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digitalmars.D.learn - basic question about adresses and values in structs

reply "nikki" <nikkikoole gmail.com> writes:
so I am still very new to structs and & and * adress and pointer 
stuff, I have this basic code :

     struct S {
         int value = 0;
     }

     void func(S thing){
         writeln(&thing); //BFC52B44
         thing.value = 100;
     }

     S guy = {value:200};
     writeln(&guy); //BFC52CCC
     func(guy);
     writeln(guy.value);// this prints 200, because the adress was 
not the same

I think I see whats going on but I don't know how to fix it?
Sep 01 2014
next sibling parent "nikki" <nikkikoole gmail.com> writes:
sorry could have quicker just googled it thanks!
Sep 01 2014
prev sibling parent reply "Gary Willoughby" <dev nomad.so> writes:
On Monday, 1 September 2014 at 18:08:48 UTC, nikki wrote:

 so I am still very new to structs and & and * adress and 
 pointer stuff, I have this basic code :

     struct S {
         int value = 0;
     }

     void func(S thing){
         writeln(&thing); //BFC52B44
         thing.value = 100;
     }

     S guy = {value:200};
     writeln(&guy); //BFC52CCC
     func(guy);
     writeln(guy.value);// this prints 200, because the adress 
 was not the same

 I think I see whats going on but I don't know how to fix it?


void func(ref S thing){
	writeln(&thing);
	thing.value = 100;
}

The ref keyword passes the variable into the function by 
reference, so that it is not copied.
Sep 01 2014
parent reply "nikki" <nikkikoole gmail.com> writes:
ah so much cleaner then the mess I was almost into ;)

thanks
Sep 01 2014
parent reply =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
On 09/01/2014 11:23 AM, nikki wrote:

 ah so much cleaner then the mess I was almost into ;)

 thanks


In case they are useful to you or somebody else, the following chapters 
are relevant.

Value Types and Reference Types:

   http://ddili.org/ders/d.en/value_vs_reference.html

Pointers:

   http://ddili.org/ders/d.en/pointers.html

Ali
Sep 01 2014
parent reply "nikki" <nikkikoole gmail.com> writes:
thanks! just what I needed, with some stumbling I managed to get 
everything working as intended: using a pointer variable to save 
an adres of a function, then dereferencing to use it.

Now I am wondering when to use the ** ?

for example I found this function over at 
https://github.com/d-gamedev-team/gfm/blob/master/core/gfm/core/memory.d

void* storeRawPointerAndReturnAligned(void* raw, size_t 
alignment) nothrow
     {
         enum size_t pointerSize = size_t.sizeof;
         char* start = cast(char*)raw + pointerSize;
         void* aligned = nextAlignedPointer(start, alignment);
         void** rawLocation = cast(void**)(cast(char*)aligned - 
pointerSize);
         *rawLocation = raw;
         return aligned;
     }

it's a little over my head yet..
Sep 04 2014
next sibling parent "evilrat" <evilrat666 gmail.com> writes:
On Thursday, 4 September 2014 at 09:54:57 UTC, nikki wrote:

 thanks! just what I needed, with some stumbling I managed to 
 get everything working as intended: using a pointer variable to 
 save an adres of a function, then dereferencing to use it.

 Now I am wondering when to use the ** ?

 for example I found this function over at 
 https://github.com/d-gamedev-team/gfm/blob/master/core/gfm/core/memory.d

 void* storeRawPointerAndReturnAligned(void* raw, size_t 
 alignment) nothrow
     {
         enum size_t pointerSize = size_t.sizeof;
         char* start = cast(char*)raw + pointerSize;
         void* aligned = nextAlignedPointer(start, alignment);
         void** rawLocation = cast(void**)(cast(char*)aligned - 
 pointerSize);
         *rawLocation = raw;
         return aligned;
     }

 it's a little over my head yet..


void** (double ptr) is a pointer to array of pointers(just 
imagine a crossword where each horizontal letter is part of 
vertical word) there is little reason to use them in D, mostly to 
C/C++ interfacing
Sep 04 2014
prev sibling parent reply =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
On 09/04/2014 02:54 AM, nikki wrote:


 a pointer variable to save an adres of a function, then dereferencing 


to use

 it.


If possible, even in C, I would recommend using a 'function pointer' for 
that. However, there are cases where the signature of the function 
should be unknown to the code that is storing it so a void* is used. 
(Note that, as discussed on these forums in the past, void* has always 
been intended to be a data pointer. The fact that it works for function 
pointers is something we get as lucky accidents, which will most 
probably always supported by compilers and CPUs.)

Here is how D does function pointers:

   http://dlang.org/expression.html#FunctionLiteral

And a chapter that expands on those:

   http://ddili.org/ders/d.en/lambda.html


 Now I am wondering when to use the ** ?


The simple answer is when dealing with the address of a type that is 
'void*' itself. In other words, there is nothing special about **: It 
appears as the type that is "a pointer to a pointer". Inserting spaces:

     int * p;      // A pointer to an int
     void* * q;    // A pointer to a void*

     // (untested)
     static assert (is (typeof(*p) == int));
     static assert (is (typeof(*q) == void*));

     int i;
     *p = i;    // Can store an int

     void* v;
     *q = v;    // Can store a void*

Ali
Sep 04 2014
parent "nikki" <nikkikoole gmail.com> writes:
On Thursday, 4 September 2014 at 14:00:14 UTC, Ali Çehreli wrote:

 On 09/04/2014 02:54 AM, nikki wrote:


 a pointer variable to save an adres of a function, then


 dereferencing to use

 it.


 If possible, even in C, I would recommend using a 'function 
 pointer' for that. However, there are cases where the signature 
 of the function should be unknown to the code that is storing 
 it so a void* is used. (Note that, as discussed on these forums 
 in the past, void* has always been intended to be a data 
 pointer. The fact that it works for function pointers is 
 something we get as lucky accidents, which will most probably 
 always supported by compilers and CPUs.)
 ...
 Ali


Ah right I was so busy with these * and & ;)

Thanks!
Sep 04 2014