reverse string using char * and string class

I try to reverse a string by pointer walking. Logic is simple that I have two pointers to char one of them head that points to first char of the string. The other tail points to second-last character(before \0) of the string. Swap then char-by-char. C-style it works well, but C++-style doesn't work. I wonder its reason.

C-style

#include <iostream>
#include <cstring>
using std::cin;
using std::cout;
using std::endl;
using std::string;

void reverse(char *str)
{
    char *tail, *head;
    head = &str[0];
    tail = &str[strlen(str)];

    cout << "String inverted is: ";

    while ((head!=tail)&&(head!=--tail))
    {
        char temp=*head;
        *head++=*tail;
        *tail=temp;
    }
}

int main(int argc, char const *argv[])
{
    char str[100];
    cout << "Enter a string: ";
    cin.getline(str,100);
    reverse(str);
    cout << str;
    cout <<"\n";

    return 0;
}

C++-style

#include <iostream>
#include <string>
using std::cin;
using std::cout;
using std::endl;
using std::string;

void reverse(string str)
{
    char *tail, *head;
    head = &str[0];
    tail = &str[str.size()];

    cout << "String inverted is: ";

    while ((head!=tail)&&(head!=--tail))
    {
        char temp=*head;
        *head++=*tail;
        *tail=temp;
    }
}

int main(int argc, char const *argv[])
{
    string str;
    cout << "Enter a string: ";
    getline(cin,str);
    reverse(str);
    cout << str;
    cout <<"\n";

    return 0;
}