Why Zserio parameters are stored as raw pointers in C++?

[mikir]

Zserio parameters in parameterized types are stored in C++ as raw pointers. This decision has been chosen after investigation of the following three different solutions:

1. Passing by value

   This is easy for applications but it would bring significant performance and memory penalties for bigger parameters. Besides of that, it is wrong from Zserio point of view because Zserio allows setting offsets which are stored in parameters. Example:

   BlockHeader
   {
       uint16 numItems;
       uint32 offset;
   };
   
   Block(BlockHeader header)
   {
   header.offset:
       int64 items[header.numItems];
   };
   

   The offset header.offset means that offset field in parameter header must be updated within of Block scope. This is of course not possible if parameter header is passed by value because only local copy can be updated not original BlockHeader.

   To overcome this, we would have to disable completely offsets which use parameters.

2. Passing by shared pointers

   This seems to be a clear, proper solution but it would bring as well performance and memory penalties. Moreover, this solution would change interface for applications. Because each field can be potentially used as a parameter, it would need to be kept by shared pointer instead of by value, regardless if it used as parameter or not.

   This would be possible to improve by global analysis of the schema to detect fields which are used as parameters and only that ones kept by shared pointers. But this approach would lead to surprising interface differences after some changes in schema which introduce new parameters of already existing Zserio types.

3. Passing by raw pointers

   This is not intuitive for developers and very fragile solution which is currently implemented in Zserio. Parameters are stored as raw pointers internally but they are not set by applications in constructors. As a consequence, the following methods must be generated:

   • New empty constructor which leaves Zserio object in uninitialized state
   • New initializeChildren() method which sets parameters for all children of the Zserio object recursively
   • New initialize() method which sets all parameters for the Zserio object and which calls internally initializeChildren() method
   • copy/move constructors and copy/move assignment operators not to break internal raw pointers