Sizeof - Writing past the end of an array

**milkydoo** · 07-14-2003

I'm new to c++ and have a question about a seeming inconsitency I've found in one of my books.

When taking string input from the keyboard for structures, the examples in my book use the sizeof operator like so:

cin.getline(Mystruct.name, sizeof(Mystruct.name) - 1);

The '-1' is used to allow room for the terminating null character.

Now, I have skipped forward a little in my book so I can add some file I/O to my practice programs.

However, the I/O examples (using the fstream class) do not subtract 1 from the char array when reading from a file. Why is this? Does this class automatically stop reading 1 byte early, or is this a typo?

**HybridM** · 07-14-2003

I'm not 100% sure but I think you're right. The file I/O functions take a number but inside the function they will terminate at number-1.

**Hammer** · 07-15-2003

It says so here:
http://www.cppreference.com/cppio_details.html#getline

Yep, num-1 characters will be read, meaning the getline function will not overrun your buffer:

Code:

#include <iostream>
using namespace std;
int main(void)
{
  char a[5];
  cin.getline(a, 5);
  cout <<a <<endl;
  return(0);
}

Input: 123456
Output:1234

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