C++ STL Uppercase/ Lowercase string

uppercase_lowercase_str.cpp

// Input  : abHDbGd
// Output : ABHDBGD - abhdbgd

#include <string>

std::string UppercaseStr(std::string str)
{
    int str_length = str.length();
    
    for (int i = 0; i < str_length; ++i)
        str[i] = std::toupper(str[i]);
    
    return str;
}

std::string LowercaseStr(std::string str)
{
    int str_length = str.length();
    
    for (int i = 0; i < str_length; ++i)
        str[i] = std::tolower(str[i]);
    
    return str;
}

std::cout << UppercaseStr("abHDbGd"); // ABHDBGD
std::cout << LowercaseStr("abHDbGd"); // abhdbgd