Question

I am new to cpp and have a question regarding arrays. The code I have below should create a reversed version of str and have it be stored in newStr. However, newStr always comes up empty. Can someone explain to me why this is happening even though I am assigning a value from str into it?

void reverse (char* str) {
    char* newStr = (char*)malloc(sizeof(str));

    for (int i=0;i<sizeof(str)/sizeof(char);i++) {
       int index = sizeof(str)/sizeof(char)-1-i;
       newStr [i] = str [index];
    }
}

PS: I know that it is much more efficient to reverse an array by moving the pointer or by using the std::reverse function but I am interested in why the above code does not work.

Answer 1

As above commenters pointed out sizeof(str) does not tell you the length of the string. You should use size_t len = strlen(str);

void reverse (char* str) {
   size_t len = strlen(str);
   char* newStr = (char*)malloc(len + 1);

   for (int i=0; i<len;i++) {
      int index = len-1-i;
      newStr[i] = str[index];
   }
   newStr[len] = '\0'; // Add terminator to the new string.
}

Don’t forget to free any memory you malloc. I assume your function is going to return your new string?

Edit: +1 on the length to make room for the terminator.