Never defined structure

Question

Is there any benefit in having never-defined structures in C ?

Example in SQLite source code :

/* struct sqlite3_stmt is never defined */
typedef struct sqlite3_stmt sqlite3_stmt;

And the object is manipulated like so :

typedef struct Vdbe Vdbe;
struct Vdbe {
    /* lots of members */
};


int sqlite3_step(sqlite3_stmt *pStmt) {
    Vdbe *v = (Vdbe*) pStmt;
    /* do stuff with v... */
}

So why not just use a usual abstract type, with the actual structure privately defined in foo.c source and a public typedef in foo.h header ?

Answer 1

It is defined like this to hide the implementation detail of sqlite3_stmt from the user, thus avoiding the internal states from being messed around. See Opaque pointer.

(This also forces the user only to use the type as a pointer since the structure sqlite3_stmt itself has incomplete implementation.)

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Edit: VDBE (virtual database engine) is just "a" back-end of the SQLite3 API. I believe the back-end is changeable, thus a sqlite3_stmt* is not necessarily a Vdbe*. Not exposing Vdbe* in the API because the back-end detail should not be exposed.