对char数组感到困惑

Question

I am trying to implement the functions below, but printing out the output of Printme(), the program that freezes . Any ideas why?

int main() 
{
   for (int i=0; i<12; i++)
   {
        cout<< PrintMe()[i];
   }
   return 0;
}

char * PrintMe() {
  char str[12];
  strncpy (str, "hello world", 11);
  str[11] = 0;
  return str;
}

Answer 1

Your code invokes Undefined Behaviour as you are returning a pointer to a temporary. As soon PrintMe ends its execution, str becomes destroyed. Thus the pointer your accessing with PrintMe()[i] is invalid.

To fix this you have to return a heap allocated string (no automatic storage duration). But dont forget to destroy it afterwards:

char * PrintMe() {
  char* str = new char[12];
  strncpy (str, "hello world", 11);
  str[11] = 0;
  return str;
}

---

char* str = PrintMe();

for (int i=0; i<12; i++)
{
   cout<< str[i];
}

delete[] str;

Or as you're writing c++ nontheless, why dont you go with a std::string ?

Answer 2

The program has undefined behaviour because function PrintMe returns a pointer to the first element of a local array that will be destroyed after exiting the function. So the pointer will not be valid.

Change the function the following way that to get the predicted result

char * PrintMe() {
  static char str[12];
  strncpy (str, "hello world", 11);
  str[11] = 0;
  return str;
}

Answer 3

In your PrintMe function, you are returning a pointer to a local variable. This is undefined behavior:

char* PrintMe() {
    char str[12];
    //...
    return str;  // cannot do this, UB.
}

This works if you change the return type to std::string

std::string PrintMe() {
    char str[12];
    //...
    return str;  
}

The reason why the above now works is that a std::string is being constructed from the local array and returned instead of the local variable.