大文字を小文字に、小文字を大文字に変換(C++版)

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

char toupper_and_tolower(char c)
{
    if (std::isupper(c)) {
        c = (char)std::tolower(c);
    }
    else if (std::islower(c)) {
        c = (char)std::toupper(c);
    }
    return c;
}

int main()
{
    std::string s = "Yahoo Google";
    std::string d;
    d.resize(s.size());

    std::transform(s.begin(), s.end(), d.begin(), toupper_and_tolower);
    std::cout << s << std::endl;
    std::cout << d << std::endl;

}