Segmentation fault error - c++

Question

`I am trying to write a program that reverses two strings, I though I had it done pretty well but when I run it, the program runs till line 26, then I get a segmentation fault error. The program compiles fine. I am wondering if there is a simple or obvious problem in my functions that I am not seeing, Any help would be appreciated!!
Thanks in advance
#include <iostream>
#include <string>
using namespace std;
// Reversing the characters in strings.
void reverse(string str);
void swap(char * first, char *last);
int main() {
// declarations and initialization
string str1;
string str2;
cout << "Please enter the first string of characters:\n";
cin >> str1;
cout << "Please enter the second string of characters:\n";
cin >> str2;
cout << "The strings before reversing are:" << endl;
cout << str1 << " " << str2 << endl;
// reverse str1
reverse(str1);
// reverse str2
reverse(str2);
// output
cout << "The strings after reversing: " << endl;
cout << str1 << " " << str2 << endl;
return 0;
}
void reverse(string str) {
int length = str.size();
char *first = NULL;
char *last = NULL;
first = &str[0];
last = &str[length - 1];
for (int i = 0; first < last; i++) {
swap(first, last);
first++;
last--;
}
}
void swap(char *first, char *last) {
char * temp;
*temp = *first;
*first = *last;
*last = *temp;
}

Answer 1

I don't know where line 26 is, but
char * temp;
*temp = ...
is not valid. temp should be pointed at a char, or (better yet) rewrite the function to where temp is a char.
Seth Carnegie observes that you'll have to pass the strings by reference if you want to modify the originals.
void reverse(string& str) { //pass by reference, so origional is modified

Answer 2

In your swap function, you are assigning a value to *temp when temp is not pointing to anything (it's uninitialized). Thus, your segmentation fault.
You want this:
void swap(char* first, char* last)
{
char temp = *first;
*first = *last;
*last = temp;
}

Answer 3

The other answers are valid in regards to the segfault cause.
I just think you may be interested in knowing that you can reverse a string easily using std::string's reverse_iterator:
std::string reverse(std::string str) {
std::string out;
for (std::string::reverse_iterator it = str.rbegin(); it != str.rend(); it++) {
out += *it;
}
return out;
}
So, calling:
reverse("foo");
...will return oof.

Answer 4

You're passing the strings by value, which means only a local copy of the string will be reversed in the reverse function. You should pass them by reference.
Also, don't alter the string's memory directly. Use operator[] like this:
for (size_t beg = 0, size_t end = str.size() - 1; beg < end; ++beg, --end)
str[beg] = str[end];
So all together:
void reverse(string& str); // prototype
....
void reverse(string& str) { // note the & which is pass by reference
int length = str.size();
for (size_t beg = 0, size_t end = str.size() - 1; beg < end; ++beg, --end)
str[beg] = str[end];
}
And as stated by Mooing Duck, the place you're probably crashing from is dereferencing a pointer which has a garbage value here:
char * temp;
*temp = ...
You're trying to assign some random memory a value, which is probably segfaulting you.

Answer 5

Again, others have pointed out the what the problem is and would like to show you this:
void reverse(std::string & str) {
for (int i = 0, last = str.size() - 1, lim = str.size() / 2 ; i < lim;) {
std::swap(str[i++], str[last--]);
}
}
I have not tested it thoroughly though.