Convertir std:string a mayusculas o minusculas con stl

#include <algorithm>

#include <string>

//Para convertir a minuscula:

std::string str("Hola Caracola"),final("");
final.resize(str.length());
std::transform(str.begin(),str.end(),final.begin(),::tolower);
std::cout << "str= \"" << str << "\" final = \"" << final << "\""<< std::endl;


//Para convertir a mayuscula:
std::transform(str.begin(),str.end(),final.begin(),::toupper);
std::cout << "str= \"" << str << "\" final = \"" << final << "\"" << std::endl;


Salida:
str= "Hola Caracola" final = "hola caracola"
str= "Hola Caracola" final = "HOLA CARACOLA"