cppyy does not retrieve correct size of type when pythonizing vector

[vepadulano]

root [0] auto a = std::vector<const char*>::value_type();
root [1] a
(const char *) nullptr

Whereas it should return the correct const char* type https://godbolt.org/z/a3s95cex3

Setup

ROOT master and 6.26
Fedora 36
GCC 12

Feel free to change assignees if needed

[vepadulano]

This was the original cause for #11581

[hahnjo]

I may be missing the point here, but to me it seems Cling is doing exactly what you ask it to do: value_type in std::vector is a typedef to the templated type, in this case const char *. Thus, "constructing" a std::vector<const char*>::value_type() yields the default value, a (correctly typed) nullptr. AFAICT you get the equivalent result if you ask for std::vector<int>::value_type(), which is (int) 0.

[vepadulano]

Indeed, it's a bit more subtle. There is a pythonization happening here, which transforms the type given by ::value_type to a string and sets the corresponding Python attribute. Probably for const char * the type is not properly parsed thus the attribute is not set.

[vepadulano]

The issue seems to be with this line in particular:

const std::string& vtype = Cppyy::ResolveName(name+"::value_type");
size_t typesz = Cppyy::SizeOf(vtype);

In the reproducer case, this is equivalent to calling Cppyy::SizeOf("const char *"). This will call SizeOf(const std::string &). In turn, this first calls gROOT->GetType("const char *"), which fails, then falls back to calling SizeOf(GetScope(type_name)).

In GetScope, we reach this part that calls TClass::GetClass. As we can see, this returns nullptr:

root [0] auto cl = TClass::GetClass("const char *", true, true);
root [1] cl
(TClass *) nullptr

Thus, the final result of the initial call to Cppyy::SizeOf(vtype) is 0. This means that cppyy thinks that the size of these types (including const char *, const int * and so on) is zero, thus the attribute is never pythonized.

In upstream cppyy, this is fixed by calling SizeOf with the original type, instead of the resolved one: https://github.com/wlav/CPyCppyy/blob/master/src/Pythonize.cxx#L1789-L1802

So this should be done in our cppyy too. Reassigning the issue to me, it's not related to cling but cppyy itself

[guitargeek]

Since this is fixed upstream, this will the fixed by the cppyy sync PR:
#14507

[github-actions]

Hi @guitargeek, @vepadulano,

It appears this issue is closed, but wasn't yet added to a project. Please add upcoming versions that will include the fix, or 'not applicable' otherwise.

Sincerely,
🤖