What are canonical types in Clang?

Question

I have a simple header parser based on clang and I get the typedefs from some source.

struct _poire {
int g;
tomate rouge;
};
typedef struct _poire kudamono;
         


        

Answer 1

It seems that you have raised an interesting point. I have figured out something, but since I can't actually test my intuition right now, I can't be 100% sure. Anyway here is what I would do :

If I parse your code (with a little extension to declare a kudamono variable), here is what I can say from this:

struct _poire {
    int g;
    char rouge; // tomate is probably one of your classes so I just changed the type of the field.
};
typedef struct _poire kudamono;

int maFonction(){
    kudamono une_poire;
    return 0;
}

When the typedef is parsed, here is what is yielded :

-TypedefDecl 0x23b4620 kudamono 'struct _poire':'struct _poire'

When I declare a variable of type kudamono, here is below its AST-dump :

-VarDecl 0x2048040 une_poire 'kudamono':'struct _poire'

NB : You can get the AST Dump of your code with this command line, it can be really handy to understand how your code will be parsed :

clang -Xclang -ast-dump -std=c++11 -fsyntax-only test.cpp (just remove -std=c++11 if you want to compile a file_name.c file)

Now, from what I understand, I will make a comparaison between the VarDecl and the TypedefDecl :

1°) This VarDecl is named une_poire and has the type kudamono which is a typedef from the type struct _poire.

2°) This TypedefDecl is named kudamono and has the type struct _poire which is a typedef from the type struct _poire

So, the weird part is right here. struct _poire is considered as typedef from struct _poire.

You'll note that I tried to make a typedef with a usual type :

typedef int numbers;

And this time, AST-dump yields :

TypedefDecl 0x25d9680 numbers 'int', so I guess the parser may have some troubles with handmade types (typically structs).

I can see one dirty way to know if your type is canonical or not (without getting false positives or false negatives) :

Check that the QualType and the canonical QualType are not the same

I don't know if a simple '=' between Qualtype will make false positives or false negatives (as I can't test), but you can still compare the names of the types with strcmp

So, to sum up a little bit :

• Your understanding of a canonical type is fine.
• Clang seems to have some trouble with handmade types, but it should be fine with Typedef from usual types (such as typedef int int32_t).
• When you want to know if a type is canonical or not, you can compare the name of the type and the name of the canonical type, but it's quite dirty. On usual type, isCanonical() works well.