[Zeek-Dev] Question about IntrusivePtr and incomplete types

[Dominik Charousset]

> I just tried to use our new IntrusivePtr type for the first time - and 
> encountered a (for me) unexpected problem.
> 
> In my specific case, I want to introduce a new member variable for 
> TableVal. The type of it is:
> 
>   IntrusivePtr<Expr> change_func;
> 
> Compiling Zeek with this addition to the TableVal class will yield in 
> the following compilation error:
> 
> /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++ 
>  -DDOCTEST_CONFIG_DISABLE -Isrc/zeekygen -I../src/zeekygen -Isrc 
> -I../src -I/opt/local/include -Iaux/binpac/lib -I../aux/binpac/lib 
> -I../aux/paraglob/include -I../aux/broker/3rdparty/caf/libcaf_test 
> -I../aux/broker/3rdparty/caf/libcaf_openssl 
> -I../aux/broker/3rdparty/caf/libcaf_io 
> -Iaux/broker/caf-build/libcaf_core 
> -I../aux/broker/3rdparty/caf/libcaf_core -Iaux/broker/include 
> -I../aux/broker/include -I. -Wall -Wno-unused -g -DDEBUG -DBRO_DEBUG 
> -Wno-register -std=c++17 -g -isysroot 
> /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk 
> -mmacosx-version-min=10.14 -MD -MT 
> src/zeekygen/CMakeFiles/bro_zeekygen.dir/ScriptInfo.cc.o -MF 
> src/zeekygen/CMakeFiles/bro_zeekygen.dir/ScriptInfo.cc.o.d -o 
> src/zeekygen/CMakeFiles/bro_zeekygen.dir/ScriptInfo.cc.o -c 
> ../src/zeekygen/ScriptInfo.cc
> In file included from ../src/zeekygen/ScriptInfo.cc:7:
> In file included from ../src/zeekygen/Manager.h:14:
> In file included from ../src/Val.h:23:
> ../src/IntrusivePtr.h:88:4: error: no matching function for call to 
> 'Unref'
>                         Unref(ptr_);
>                         ^~~~~
> ../src/Val.h:903:2: note: in instantiation of member function 
> 'IntrusivePtr<Expr>::~IntrusivePtr' requested here
>         TableVal()      {}
>         ^
> ../src/Obj.h:199:13: note: candidate function not viable: cannot convert 
> argument of incomplete type 'IntrusivePtr<Expr>::pointer' (aka 'Expr *') 
> to 'BroObj *' for 1st argument
> inline void Unref(BroObj* o)
>             ^
> Including Expr.h in Val.h is not trivially possible due to other 
> ordering conflicts breaking the build.
> 
> Is there some way to use an IntrusivePtr in this case - or is is just 
> not possible to use it in cases where it is not possible to use complete 
> types?

You should be able to get away with forward declarations as long as you provide matching prototypes for Ref and Unref.