digitalmars.D.learn - Generic functions to convert to void* and from void*
- TSalm (40/40) Feb 22 2009 Hello,
- TSalm (1/38) Feb 22 2009 I've forget to say that toPtr can't be call with constants.
- Daniel Keep (17/65) Feb 22 2009 First of all, I have to ask: have you looked at std.variant /
- TSalm (7/16) Feb 23 2009 I'm really a newbie concerning the use of void* ( I think you have notic...
- Daniel Keep (19/40) Feb 23 2009 The Phobos Variant will use however much space you reserve as the
- TSalm (9/50) Feb 24 2009 Thanks.
- Daniel Keep (21/31) Feb 24 2009 You still have to get the code right. There's a surprising number of
- TSalm (6/36) Feb 25 2009 You are right.
Hello, I'm trying to build function which have the hability to convert a type to void* and from void*. I must use "ref" in the "toPtr" function because of this : http://www.digitalmars.com/pnews/read.php?server=news.digitalmars.com&group=digitalmars.D.learn&artnum=15600 Do you think that what I done is correct ? /* --------- CODE --------- */ /*** * Convert a value to a void* * Params: * val = * Returns: */ void* toPtr(T)(ref T val) { void* p ; static if ( is( T b : Type ) ) { p = new T ; p = &val ; } else { p = &val ; } return p ; } /*** * Convert a void* to his value * Params: * ptr = * Returns: */ T fromPtr(T)(void* ptr) { return *(cast(T*)ptr) ; } /* ------- END CODE -------- */ Thanks in advance, TSalm
Feb 22 2009
I'm trying to build function which have the hability to convert a type to void* and from void*. I must use "ref" in the "toPtr" function because of this : http://www.digitalmars.com/pnews/read.php?server=news.digitalmars.com&group=digitalmars.D.learn&artnum=15600 Do you think that what I done is correct ? /* --------- CODE --------- */ /*** * Convert a value to a void* * Params: * val = * Returns: */ void* toPtr(T)(ref T val) { void* p ; static if ( is( T b : Type ) ) { p = new T ; p = &val ; } else { p = &val ; } return p ; } /*** * Convert a void* to his value * Params: * ptr = * Returns: */ T fromPtr(T)(void* ptr) { return *(cast(T*)ptr) ; } /* ------- END CODE -------- */I've forget to say that toPtr can't be call with constants.
Feb 22 2009
TSalm wrote:First of all, I have to ask: have you looked at std.variant / tango.core.Variant?I'm trying to build function which have the hability to convert a type to void* and from void*.What is this test doing? You're checking to see if T can be implicitly cast to "Type"... but "Type" isn't defined. And what is 'b' doing there?I must use "ref" in the "toPtr" function because of this : http://www.digitalmars.com/pnews/read.php?server=news.digitalmars.com&group=digitalmars.D.learn&artnum=15600 Do you think that what I done is correct ? /* --------- CODE --------- */ /*** * Convert a value to a void* * Params: * val = * Returns: */ void* toPtr(T)(ref T val) { void* p ; static if ( is( T b : Type ) )You never use the freshly-allocated T. You assign it to p and then immediately overwrite it with &val.{ p = new T ; p = &val ;So the function is basically doing this: T fromPtr(T)(ref T val) { return &val; }} else { p = &val ; } return p ; }I get the distinct impression that you're seriously over-thinking this. Both of these functions could be rewritten as casts. Aside from that, you've given no context for me to have any idea what you're trying to accomplish here. -- Daniel/*** * Convert a void* to his value * Params: * ptr = * Returns: */ T fromPtr(T)(void* ptr) { return *(cast(T*)ptr) ; } /* ------- END CODE -------- */I've forget to say that toPtr can't be call with constants.
Feb 22 2009
Yes, but it seems that Variant class uses more memory than void* .First of all, I have to ask: have you looked at std.variant / tango.core.Variant?I'm trying to build function which have the hability to convert a type to void* and from void*.[...]I get the distinct impression that you're seriously over-thinking this. Both of these functions could be rewritten as casts. Aside from that, you've given no context for me to have any idea what you're trying to accomplish here.I'm really a newbie concerning the use of void* ( I think you have notice this ;-) ) Thanks for your usefull remarks. I'm simply trying to make a little and lightweight "DBMS in memory". Simply made by classes like TableMem, RowMem and ColumnMem(T). There's also a "private" class which aims to store datas, using void*[][].
