CODEWITHSUNDEEP
cstructtypedef
anacy
anacy

Reputation: 123

What is the advantage of this C structure definition?

I don't quite understand why this structure is defined this way.

here is the block of code in question... 

typedef struct Except_Frame Except_Frame;
struct Except_Frame {
    Except_Frame *prev;
    jmp_buf env;
    const char *file;
    int line;
    const T *exception;
};

Why is this struct defined this way, as opposed to just ...

typedef struct {
    Except_Frame *prev;
    jmp_buf env;
    const char *file;
    int line;
    const T *exception;
} Except_Frame;

and what are the advantages?

Upvotes: 2

Views: 221

Answers (4)

Viswesn
Viswesn

Reputation: 4880

First we need to understand the use of typedef; typedef can be used to indicate how a variable represents something; Example

typedef int km_per_hour ;
typedef int points ;

Now, coming to your case, you are defining the struct and you want it to typedef to call by something; We need to PREDEFINE this before using it thus we are declaring before defining the struct 

1  typedef struct Except_Frame t_Except_Frame;
2  struct Except_Frame {
3      t_Except_Frame *prev;
4      ...
5 }

line 1) Now the compiler understand that there will be struct of name "struct Except_Frame" and we need to typedef to "t_Except_Frame"; Did you note that I added t_ for typedef; it is good practice to follow this so that it would be easy for programmer to understand that this value is typedef;

line 3) The system understand it is typedef variable of struct Except_Frame and compile the program accordingly.

Upvotes: 0

AnT stands with Russia
AnT stands with Russia

Reputation: 320777

If you need that typedef name, then the two popular stylistic variants available to you actually look as follows

typedef struct Except_Frame Except_Frame;
struct Except_Frame {
    Except_Frame *prev;
    ...
};

or 

typedef struct Except_Frame {
    struct Except_Frame *prev;
    ...
} Except_Frame;

Note the difference from your second variant (and your original second variant will not even compile).

Now, which one you want to use is largely a matter of your personal preference. The first variant makes the "short" version of the type name (just Except_Frame) available earlier than the second variant.

Upvotes: 0

nerez
nerez

Reputation: 447

By using 

typedef struct Except_Frame Except_Frame;

You are renaming the struct "struct Except_Frame" into "Except_Frame".

First, it's more convenient to type Except_Frame rather than struct Except_Frame. Second, in this case, the field "Except_Frame *prev" of the struct will fail in compilation as the compiler is not familiar with a struct called "Except_Frame" (It is familiar with a struct called struct Except_Frame)

Cheers, N

Upvotes: 1

R Sahu
R Sahu

Reputation: 206747

If you don't use:

typedef struct Except_Frame Except_Frame;

then, the struct will need to be defined using:

struct Except_Frame {

    // The keyword struct is necessary without the typedef
    struct Except_Frame *prev;

    jmp_buf env;
    const char *file;
    int line;
    const T *exception;
};

If you want to define the struct and the typedef in one statement, it will be:

typedef struct Except_Frame {

    // The keyword struct is necessary without the typedef
    // being defined ahead of the definition of the struct.
    struct Except_Frame *prev;

    jmp_buf env;
    const char *file;
    int line;
    const T *exception;
} Except_Frame;

Upvotes: 4

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