propper way of declaring a struct for a linked list or binary tree

**cooper1200** · 06-12-2019

i have seen several ways of doing this

Code:

typedef struct some_name
{
   // variables here
} some_name;

typedef struct some_name
{
    //variables here
} some_other_name; // this is used later on to address this struct

struct some_name // to use this they write struct some_name myvar;
{
    //variables here
};

struct some_name 
{
    //variables here
};

typedef struct some_name myvar
//then type my_var->some memeber

which is the correct way or are they all equally valid and down to preference
many thanks
coop

**GReaper** · 06-12-2019

Unless I'm referring to the struct inside itself, I always use this:

Code:

typedef struct {
   // variables here
} some_name;

If I have to though, I give it a "reserved" name:

Code:

typedef struct _some_name {
   // variables here
   struct _some_name *ptr;
} some_name;

**laserlight** · 06-12-2019

Originally Posted by GReaper

If I have to though, I give it a "reserved" name:

You should be aware though:

Originally Posted by C11 7.1.3 Paragraph 1 (part)

All identifiers that begin with an underscore are always reserved for use as identifiers with file scope in both the ordinary and tag name spaces.

For struct _some_name, the name is presumably declared at file scope, and _some_name is in the tag name space of struct. Therefore, the name is not merely "reserved", but really is reserved to the implementation.

**Niccolo** · 06-12-2019

Most regard this as the "modern" C style

Code:

struct some_name // to use this they write struct some_name myvar;

{

    //variables here

};

Yet, it requires the use of struct to use it to declare a variable.

The typedef defines a type so the struct keyword isn't required to declare a variable.

Code:

typedef struct        
{
  int x;
  int array[100];
} Foo;

Foo a;

typdef Foo AnotherFoo;

AnotherFoo a;

typdef some_name myname;

myname b;
struct some_name c;

**flp1969** · 06-12-2019

Originally Posted by cooper1200

Code:

struct some_name 
{
    //variables here
};

typedef struct some_name myvar
//then type my_var->some memeber

This is wrong... You are defining a type, not a var.

**cooper1200** · 06-12-2019

[QUOTE=flp1969;1287511]This is wrong... You are defining a type, not a var.[/QUOTE

sorry line 7 of your post should read then they can type: myvar->some_member

**laserlight** · 06-12-2019

To understand flp1969's objection, try compiling:

Code:

#include <stdio.h>

struct some_name 
{
    int some_member;
};

typedef struct some_name myvar;

int main(void)
{
    myvar->some_member = 123;
    printf("%d\n", myvar->some_member);
    return 0;
}

Then compile and run:

Code:

#include <stdio.h>

struct some_name 
{
    int some_member;
};

typedef struct some_name myvar;

int main(void)
{
    myvar my_real_var;
    myvar *my_real_pointer = &my_real_var;
    my_real_pointer->some_member = 123;
    printf("%d\n", my_real_pointer->some_member);
    return 0;
}

Basically, you cannot validly write myvar->some_member because myvar is a type, not a variable of pointer type.

**cooper1200** · 06-12-2019

oh i get it now sorry.... i meant the latter sorry i didn't word it correctly at all

it was written on the fly quickly because every time i watch a video one of versions 2, 3 or 4 (the correct version of 4) but never the way i do it (version 1) i guess its application and user dependant

**GReaper** · 06-12-2019

Originally Posted by laserlight

You should be aware though:

Originally Posted by C11 7.1.3 Paragraph 1 (part)

All identifiers that begin with an underscore are always reserved for use as identifiers with file scope in both the ordinary and tag name spaces.

For struct _some_name, the name is presumably declared at file scope, and _some_name is in the tag name space of struct. Therefore, the name is not merely "reserved", but really is reserved to the implementation.

I knew that, but I've shamelessly ignored it so far. I guess it hasn't presented as a problem since I haven't been a part of any major project. Could it actually create problems? What might you suggest as an alternative?

**flp1969** · 06-12-2019

Originally Posted by cooper1200

Originally Posted by flp1969

This is wrong... You are defining a type, not a var.

sorry line 7 of your post should read then they can type: myvar->some_member

Note, you don't get it... the right way, in this case, is:

Code:

struct some_name { ... };
typedef struct some_name mytype;

mytype myvar; // myvar.some_member

**Niccolo** · 06-12-2019

@GReaper

What might you suggest as an alternative?

A trailing underscore.

**christop** · 06-12-2019

Originally Posted by GReaper

I knew that, but I've shamelessly ignored it so far. I guess it hasn't presented as a problem since I haven't been a part of any major project. Could it actually create problems? What might you suggest as an alternative?

I generally give the struct and the typedef type the same name:

Code:

typedef struct some_name {
    // variables here
    struct _some_name *ptr;
} some_name;

There's no name conflict because "struct" is a separate namespace from other types. It works in C++ too (though it's unnecessary because C++ automatically lets you use a struct tag without "struct" in front of it).

**cooper1200** · 06-12-2019

christop that's exactly what i do but in all the videos i have watched i have never seen it done

**stahta01** · 06-12-2019

Some old C compiler had a name clash when you did it that way.

Edit: This might have been in the K&R C days

Tim S.

**christop** · 06-12-2019

Originally Posted by stahta01

Some old C compiler had a name clash when you did it that way.

Edit: This might have been in the K&R C days

Tim S.

Yeah, it might be a problem with a pre-ANSI compiler, but fortunately it should work fine with almost any standard C compiler made in the last 30 years.

(It's definitely something to be aware of if you do software archaeology, like looking through the ancient Unix source code or something.)

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