explanation of code

[jub4jive]

Hi, I am learning about pointers right now in c++, and i came across a few programs I didn't quite understand. Here is the first program:

 // ptrstr.cpp -- using pointers to strings
#include <iostream>
#include <cstring>        // declare strlen(), strcpy()
int main()
{
  using namespace std;
  char animal[20] = "bear";  // animal holds bear
  const char * bird = "wren"; // bird holds address of string
  char * ps;
  
  cout << animal << " and "; // display bear
  cout << bird << "\n";     // display wren
  // cout << ps << "\n";    // may display garbage, may cause a crash
  
  cout << "Enter a kind of animal: ";
  cin >> animal;             // ok if input < 20 chars 
  // cin >> ps;   Too horrible a blunder to try; ps doesn't 
  //              point to allocated space

  ps = animal;           // set ps to point to string
  cout << ps << "s!\n";  // ok same as using animal
  cout << "Before using strcpy():\n";
  cout << animal << " at " << (int *) animal << endl;
  cout << ps << " at " << (int *) ps << endl;

  ps = new char[strlen(animal) + 1];  // get new storage
  strcpy(ps, animal);       // copy string to new storage
  cout << "After using strcpy():\n";
  cout << animal << " at " << (int *) animal << endl;
  cout << ps << " at " << (int *) ps << endl;
  delete [] ps;
  return 0;
}

I don't seem to understand the code after the following line:
cout << "Before using strcpy():\n";
Few specific questions I have:
what does (int *) animal tell us? Is it done correctly?
Correct me if I'm wrong:
1) new char[strlen(animal) + 1] means that whatever size is taken for animal (say 4 bits) new char will allot 5 bits in memory?
2) in strcpy(ps, animal) the address of animal is stored in ps?

Thanks in advance, any help will be appreciated!

[Richard 4234]

You know what a pointer is, right?

[jub4jive]

yeah, a pointer refers to the memory address.

[Disch]

what does (int *) animal tell us? Is it done correctly?

Let's look at the similar line after that first, since that's easier to understand and explain.

(int*)ps

ps is a pointer to a char, as you know.

The thing is... when you give a char pointer to cout... it treats it special... like a string. Therefore when you say cout << ps; it will not print the pointer, but instead will go to whatever address you gave it and will print characters from that address until it finds a null. Effectively, it will print the string data pointed to by the pointer.\

But that is the special case. If you give cout any other kind of pointer, it will not do that, and instead will just print the pointer as an address.

By doing (int*)ps we are casting ps from a char pointer to an int pointer. This "tricks" cout so that it will no longer print the string data... but instead will print the actual address that the pointer contains.


(int*)animal is similar, but also has an implicit cast from an array name to a pointer. You can think of it as doing the same thing.

new char[strlen(animal) + 1] means that whatever size is taken for animal (say 4 bits) new char will allot 5 bits in memory?

strlen follows whatever pointer you gives it and counts the number of characters it finds until it reaches the null terminator. It effectively gives you the length of the string.

Since animal contains "bear" -- which is 4 characters long -- strlen will return 4... giving you the length of the string.

Note the length is in characters.. not in bits.

new char[x] will create 'x' characters. In this case... since strlen(animal) gives us 4, and we are adding 1 to that...

ps = new char[strlen(animal) + 1]; <- this will allocate 5 characters with new[], and will assign a pointer to those 5 characters to our 'ps' pointer.

2) in strcpy(ps, animal) the address of animal is stored in ps?

strcpy follows the 2nd pointer you give it (in this case, 'animal') and copies characters one by one to the 1st pointer (in this case, 'ps') until the null is found. It effectively copies the string data from one array to another.

[jub4jive]

Thanks a lot, I got it!