关于#c++#的问题：输入字符转成小写字母

1.输入一个字符，将其转换成小写字母后输出的程序，怎么编译呀

#include<iostream>
using namespace std;
int main()
{
    char ch;
    std::cin >> ch;
    if ('A' <= ch && ch <= 'Z')
        ch += 32;
    std::cout << ch << endl;
}

或者这样：

#include<iostream>
#include<cctype>
using namespace std;
int main()
{
    char ch;
    std::cin >> ch;
    if (isupper(ch))
    {
        ch = tolower(ch);
    }
    std::cout << ch << endl;
}

或者这样

#include<iostream>
using namespace std;
int main()
{
    char ch;
    std::cin >> ch;
    switch (ch)
    {
        case 'A':
            ch = 'a';
            break;
        case 'B':
            ch = 'b';
            break;
        case 'C':
            ch = 'c';
            break;
        case 'D':
            ch = 'd';
            break;
        case 'E':
            ch = 'e';
            break;
        case 'F':
            ch = 'f';
            break;
        case 'G':
            ch = 'g';
            break;
        case 'H':
            ch = 'h';
            break;
        case 'I':
            ch = 'i';
            break;
        case 'J':
            ch = 'j';
            break;
        case 'K':
            ch = 'k';
            break;
        case 'L':
            ch = 'l';
            break;
        case 'M':
            ch = 'm';
            break;
        case 'N':
            ch = 'n';
            break;
        case 'O':
            ch = 'o';
            break;
        case 'P':
            ch = 'p';
            break;
        case 'Q':
            ch = 'q';
            break;
        case 'R':
            ch = 'r';
            break;
        case 'S':
            ch = 's';
            break;
        case 'T':
            ch = 't';
            break;
        case 'U':
            ch = 'u';
            break;
        case 'V':
            ch = 'v';
            break;
        case 'W':
            ch = 'w';
            break;
        case 'X':
            ch = 'x';
            break;
        case 'Y':
            ch = 'y';
            break;
        case 'Z':
            ch = 'z';
            break;
    }
    std::cout << ch << endl;
}

经过验证，题不对（没说明规则）。或者
std::cin >> ch;
std::cout << ‘a' << endl;