» Beispiel zur Angwendung von tolower bei cpp-Strings

/** Testet eigenen tolower-Ersatz*/
/// std::transform applies the given function to a range and stores the result in another range, beginning at d_first.
/// std::transform: transformiert einen Datensatz mit einer Funktion in einen anderen
//
/// begin()   This function gives a reference to the first element.
/// end()     Returns an iterator to the character following the last character of the string.
///
/// OutputIt transform( InputIt first1, InputIt last1, OutputIt d_first,
///                     UnaryOperation unary_op );
///
///   std::transform applies the given function to a range and stores the result in another range, beginning at d_first.
///
///1) The unary operation unary_op is applied to the range defined by [first1, last1).
///3) The binary operation binary_op is applied to pairs of elements from two ranges: one defined by [first1, last1) and the other beginning at first2.
///2,4) Same as (1,3), but executed according to policy. This overload only participates in overload resolution if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true

#include <iostream>
#include <algorithm>  /// for transform
/* Original:
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;
}*/

///Erste Version 
std::string str_tolower(std::string s)
{
 std::transform(s.begin(), s.end(), s.begin(), [](unsigned char c){ return std::tolower(c); } // correct
                  );
    return s;
}
/// Version tastet Quell-String nicht an
std::string str_toupper(std::string s)
{
 std::transform(s.begin(), s.end(), s.begin(),[](unsigned char c){ return std::toupper(c); } // correct
                 );
    return s;
}
/// Version tastet Quell-String an
void str_tolowerb(std::string *s)
{
 std::transform(s->begin(), s->end(), s->begin(), [](unsigned char c){ return std::tolower(c); } // correct
                  );
}

int main(int argc, char **argv)
{
    std::string s("Exemplar");
    std::string st("TestSTRING ÖÄÜß²³ÖÄÜ tEST");
    std::string sta("");
    std::string stb("");
    std::string stc("");
    *s.begin() = 'e';
    std::cout << s <<'\n';

    auto i = s.cbegin();
    std::cout << *i << '\n';
    ///  *i = 'E'; // error: i is a constant iterator

    auto j = s.end();         //Zeichen nach letzten String-Zeichen ist newline-Zeidchen
    std::cout << *j << '\n';

    sta = st;

    std::cout << "Teststring vorher : " << st << std::endl;
    /// ursprüngliche Version
    std::transform(st.begin(), st.end(), st.begin(), (int (*)(int))std::toupper);
    std::cout << "Teststring nachher: " << st << std::endl;
    stb = str_tolower(sta);
    std::cout << "nach str_tolower  : " << stb << std::endl;
    stc = str_toupper(stb);
    std::cout << "nach str toupper  : " << stc << std::endl;
    str_tolowerb(&stc);
    std::cout << "nach str_tolowerb  : " << stc << std::endl;

 return 0;
}

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