Difference between string.empty and string[0] == '\0'

Question

Suppose we have a string

std::string str; // some value is assigned

What is the difference between str.empty() and str[0] =

Answer 1

C++ string has the concept of whether it is empty or not. If the string is empty then str[0] is undefined. Only if C++ string has size >1, str[0] is defined.

str[i] == '\0' is a concept of the C-string style. In the implementation of C-string, the last character of the string is '\0' to mark the end of a C-string.
For C-string you usually have to 'remember' the length of your string with a separate variable. In C++ String you can assign any position with '\0'.

Just a code segment to play with:

#include 
#include 

using namespace std;

int main(int argc, char* argv[]) {
   char str[5] = "abc";
   cout << str << " length: " << strlen(str) << endl;
   cout << "char at 4th position: " << str[3] << "|" << endl;
   cout << "char at 5th position: " << str[4] << "|" << endl;
   str[4]='X'; // this is OK, since Cstring is just an array of char!
   cout << "char at 5th position after assignment: " << str[4] << "|" << endl;
   string cppstr("abc");
   cppstr.resize(3);
   cout << "cppstr: " << cppstr << " length: " << cppstr.length() << endl;
   cout << "char at 4th position:" << cppstr[3] << endl;
   cout << "char at 401th positon:" << cppstr[400] << endl;
   // you should be getting segmentation fault in the
   // above two lines! But this may not happen every time.

   cppstr[0] = '\0';
   str[0] = '\0';
   cout << "After zero the first char. Cstring: " << str << " length: " << strlen(str) << " | C++String: " << cppstr << " length: " << cppstr.length() << endl;
   return 0;
}

On my machine the output:

abc length: 3
char at 4th position: |
char at 5th position: |
char at 5th position after assignment: X|
cppstr: abc length: 3
char at 4th position:
char at 401th positon:?
After zero the first char. Cstring:  length: 0 | C++String: bc length: 3