Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>You seem to be misunderstanding the way an instance pointer works. You don't store a pointer to the field, you store a pointer to the class or the record that it's a field of. Object references are pointers already, so no casting is needed there. For records, you need to obtain a pointer to them with the @ symbol.</p> <p>Once you have your pointer, and a TRttiField object that refers to that field, you can call SetValue or GetValue on the TRttiField, and pass in your instance pointer, and it takes care of all the offset calculations for you.</p> <p>In the specific case of arrays, GetValue it will give you a TValue that represents an array. You can test this by calling <code>TValue.IsArray</code> if you want. When you have a TValue that represents an array, you can get the length of the array with <code>TValue.GetArrayLength</code> and retrieve the individual elements with <code>TValue.GetArrayElement</code>.</p> <p><strong>EDIT:</strong> Here's how to deal with record members in a class.</p> <p>Records are types too, and they have RTTI of their own. You can modify them without doing "GetValue, modify, SetValue" like this:</p> <pre><code>procedure ModifyPoint(example: TExampleClass; newXValue, newYValue: integer); var context: TRttiContext; value: TValue; field: TRttiField; instance: pointer; recordType: TRttiRecordType; begin field := context.GetType(TExampleClass).GetField('FPoint'); //TValue that references the TPoint value := field.GetValue(example); //Extract the instance pointer to the TPoint within your object instance := value.GetReferenceToRawData; //RTTI for the TPoint type recordType := context.GetType(value.TypeInfo) as TRttiRecordType; //Access the individual members of the TPoint recordType.GetField('X').SetValue(instance, newXValue); recordType.GetField('Y').SetValue(instance, newYValue); end; </code></pre> <p>It looks like the part you didn't know about is TValue.GetReferenceToRawData. That will give you a pointer to the field, without you needing to worry about calculating offsets and casting pointers to integers.</p>
    singulars
    1. This table or related slice is empty.
    1. PORtti accessing fields and properties in complex data structures
      singulars
      1. PTQuestion
    1. PTAnswer
    1. USMason Wheeler
    1. USMason Wheeler
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. COI do understand the idea of the instance pointer. And I do know how to access fields of a record or a class. The reason, why I do this calculation of the instance pointer above is, that the Field FPoint again contains a record. I do not want to first use GetValue for FPoint, then modify it and then again use SetValue to write it back. Therefore I need to know the instance pointer of FPoint. So I can access the values without having to care about the instance of TExampleClass.
      singulars
      1. PO
      1. USChristian Metzler
    2. CO@Coco: I see. There's still a simpler way. I'll add to my answer.
      singulars
      1. PO
      1. USMason Wheeler
    3. COUnfortunately this does not work as expected. Modifying a Value like this will not affect the original data. The reason for this is, that TValue ALWAYS makes a copy (I tried this before). You can easily prove this by watching the adress of FPoint and and Instance. GetReferenceToRawData does not mean a reference to the original data. Perhaps this is a bug, but if you try my offset calculation, you will get a correct result.
      singulars
      1. PO
      1. USChristian Metzler
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload