This adds a new `DNA_sdna_type_ids.hh` header:
```cpp
namespace blender::dna {
/**
* Each DNA struct has an integer identifier which is unique within a specific
* Blender build, but not necessarily across different builds. The identifier
* can be used to index into `SDNA.structs`.
*/
template<typename T> int sdna_struct_id_get();
/**
* The maximum identifier that will be returned by #sdna_struct_id_get in this
* Blender build.
*/
int sdna_struct_id_get_max();
} // namespace blender::dna
```
The `sdna_struct_id_get` function is used as replacement of
`SDNA_TYPE_FROM_STRUCT` in all places except the DNA defaults system. The
defaults system is C code and therefore can't use the template. There is ongoing
work to replace the defaults system as well though: #134531.
Using this templated function has some benefits over the old approach:
* No need to rely on macros.
* Can use type inferencing in functions like `BLO_write_struct` which avoids
redundancy on the call site. E.g. `BLO_write_struct(writer, ActionStrip,
strip);` can become `BLO_write_struct(writer, strip);` which could even become
`writer.write_struct(strip);`. None of that is implemented as part of this
patch though.
* No need to include the generated `dna_type_offsets.h` file which contains a
huge enum.
Implementation wise, this is done using explicit template instantiations in a
new file generated by `makesdna.cc`: `dna_struct_ids.cc`. The generated file
looks like so:
```cpp
namespace blender::dna {
template<typename T> int sdna_struct_id_get();
int sdna_struct_id_get_max();
int sdna_struct_id_get_max() { return 951; }
}
struct IDPropertyUIData;
template<> int blender:🧬:sdna_struct_id_get<IDPropertyUIData>() { return 1; }
struct IDPropertyUIDataEnumItem;
template<> int blender:🧬:sdna_struct_id_get<IDPropertyUIDataEnumItem>() { return 2; }
```
I tried using static variables instead of separate functions, but I didn't
manage to link it properly. Not quite sure yet if that's an actual limitation or
if I was just missing something.
Pull Request: https://projects.blender.org/blender/blender/pulls/138706
9fd7a093c9