C++将字符串转换成大写、小写的函数

#include <string>
 
std::string toLower( std::string str )
{
    for ( int i = 0; i < str.length(); i++ )
    {
        /* Only convert upper-case letters */
        if ( str[i] > 64 && str[i] < 91 )
            str[i] += 32;
    }
    return str;
}
 
std::string toUpper( std::string str )
{
    for ( int i = 0; i < str.length(); i++ )
    {
        /* Only convert lower-case letters */
        if ( str[i] > 96 && str[i] < 123 )
            str[i] -= 32;
    }
    return str;
}
 
bool isNpos( size_t iter )
{
    return iter == std::string::npos;
}