my compiler says I have to have a variable at the end of my structure

ambeco · #1 20-Feb-2004, 10:41

I'm just started to write C++ programs, and my compiler says I have to have a variable at the end of my structure:

CPP / C++ / C Code:

struct person
{
      int height;
}variable;  <------- what is this variable

The book that I was using (is a piece of trash, and) doesnt mention a variable there when it teaches about structures. What is this?!?!?!?!?

dsmith · #2 20-Feb-2004, 11:37

Quote:

CPP / C++ / C Code:

struct person
{
      int height;
}variable;  <------- what is this variable

The book that I was using (is a piece of trash, and) doesnt mention a variable there when it teaches about structures. What is this?!?!?!?!?

This creates a variable type called person that is made up of your structure. This is not your variable at this point just a type, like int. Then it goes on to say that you want to creat a variable of this type. This is a short-hand way to write:

CPP / C++ / C Code:

struct person{         //define the structure (type)
      int height;
};

struct person variable;  //Make a variable of this type (struct person)

Does that help or is it that more confusing?

EDIT: Ambeco, I noticed that you changed the name of this thread. For the record, I have never seen a compiler require a variable after the definition of a structure, but it does require a semicolon. I have had many a compile error because of this little detail

WaltP · #3 20-Feb-2004, 13:52

Quote:

Originally Posted by ambeco

I'm just started to write C++ programs, and my compiler says I have to have a variable at the end of my structure:

CPP / C++ / C Code:

struct person
{
      int height;
}variable;  <------- what is this variable

The book that I was using (is a piece of trash, and) doesnt mention a variable there when it teaches about structures. What is this?!?!?!?!?

As dsmith said, your struct person is in effect creating a new type, like int.

But if you want to create a "type" you use:

CPP / C++ / C Code:

typedef struct PERSON
{
      int height;
};

Now you can use PERSON the same way you use int

CPP / C++ / C Code:

PERSON person, people[5]; // define 2 vars with the PEOPLE type

I reccommend making the type name all caps as shown.

But in your case you were simply defining a variable with the specified structure, not making a type. If you leave off typedef you need to specify the variable itself:

CPP / C++ / C Code:

struct PERSON
{
      int height;
} person, preople[5];  // define 2 vars with this structure

ambeco · #4 23-Feb-2004, 10:17

Quote:

Originally Posted by WaltP

But in your case you were simply defining a variable with the specified structure, not making a type. If you leave off typedef you need to specify the variable itself:

CPP / C++ / C Code:

struct PERSON
{
      int height;
} person, preople[5];  // define 2 vars with this structure

I dont think you guys understood my question, but the part I have marked in the quote answered my question. The two words right after the } are new structures of the person type. If I want to leave them off I have to use typedef. Got it. Thanks for your help guys!

aaroncohn · #5 23-Feb-2004, 14:03

Hey Walt, I have a question for you. Let's say I define a structure like this:

CPP / C++ / C Code:

struct student
{
    char *name;
    float gpa;
    int score[3];
};

Then I create a variable with it:

CPP / C++ / C Code:

student aaron;

What is the difference between doing what I did, and adding the typedef statement before my structure definition?

WaltP · #6 23-Feb-2004, 21:38

Well, other than the creation of the variable showing the programmer's healthy ego

, it doesn't compile -- at least not in Borland 5.5 and Turbo 1.0

According to my tests:

CPP / C++ / C Code:

struct STRUCTURE1
{
    int  val1a;
    int  val1b;
};
struct STRUCTURE1 test1b;
STRUCTURE1 test1a;  // declaration syntax error

without the typedef you must specify the struct structname when defining the variable otherwise a declaration syntax error occurs.

By creating a typedef:

CPP / C++ / C Code:

typedef struct 
{
    int  val4a;
    int  val4b;
} STRUCTURE4;
STRUCTURE4 test4a;

or

CPP / C++ / C Code:

struct structure3 
{
    int  val3a;
    int  val3b;
};
typedef struct structure3 STRUCTURE3;
STRUCTURE3 test3a;

you don't need the struct qualifier when defining the variable.

I haven't checked whether there is a difference in the actual execuatable, but I doubt there is. All the test files I created (4) to see what happens are identical in size.

If I have time, I'll run these thru VC6, too, but that's on a different machine.

FYI, Tubo 1.0 created files 8K is size, Borland 5.5 files were 55K.

PS -- I just looked at my first post and I had the syntax wrong on the [i]typedef[i]inition. Follow this post. It was tested by the compiler.

dsmith · #7 23-Feb-2004, 22:04

My experience has been that C++ allows a variable definition from a struct whereas straight C does not.

So in C++, this works:

CPP / C++ / C Code:

struct STRUCTURE1
{
    int  val1a;
    int  val1b;
};
STRUCTURE1 test1a;

Whereas in a straight C compiler it doesn't. YMMV. It may be that the newer C++ compilers put in an implicit "struct" when it sees this.

aaroncohn · #8 23-Feb-2004, 22:36

Yes, that's what I was thinking. I'm running Borland C++ builder 6 right now, and I have never had a problem doing what I did in my example above, which is why I asked. Should I make it a habit to perform a typedef statement, or just take advantage of the C++ compiler feature? I think I'll try to do it in VC6 and see what happens.

WaltP · #9 23-Feb-2004, 23:30

Interesting. I didn't realize C++ allows you to leave the struct off the definition. I was compiling as C.

dsmith · #10 24-Feb-2004, 07:31

Quote:

Originally Posted by aaroncohn

Should I make it a habit to perform a typedef statement, or just take advantage of the C++ compiler feature?

For portability reasons, it would not be a bad idea to have a typedef statement. If this really is a C++ "feature" and you are writing C++ code it probably doesn't really matter.

For what it's worth, I have gotten lazy and ussually don't use the typedef or the explicit struct, but this is for C++ programs. Of the three compilers I use, none of them complain (Visual C++, gnu g++, bloodshed dev-c++).

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my compiler says I have to have a variable at the end of my structure

I think I figured it out