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how can i return multiple values from vc++  dll

Posted on 2008-10-23
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Last Modified: 2012-05-05
i have written a dll in vc++ 6.
i have two variables to return one is _bstr_t and another one is short
how do i retun these variables
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Question by:hp746
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Expert Comment

by:HooKooDooKu
ID: 22793551
It's only possible to return one variable value.

When you have to return multiple values, the only way to do it is to pass a parameter to the function by reference and set the value of that parameter inside the function.
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numberkruncher earned 500 total points
ID: 22794342
There are several choices that you could use.

If you only have a small amount of data, like you do, you could use a struct. If your returning large objects, remember that they get copied each time you return them.



// Using a struct, or class:
struct MYDATA
{
   int valueA;
   int valueB;
}
 
MYDATA DoSomething()
{
   MYDATA result;
   result.valueA = 1;
   result.valueB = 2;
   return result;
}
 
void foo()
{
   MYDATA result = DoSomething();
   // result.valueA == 1 && result.valueB == 2
}
 
// Using attributes
void DoSomething(int *pValueA, int *pValueB)
{
   *pValueA = 1;
   *pValueB = 2;
}
 
void foo()
{
   int valueA = 0, valueB = 0;   // Don't forget to initialize here!
   DoSomething(&valueA, &valueB);
   // valueA == 1 && valueB == 2
}
 
Or you could use references instead of pointers.
 
void DoSomething(int& rValueA, int& rValueB)
{
   rValueA = 1;
   rValueB = 2;
}
 
void foo()
{
   int valueA = 0, valueB = 0;   // Don't forget to initialize here!
   DoSomething(valueA, valueB);
   // valueA == 1 && valueB == 2
}

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Expert Comment

by:numberkruncher
ID: 22794374
If your two variables are global, and your just trying to access them from your executable (or another library) then you need to export them.

// .h Header of Library
extern MY_API int globalA;
extern MY_API int globalB;

// .cpp Source of Library
int globalA = 1;
int globalB = 2;
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LVL 19

Expert Comment

by:mrwad99
ID: 22794817
>>

MYDATA DoSomething()
{
   MYDATA result;
   result.valueA = 1;
   result.valueB = 2;
   return result;
}

I would personally pass a MYDATA into that function, then modify it.  This saves the copying on return.

void DoSomething ( MYDATA& result )
{
   result.valueA = 1;
   result.valueB = 2;
}

But that is just me :)
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LVL 5

Expert Comment

by:Kimpan
ID: 23811726
Just wondering. Since you are using _bstr_t, is it a COM-specific solution you need? Maybe even want it to be OLE Automation?
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