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digitalmars.D.learn - Overloading struct members (or cast?)

reply "Wsdes" <valentino.dileonardo bccgi.com> writes:
Hello again and welcome to another tale of mystery :)

I, again, am facing problems connecting my D program to the C API 
as seen in the callback thread before.

I want to write a callback function that is assigned to a struct 
member. The function should write data from members of another 
struct to the command line. Please consider my code:


    void MyCB(void* pVoid) {
      DataInfo_t* userData; //DataInfo_t is a struct
      userData = cast(DataInfo_t*) pVoid;

      writeln("Value: ", pMsgUserData.Values[i].value.string);
    }

Here the DataInfo_t struct has a member Values which itself is a 
struct. The programmer of the API called the members of the union 
inside the Values struct after simple data types or pointers to 
them, specifically

      union value {
           const char       *string;
      }

When I try to compile I get the following error:

            Error: need 'this' for 'string' of type 'const(char*)'

I'm not quite sure what the error wants to tell me. The way I see 
it is that there is some kind of problem with the instance of the 
struct and hence the reference to that specific member value does 
not work.

Any ideas?
Nov 18 2014
next sibling parent "bearophile" <bearophileHUGS lycos.com> writes:
Wsdes:

union value {

           const char       *string;
      }


Try:

union Value {
     const char* myString;
}

Note the upper caseing, the spacing after the star, and the 
"string" name replaced with something else.

Bye,
bearophile
Nov 18 2014
prev sibling parent reply =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
On 11/18/2014 07:32 AM, Wsdes wrote:


             Error: need 'this' for 'string' of type 'const(char*)'


Reduced:

struct S
{
     union Value
     {
         const char* myString;
     }
}

void main()
{
     auto s = S();
     auto mine = s.Value.myString;    // ERROR
}

You may be thinking that Value introduces a name scope so that you can 
reference its myString by Value.myString.

You have two options:


a) Remove Value so that it is an anonymous union. myString becomes a 
direct member of S:

struct S
{
     union    // <-- Anonymous
     {
         const char* myString;
     }
}

void main()
{
     auto s = S();
     auto mine = s.myString;    // <-- No more Value
}


b) Make S have an actual member of type Value:

struct S
{
     union Value
     {
         const char* myString;
     }

     Value value;    // <-- Added
}

void main()
{
     auto s = S();
     auto mine = s.value.myString;    // <-- Note lower initial
}

Ali
Nov 18 2014
parent reply Steven Schveighoffer <schveiguy yahoo.com> writes:
On 11/18/14 1:28 PM, Ali Çehreli wrote:

 On 11/18/2014 07:32 AM, Wsdes wrote:

  >             Error: need 'this' for 'string' of type 'const(char*)'

 Reduced:

 struct S
 {
      union Value
      {
          const char* myString;
      }
 }

 void main()
 {
      auto s = S();
      auto mine = s.Value.myString;    // ERROR
 }

 You may be thinking that Value introduces a name scope so that you can
 reference its myString by Value.myString.

 You have two options:


 a) Remove Value so that it is an anonymous union. myString becomes a
 direct member of S:

 struct S
 {
      union    // <-- Anonymous
      {
          const char* myString;
      }
 }

 void main()
 {
      auto s = S();
      auto mine = s.myString;    // <-- No more Value
 }


 b) Make S have an actual member of type Value:

 struct S
 {
      union Value
      {
          const char* myString;
      }

      Value value;    // <-- Added
 }

 void main()
 {
      auto s = S();
      auto mine = s.value.myString;    // <-- Note lower initial
 }

 Ali


This works in C++:

struct S
{
    union
    {
       const char *myString;
    } Value;
};

But the issue is that D does not require the semicolon at the end of the 
struct, so by the closing brace, it is done with the union.

the closest solution you can get is the last one you said.

I never even considered that you can't make an anonymous struct or union 
type and assign it to a member in D. Not sure if we really need to 
support it though.

-Steve
Nov 18 2014
parent =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
On 11/18/2014 11:01 AM, Steven Schveighoffer wrote:


 This works in C++:

 struct S
 {
     union
     {
        const char *myString;
     } Value;
 };

 But the issue is that D does not require the semicolon at the end of the
 struct, so by the closing brace, it is done with the union.


Interesting. It never occurred to me that the lack of semicolons could 
have that effect.


 the closest solution you can get is the last one you said.

 I never even considered that you can't make an anonymous struct or union
 type and assign it to a member in D. Not sure if we really need to
 support it though.


I wouldn't miss that feature. :)

If I am not mistaken, the only "benefit" of anonymous types is that they 
cannot refer to themselves. I remember reading in a C++ thread years ago 
that anonymous types could not have contructors, destructors, and 
assignment operators either because they could not spell out the name of 
the type to define such functions.

Ali
Nov 18 2014