(char ch)
    {
        return static_cast<char>(std::tolower(static_cast<unsigned char>(ch)));
    }

Similarly, they should not be directly used with standard algorithms when the iterator's value type is char or signed char. Instead, convert the value to unsigned char first:

    std::string str_tolower(std::string s) {
        std::transform(s.begin(), s.end(), s.begin(),
                    // static_cast<int(*)(int)>(std::tolower)         // wrong
                    // [](int c){ return std::tolower(c); }           // wrong
                    // [](char c){ return std::tolower(c); }          // wrong
                       [](unsigned char c){ return std::tolower(c); } // correct
                      );
        return s;
    }

### Example

Run this code

    #include <iostream>
    #include <cctype>
    #include <clocale>

    int main()
    {
        unsigned char c = '\xb4'; // the character Ž in ISO-8859-15
                                  // but ´ (acute accent) in ISO-8859-1

        std::setlocale(LC_ALL, "en_US.iso88591");
        std::cout << std::hex << std::showbase;
        std::cout << "in iso8859-1, tolower('0xb4') gives "
                  << std::tolower(c) << '\n';
        std::setlocale(LC_ALL, "en_US.iso885915");
        std::cout << "in iso8859-15, tolower('0xb4') gives "
                  << std::tolower(c) << '\n';
    }

Output:

    in iso8859-1, tolower('0xb4') gives 0xb4
    in iso8859-15, tolower('0xb4') gives 0xb8

### See also

toupper | converts a character to uppercase (function)
tolower(std::locale) | converts a character to lowercase using the ctype facet of a locale (function template)
towlower | converts a wide character to lowercase (function)

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