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std::islower(std::locale) - cppreference.com

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template< class CharT > bool islower( CharT ch, const locale& loc );

Checks if the given character is classified as a lowercase alphabetic character by the given locale's std::ctype facet.

Parameters

ch - character
loc - locale

Return value

Returns true if the character is classified as lowercase, false otherwise.

Possible implementation

template<class CharT>
bool islower(CharT ch, const std::locale& loc)
{
    return std::use_facet<std::ctype<CharT>>(loc).is(std::ctype_base::lower, ch);
}

Example

Demonstrates the use of islower() with different locales (OS-specific).

#include <iostream>
#include <locale>

int main()
{
    const wchar_t c = L'\u03c0'; // GREEK SMALL LETTER PI

    std::locale loc1("C");
    std::cout << std::boolalpha
              << "islower('π', C locale) returned "
              << std::islower(c, loc1) << '\n'
              << "isupper('π', C locale) returned "
              << std::isupper(c, loc1) << '\n';

    std::locale loc2("en_US.UTF8");
    std::cout << "islower('π', Unicode locale) returned "
              << std::islower(c, loc2) << '\n'
              << "isupper('π', Unicode locale) returned "
              << std::isupper(c, loc2) << '\n';
}

Possible output: 

islower('π', C locale) returned false
isupper('π', C locale) returned false
islower('π', Unicode locale) returned true
isupper('π', Unicode locale) returned false

See also