Re: [Dev-C++] Struct tag

[Jason H. <jas...@bt...>]

The struct tag is a remnant of C carried through and extended in C++. =
Structs in C carry data only and don't allow methods and such to added, =
but a struct in C++ is equivalent to a class but with it's member =
automatically declared public by default, whereas in a class the default =
is private.
The only reason I can think of for using it in C++ is if it lends a =
clearer meaning to the code you are writing and the way it is =
represented, and possibly in compatibility with C, i.e. porting old C =
code to C++. Another difference between the C struct and the C++ struct =
is that in C a struct is a tag for a set of data formatted in a specific =
manner, and in C++ a struct represents a type. Mainly meaning that in C =
each declaration of a struct required you to specify that it was a =
struct, i.e.

struct a_structure data; // creates an a_structure struct in data.

In C++ this is still valid but you could also say:

a_structure data; // same

"a_structure" can be any structure previously defined. A work around you =
will see often in things like Win32 API is the use of a typedef for a =
struct, i.e.:

typedef struct tagPOINT { // pt=20
    LONG x;=20
    LONG y;=20
} POINT;

Where POINT is a typedef of the tagPOINT struct, and means you can =
declare a POINT directly.

An array is a primitive data representation used in C and C++ as the =
basic structured container available. An array of 1000 bytes directly =
represents 1000 consecutive memory locations in the underlying system. =
AFAIK the underlying code is supported by direct assembly =
representations of arrays (CISC), but don't quote me on that. One of the =
drawbacks of the basic array is that it has no bounds checking built in =
to it, so you could create an array of 1000 bytes and then go on to =
access 2000 bytes (and most likely crash your system). These errors can =
often be hard to find and require an extra investment in checking of =
bounds in your code.
On the other hand the Vector class (std::vector<type>) is a C++ =
container class that mimics much of what an array is, and most likely =
(although not guaranteed) is internally represented as an array or group =
of arrays. On top of that the class supports automatic resizing and =
bounds checking and a whole host of other manipulation methods and =
algorithmic functions. This extra functionality will incur an extra =
overhead on a system, but in modern C++ standard library implementations =
this cost will be negligible, and unlikely to be significant. As a C++ =
coder more often than not you would be recommended to use a vector =
although basic arrays will still have their uses occasionally.

Maybe this is more information that you wanted to know, but it may be =
valuable in future use.

Jason.
  ----- Original Message -----=20
  From: Daniel Glenfield=20
  To: dev...@li...=20
  Sent: Saturday, March 23, 2002 8:43 PM
  Subject: [Dev-C++] Struct tag


  Hi, just been going over my course notes and I've come across the =
Struct tag. An example is below.
  Looking at it, how is this different to a class? And why use it?

  Also, what's the difference between an array, and a vector, which I =
understand is its replacement but know little about them.

  // A demonstration of the Employee structure

  #include <iostream.h>

  struct Employee // declaration of a structure type

  {

  int EmpID;

  char EmpName[30];

  char EmpAddr[40];

  float Salary;

  void GetData();

  }EmpOne; // Optional declaration of a structure variable

  void main()

  {

  }



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