Feb 23 2009
TSalm wrote:The Phobos Variant will use however much space you reserve as the maximum, plus 4 bytes for a function pointer, but it can only store types as big as you allow for. The Tango version will use max(real.sizeof,void[].sizeof) + 4 bytes for the typeid and can store anything you throw at it. For that extra space, both of these will give you runtime type safety, meaning you can't accidentally get the types wrong. They're MUCH safer than void*.Yes, but it seems that Variant class uses more memory than void* .First of all, I have to ask: have you looked at std.variant / tango.core.Variant?I'm trying to build function which have the hability to convert a type to void* and from void*.void* is just a pointer like any other. It doesn't have any special properties except that you cannot dereference it; that's it. If you're not sure how to use pointers, then don't. For example, you could store objects instead; this takes the same amount of storage in the DBMS, and allows for safe casting back to the original type. Plus, you don't have to stuff about with casting things to void* and back.[...]I get the distinct impression that you're seriously over-thinking this. Both of these functions could be rewritten as casts. Aside from that, you've given no context for me to have any idea what you're trying to accomplish here.I'm really a newbie concerning the use of void* ( I think you have notice this ;-) ) Thanks for your usefull remarks.I'm simply trying to make a little and lightweight "DBMS in memory". Simply made by classes like TableMem, RowMem and ColumnMem(T). There's also a "private" class which aims to store datas, using void*[][].Unless you really need to store small value types like integers, etc. in that private data, objects might be the best bet for now. -- Daniel
Feb 23 2009
TSalm wrote:In my case, there's also no possibility to get the wrong type, because it is managed by the type of the ColumnMem.The Phobos Variant will use however much space you reserve as the maximum, plus 4 bytes for a function pointer, but it can only store types as big as you allow for. The Tango version will use max(real.sizeof,void[].sizeof) + 4 bytes for the typeid and can store anything you throw at it. For that extra space, both of these will give you runtime type safety, meaning you can't accidentally get the types wrong. They're MUCH safer than void*.Yes, but it seems that Variant class uses more memory than void* .First of all, I have to ask: have you looked at std.variant / tango.core.Variant?I'm trying to build function which have the hability to convert a type to void* and from void*.void* is just a pointer like any other. It doesn't have any special properties except that you cannot dereference it; that's it. If you're not sure how to use pointers, then don't. For example, you could store objects instead; this takes the same amount of storage in the DBMS, and allows for safe casting back to the original type. Plus, you don't have to stuff about with casting things to void* and back.[...]I get the distinct impression that you're seriously over-thinking this. Both of these functions could be rewritten as casts. Aside from that, you've given no context for me to have any idea what you're trying to accomplish here.I'm really a newbie concerning the use of void* ( I think you have notice this ;-) ) Thanks for your usefull remarks.Thanks. And about Object, if I want to store base type like int,double,etc..., if I do something like : Object o; int a = 30 ; o = cast(Object) &a ; is this syntax is GC safe ?I'm simply trying to make a little and lightweight "DBMS in memory". Simply made by classes like TableMem, RowMem and ColumnMem(T). There's also a "private" class which aims to store datas, using void*[][].Unless you really need to store small value types like integers, etc. in that private data, objects might be the best bet for now.
Feb 24 2009
TSalm wrote:In my case, there's also no possibility to get the wrong type, because it is managed by the type of the ColumnMem.You still have to get the code right. There's a surprising number of corner cases trying to store arbitrary types.And about Object, if I want to store base type like int,double,etc..., if I do something like : Object o; int a = 30 ; o = cast(Object) &a ; is this syntax is GC safe ?It's not safe, period. If the compiler lets you do that, I'd be tremendously surprised; even more surprised if it doesn't cause major problems later. This is what I'm worried about; you're doing dangerous things with a type system you don't understand. Don't do this. Here's the problem: void* isn't going to work for everything. It'll work for Object references, other pointers, and that's it. You can't store arrays, and you can't store value types like structs or primitive types. For that, you need to allocate heap storage, copy the value and then store the pointer to that. Oh, and don't forget that fixed-length arrays have value semantics whereas dynamic arrays and slices have reference semantics; although you generally solve that issue by having a special template for your type which rewrites T[n] as T[]. Also, delegates won't fit, but function pointers will. This is why I was pointing you at Variant because I already went through the trouble to solve all this once. :P If you still want to do this with void*, build that code in isolation and test the heck out of it. -- Daniel
Feb 24 2009
Le Wed, 25 Feb 2009 01:03:32 +0100, Daniel Keep <daniel.keep.lists gmail.com> a écrit:TSalm wrote:You are right. I will first use Variant to implements all functionnalities, and insolating the storage type. And in the future, I will be able to use void* as an improvement.In my case, there's also no possibility to get the wrong type, because it is managed by the type of the ColumnMem.You still have to get the code right. There's a surprising number of corner cases trying to store arbitrary types.And about Object, if I want to store base type like int,double,etc..., if I do something like : Object o; int a = 30 ; o = cast(Object) &a ; is this syntax is GC safe ?It's not safe, period. If the compiler lets you do that, I'd be tremendously surprised; even more surprised if it doesn't cause major problems later. This is what I'm worried about; you're doing dangerous things with a type system you don't understand. Don't do this. Here's the problem: void* isn't going to work for everything. It'll work for Object references, other pointers, and that's it. You can't store arrays, and you can't store value types like structs or primitive types. For that, you need to allocate heap storage, copy the value and then store the pointer to that. Oh, and don't forget that fixed-length arrays have value semantics whereas dynamic arrays and slices have reference semantics; although you generally solve that issue by having a special template for your type which rewrites T[n] as T[]. Also, delegates won't fit, but function pointers will. This is why I was pointing you at Variant because I already went through the trouble to solve all this once. :P If you still want to do this with void*, build that code in isolation and test the heck out of it.
Feb 25 2009