The C++ code that using templates doesn't compile in c++ 20, but was ok in c++ 17

Question

I have some legacy code that I want to upgrade to c++20. The code compiles in c++17 without any issues but fails if I switch to C++20. I simplified the code as much as possible, please look at the example. I am using Visual Studio 2022 on Windows

 class MyFile { public: void Read(){ } }; class ArrayWrapper; class BaseClass { public: ArrayWrapper* arrayWrapper; }; template <class T> class ArrayClass : public BaseClass { public: void Add() { arrayWrapper->file.Read(); } }; class ArrayWrapper { public: ArrayWrapper(): file(new MyFile()){ } ~ArrayWrapper() { delete file; } MyFile* file; ArrayClass<int> Array{}; void Update() { Array.Add(); } }; int main() { ArrayWrapper wrapper; wrapper.Update(); }

And this is the error message that I am getting:

error C2027: use of undefined type 'ArrayWrapper' message : see declaration of 'ArrayWrapper' message : see reference to class template instantiation 'ArrayClass<T>' being compiled

I did look on the stack overflow and in the C++ docs but couldn't figure out what is wrong with this code in C++ 20 Can someone explain how to fix this issue, please?

This doesn't compile with /std:c++17 either. It gives the same expected and self-explanatory error since you have member access into the incomplete type ArrayWrapper. What in the error message is unclear? — Ted Lyngmo
– Ted Lyngmo, Commented Jul 27, 2023 at 10:06
Clang 16 gives "error: member access into incomplete type 'ArrayWrapper' void Add() { arrayWrapper->file.Read(); }" which might be more helpful - at the point where arrayWrapper->file.Read() is encountered, the compiler doesn't know anything about ArrayWrapper — Peter Hull
– Peter Hull, Commented Jul 27, 2023 at 10:15
@TedLyngmo compiles with /std:c++17 for me after fixing the error with file.Read which should be file->Read demo. — n. m. could be an AI
– n. m. could be an AI, Commented Jul 27, 2023 at 10:18
@n.m.willseey'allonReddit It seems they made it more strict in C++20 mode. Add /permissive- when compiling in C++17 mode and it fails there too. — Ted Lyngmo
– Ted Lyngmo, Commented Jul 27, 2023 at 11:07
MSVC lacks two-phase name lookup without /permissive-, and setting C++20 or C++latest implies /permissive- while setting C++17 does not. — ildjarn
– ildjarn, Commented Jul 27, 2023 at 11:26

Quimby · Accepted Answer · 2023-07-27 10:11:04Z

Move ArrayClass::Add definition after ArrayWrapper.

The method and the whole ArrayClass<int> is instantiated right after the definition of the class template, at which point ArrayWrapper is still not fully defined.

You would get the same error if ArrayClass was not a template.

class MyFile { public: void Read(){ } }; class ArrayWrapper; class BaseClass { public: ArrayWrapper* arrayWrapper; }; template <class T> class ArrayClass : public BaseClass { public: void Add(); }; class ArrayWrapper { public: ArrayWrapper(): file(new MyFile()){ } ~ArrayWrapper() { delete file; } MyFile* file; ArrayClass<int> Array{}; void Update() { Array.Add(); } }; template <class T> void ArrayClass<T>::Add() { arrayWrapper->file->Read(); } int main() { ArrayWrapper wrapper; wrapper.Update(); }

Also, it should be arrayWrapper->file->Read(); file is a pointer.

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