digitalmars.D.learn - need 'this' to access data member
- clayasaurus (3/3) Sep 26 2006 Hi, I have the following code attached, and I do not understand why I
- Chris Nicholson-Sauls (5/67) Sep 26 2006 Because 'SDL_RWops.hidden.unknown' is a struct type decleration, not a f...
- clayasaurus (3/67) Sep 26 2006 Thanks.
- Derek Parnell (64/66) Sep 26 2006 As explained already, a named union/struct must also have a data
- clayasaurus (4/76) Sep 27 2006 That would be nicer, but it is not my call. The SDL_RWops structure is
- Mike Parker (60/67) Sep 27 2006 In C, named inner structs and unions are treated as fields.
- Mike Parker (3/15) Sep 27 2006 And now that I've posted that I see that Derek was talking about
Hi, I have the following code attached, and I do not understand why I can not access this data member. Help is very much appreciated, thanks. ~ Clay
Sep 26 2006
clayasaurus wrote:
Hi, I have the following code attached, and I do not understand why I
can not access this data member. Help is very much appreciated, thanks.
~ Clay
------------------------------------------------------------------------
// datafield hacks
alias int FILE;
alias int Uint8;
// structure
struct SDL_RWops
{
int (*seek)(SDL_RWops *context, int offset, int whence);
int (*read)(SDL_RWops *context, void *ptr, int size, int maxnum);
int (*write)(SDL_RWops *context, void *ptr, int size, int num);
int (*close)(SDL_RWops *context);
uint type;
union hide
{
version(Windows)
{
struct win32io
{
int append;
void *h;
}
}
struct stdio
{
int autoclose;
FILE *fp;
}
struct mem
{
Uint8 *base;
Uint8 *here;
Uint8 *stop;
}
struct unknown
{
void *data1;
}
}
hide hidden;
}
int main()
{
SDL_RWops w;
// need 'this' to access data member data1??
w.hidden.unknown.data1 = null;
return 0;
}
Because 'SDL_RWops.hidden.unknown' is a struct type decleration, not a field
decleration.
In the same way that you had to create a field of type 'hide' in order to
access its
components ('hidden') you also need a field of type 'unknown' to do so.
-- Chris Nicholson-Sauls
Sep 26 2006
clayasaurus <clayasaurus gmail.com> Chris Nicholson-Sauls wrote:clayasaurus wrote:Thanks. ~ ClayHi, I have the following code attached, and I do not understand why I can not access this data member. Help is very much appreciated, thanks. ~ Clay ------------------------------------------------------------------------ // datafield hacks alias int FILE; alias int Uint8; // structure struct SDL_RWops { int (*seek)(SDL_RWops *context, int offset, int whence); int (*read)(SDL_RWops *context, void *ptr, int size, int maxnum); int (*write)(SDL_RWops *context, void *ptr, int size, int num); int (*close)(SDL_RWops *context); uint type; union hide { version(Windows) { struct win32io { int append; void *h; } } struct stdio { int autoclose; FILE *fp; } struct mem { Uint8 *base; Uint8 *here; Uint8 *stop; } struct unknown { void *data1; } } hide hidden; } int main() { SDL_RWops w; // need 'this' to access data member data1?? w.hidden.unknown.data1 = null; return 0; }Because 'SDL_RWops.hidden.unknown' is a struct type decleration, not a field decleration. In the same way that you had to create a field of type 'hide' in order to access its components ('hidden') you also need a field of type 'unknown' to do so. -- Chris Nicholson-Sauls
Sep 26 2006
On Tue, 26 Sep 2006 22:22:05 -0500, clayasaurus wrote:Hi, I have the following code attached, and I do not understand why I can not access this data member. Help is very much appreciated, thanks.As explained already, a named union/struct must also have a data declaration before you can refer to its members. However, have you considered anonymous union/struct ? When you use these, you can refer to the members without having to qualify their names. import std.stdio; // datafield hacks alias int FILE; alias int Uint8; // structure struct SDL_RWops { int (*seek)(SDL_RWops *context, int offset, int