A question of differences: C & C++

[SiliconHobo]

Hey Folks,

I've been into MUDing off and on now for years and I've begun to learn C in my spare time so I can eventually get under the hood and implement some of my own ideas. I've got some good C/C++ resources and I've been doing the exercises. However, I've yet to receive an articulate explanation of the difference between C and C++. I've read that all C code can be compiled in C++ but not vice versa. I also understand that the focus on OOP in C++ is a main difference.

Anyway, I'll get to the point: I've spent most of the day curled up by the fire, reading and doing exercises and I just completed a basic program from a C course that was designed to teach nested structures and passing variables between functions. I was pretty happy when I got it working but then I noticed that my compiler (Dev-C++ 4.9.9.2) uses the .cpp file extension by default when creating a new source file. I had accidentally written a C++ program when I was supposed to be learning C! I then tried to compile the source as a C program and I got a number of compiler warnings and errors. I will post the code below along with the errors in the hope that someone can tell me what's wrong and how to turn this working C++ program into a C program. Cheers.

#include <stdio.h>

struct address_tree
{
       char street[100];
       char city[25];
       char post [25];
       char state [25];
       char country[25];
};

struct top_tree
{
       char name[100];
       char email[50];
       char birth[10];
       char phone[15];
       address_tree add;
};

typedef struct top_tree entry;

entry input ()
{
      entry  en1;

      printf ("Enter a new name.\n\n");
      scanf("&#37;[^\n]", en1.name);
      fflush(stdin);
      printf ("\nEnter a street address.\n\n");
      scanf("%[^\n]", en1.add.street);
      fflush(stdin);
      printf ("\nEnter a city of residence.\n\n");
      scanf("%[^\n]", en1.add.city);
      fflush(stdin);
      printf ("\nEnter a province.\n\n");
      scanf("%[^\n]", en1.add.state);
      fflush(stdin);
      printf ("\nEnter a post code.\n\n");
      scanf("%[^\n]", en1.add.post);
      fflush(stdin);
      printf ("\nEnter a country.\n\n");
      scanf("%[^\n]", en1.add.country);
      fflush(stdin);
      printf ("\nEnter a birthdate in the form dd/mm/yy.\n\n");
      scanf("%[^\n]", en1.birth);
      fflush(stdin);
      printf ("\nEnter a phone number.\n\n");
      scanf("%[^\n]", en1.phone);
      fflush(stdin);
      printf ("\nEnter an email address.\n\n");
      scanf("%[^\n]", en1.email);
      fflush(stdin);
      
      return en1;
}

void output (entry en1)
{     
      printf ("\nDatabase Entry\n\n");
      printf ("Name: %s\n\n", en1.name);
      printf ("Address:\n%s\n%s\n%s\n%s\n%s\n\n", 
      en1.add.street, en1.add.city, en1.add.state, en1.add.post, en1.add.country);
      printf ("Phone: %s\n", en1.phone);
      printf ("Email: %s\n", en1.email);
}


main ()
{
     entry en1;
     
     en1 = input ();
     
     output (entry en1);
     getchar ();   
}

When compiled as a .c file this code generated the following errors in Dev-C++4.9.9.2:

\address practice prog.c:24: error: syntax error before "address_tree"
\address practice prog.c:24: warning: no semicolon at end of struct or union
\address practice prog.c:30: error: return type is an incomplete type
\address practice prog.c: In function `input':
\address practice prog.c:31: error: storage size of 'en1' isn't known
\address practice prog.c:61: warning: `return' with a value, in function returning void
\address practice prog.c: At top level:
\address practice prog.c:65: error: parameter `en1' has incomplete type
\address practice prog.c: In function `main':
\address practice prog.c:77: error: storage size of 'en1' isn't known
\address practice prog.c:81: error: syntax error before "entry"

Execution terminated

[matsp]

That's because the C++ language does "typedef struct name name" automagically, whilst in C you have to either perform such a typedef line, or use "struct name" instead of "name". [This applies to unions and enums and classes - the last one obviously not valid in C].

--
Mats

[SiliconHobo]

Thanks matsp for the fastest reply ever. I added another typedef line and removed the type declaration in the call to output and it works fine. Cheers.

[Salem]

Read the FAQ on why fflush(stdin) is bad.

For C and C++ differences.
http://david.tribble.com/text/cdiffs.htm

[matsp]

And this

     scanf("%[^\n]", en1.name);

is a bad way to write

fgets(en1.name, sizeof(en1.name), stdin)

Most programmers will at least have to think a bit about what the scanf() line does, and many would also have to look that up in a book - the latter is not only better because it does the same thing in a more readable way, but it's also SAFE for overentering data. [You want to wrap it into another function, as you will need to remove a newline from the end tho']

--
Mats