Natvis visualizer for c++ union/structure

Question

I'm trying to implement a personal visualizer using msvc natvis visualizer. The problem is that I don't know how to do it regarding union. A simple example with a structure ( value) containing a union of two structure (string1 and string2 ):

typedef struct value                      
{   int type;                                              /* variable type */
    union                                                         
    {
         string1    sval;                                 
         string2    sval2;                                  
    } t;
} 

typedef struct string1
{  
    int    size;
    char  *data;
} aString1;

typedef struct string2
{   
    int    size;
    char  *data;
} aString2;

I can create two type for string1 and string2 with the following code in natvis :

<Type Name="string1"> /// (A) preview
    <DisplayString>{{ string 1 }}</DisplayString>
    <Expand>
      <Item Name="text">data</Item>
    </Expand>
  </Type>
<Type Name="string2">  /// (B) preview
    <DisplayString>{{ string 2 }}</DisplayString>
    <Expand>
      <Item Name="text">data</Item>
    </Expand>
  </Type>

But how can I automatically preview these type when I have a "value" variable (the union). I’m stuck to this point : (assuming the variable type equals 1 to indicate string1, and 2 for string2 ). I have done :

 <Type Name="value">
    <DisplayString>{{Value}}</DisplayString>
    <Expand>
            <Synthetic Name="String 1 Name" Condition="type==1"> // assume type of string1 = 1
                  /// here i want to call preview I have created for string1 in (A)
            </Synthetic>

             <Synthetic Name="String 2 Name" Condition="type==2"> // assume type of string2 = 2
                 /// here i want to call preview I have created for string2 in (B)
             </Synthetic>
   </Expand>
</Type>

So I would like that depending of the type value, the debug will show the correct visualizer. Can you explain me how to deal with union with natvis ? or is there somewhere an example ? (the official msvc documentation do not consider unions..) Obvioulsy this example makes no sense but it is just to understand because I have a far more complex union.

Answer 1

The following should work:

<Type Name="value">
  <DisplayString Condition="type == 1">{t.sval}</DisplayString>
  <DisplayString Condition="type == 2">{t.sval2}</DisplayString>
  <Expand>
    <ExpandedItem Condition="type == 1">t.sval</ExpandedItem>
    <ExpandedItem Condition="type == 2">t.sval2</ExpandedItem>
  </Expand>
</Type>

ExpandedItem removes the view of the union and uses string1 resp. string2 expansion instead, depending on the value of type.

I haven't tried to use the XML I posted here, so there could be some syntax errors, but you should be able to get it to work with minor tweaks (if any).