whence); int (*read)(SDL_RWops *context, void *ptr, int size, int maxnum); int (*write)(SDL_RWops *context, void *ptr, int size, int num); int (*close)(SDL_RWops *context); uint type; union { version(Windows) { struct { int append; void *h; } } struct { int autoclose; FILE *fp; } struct { Uint8 *base; Uint8 *here; Uint8 *stop; } struct { void *data1; } } } int main() { SDL_RWops w; // need 'this' to access data member data1?? std.stdio.writefln("autoclose A:" , w.autoclose); w.data1 = &w; // sample data std.stdio.writefln("autoclose B:" , w.autoclose); version(Windows) { w.append = 42; // sample data std.stdio.writefln("autoclose B:" , w.autoclose); } return 0; } -- Derek (skype: derek.j.parnell) Melbourne, Australia "Down with mediocrity!" 27/09/2006 3:22:49 PM
Sep 26 2006
Derek Parnell wrote:On Tue, 26 Sep 2006 22:22:05 -0500, clayasaurus wrote:That would be nicer, but it is not my call. The SDL_RWops structure is from the SDL library (www.libsdl.org) and the goal is to stay as close to the original structure as possible.Hi, I have the following code attached, and I do not understand why I can not access this data member. Help is very much appreciated, thanks.As explained already, a named union/struct must also have a data declaration before you can refer to its members. However, have you considered anonymous union/struct ? When you use these, you can refer to the members without having to qualify their names. import std.stdio; // datafield hacks alias int FILE; alias int Uint8; // structure struct SDL_RWops { int (*seek)(SDL_RWops *context, int offset, int whence); int (*read)(SDL_RWops *context, void *ptr, int size, int maxnum); int (*write)(SDL_RWops *context, void *ptr, int size, int num); int (*close)(SDL_RWops *context); uint type; union { version(Windows) { struct { int append; void *h; } } struct { int autoclose; FILE *fp; } struct { Uint8 *base; Uint8 *here; Uint8 *stop; } struct { void *data1; } } } int main() { SDL_RWops w; // need 'this' to access data member data1?? std.stdio.writefln("autoclose A:" , w.autoclose); w.data1 = &w; // sample data std.stdio.writefln("autoclose B:" , w.autoclose); version(Windows) { w.append = 42; // sample data std.stdio.writefln("autoclose B:" , w.autoclose); } return 0; }
Sep 27 2006
clayasaurus wrote:In C, named inner structs and unions are treated as fields. This... typedef struct { struct foo { int x; } } MyStruct; MyStruct ms; ms.foo.x = 0; ...is identical to this... typedef struct { int x; } Foo; typedef struct { Foo foo; } MyStruct; MyStruct ms; ms.foo.x = 0; Assuming a 32 bit C compiler, both sizeof(MyStruct) in both examples above should output '4'. In D, inner structs and unions are treated as type declarations but not members. Here is the same example in D: struct MyStruct { struct foo { int x; } } MyStruct ms; ms.foo.x = 0; // Error MyStruct.sizeof will output 1 in this example. Not only is foo not accessible as a member field, MyStruct is incompatible with the C side of the house. The following version fixes both issues: struct MyStruct { struct Foo { int x; } Foo foo; } MyStruct ms; ms.foo.x = 0; Now the output of MyStruct.sizeof should be the same as the C version. DerelictSDL was a tedious port, so I'm surprised more silliness like this hasn't turned up yet. I'm glad you caught it.That would be nicer, but it is not my call. The SDL_RWops structure is from the SDL library (www.libsdl.org) and the goal is to stay as close to the original structure as possible.
Sep 27 2006
Mike Parker wrote:clayasaurus wrote:And now that I've posted that I see that Derek was talking about anonymous inner structs. Oh well, what I said is still true :)In C, named inner structs and unions are treated as fields.That would be nicer, but it is not my call. The SDL_RWops structure is from the SDL library (www.libsdl.org) and the goal is to stay as close to the original structure as possible.
Sep 27 2006