C++ module question?

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I'm trying to figure out the basics of getting the forward declaration problem solved in modules. I believe any reasonably sized project needs classes to hold references or pointers to other classes. So the bare bones of the problem, expressed without modules is:

C++

class B;

class A {
   private:
      B* b;
   public:
      void setB(B* b_) { b = b_; }
   };

class B {
   private:
      A* a;
   public:
      void setA(A* a_) { a = a_; }
   };


int main() {
   A a;
   B b;
   a.setB(&b);
   b.setA(&a);
   return 0;
   }

Since I read that modules can include header files, my first attempt in Visual Studio 2019 (updated to 16.11.9) is:

C++

//Main unit:
import B;

class A {
   private:
      B* b;
   public:
      void setB(B* b_) { b = b_; }
   };

int main() {
   A a;
   B b;
   a.setB(&b);
   b.setA(&a);
   return 0;
   }

C++

//B.ixx:
module;
export module B;

#include "A_declaration.h"

export class B {
   private:
      A* a;
   public:
      void setA(A* a_) { a = a_; }
   };

and

C++

//A_declaration.h:
#pragma once

class A;   //Just a forward declaration

Upon compilation, at the 'b.setA(&a);' line this gives: Error C2664 'void B::setA(A *)': cannot convert argument 1 from 'A *' to 'A *'

What is the correct way to do this?

What I have tried:

What I've outlined above. And some reading. But not enough to find the solution, unfortunately.

Posted 20-Jan-22 20:51pm

David O'Neil

Updated 21-Jan-22 9:55am

Sean Ewington

v2

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2 solutions

Solution 1

I am not sure where you got those import and export lines from. The usual way inb C++ is to put the declarations in the header file and the implementation in the .cpp file. Something like:

C++

// Something.h
//
// if CSomething includes a reference to COtherthing then you can add
class COtherthing; // forward reference. But it is a bad idea for COtherthing to back reference CSomething. That is a design issue.

class CSomething
{
private:
    int m_somevariable;
    COtherthing* other;

public:
    CSomething(); // constructor declarations
    ~CSomething(); // destructor declarations

    int getSomething(); // function declaration
};

The definitions (implementation) then goes into the .cpp file

C++

#nclude "Something.h"

int main()
{
    CSomething something; // instantiate an object of the class

    int foo = something.getSomething(); // gets a value
}

Posted 20-Jan-22 21:50pm

Richard MacCutchan

Comments

David O'Neil 21-Jan-22 4:04am

I understand where you are coming from, but I am trying to wrap my head around modules, which are fairly new in C++: https://docs.microsoft.com/en-us/cpp/cpp/modules-cpp?view=msvc-170. My initial code example is compilable, and I know how to split that up into .cpp/h files without any problems. It is only when I try to replicate it with modules that the problems come up.

Richard MacCutchan 21-Jan-22 5:33am

Sorry, I misunderstood the question. My experience of modules is a number approaching (but not equal to) zero. But whether you use modules, or traditional headers, the issues are very similar.

CPallini 21-Jan-22 4:16am

Hey man, you need to get more current!
https://vector-of-bool.github.io/2019/03/10/modules-1.html
(actually this is the very first time I hear about C++ modules... )

Richard MacCutchan 21-Jan-22 5:32am

I had heard of them before today but have not actually tried to use them yet. Maybe tomorrow :).

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David O'Neil · Accepted Answer · 2022-01-21T09:55:00

Thanks to c++20 - C++ Modules Forward Declaring Entity from Another Module - Stack Overflow[^], I got a solution working. It is:

C++

//main.cpp:
import A;
import B;

int main() {
   A a;
   B b;
   a.setB(&b);
   b.setA(&a);
   return 0;
   }

C++

//A.ixx:
export module A;

export class B;

export class A {
   private:
      B* b;
   public:
      void setB(B* b_) { b = b_; }
   };

C++

//B.ixx:
export module B;

export class A;

export class B {
   private:
      A* a;
   public:
      void setA(A* a_) { a = a_; }
   };

The only problem is that although it works, Visual Studio Intellisense is showing errors that "incomplete class type "A" is not allowed" on both "A a;" and "a.setB(&b);" lines. Those errors will be annoying to see, even though the compiled program doesn't treat them as errors! :rolleyes:

If I rearrange the 'import A' 'import B' lines, then the errors become 'incomplete class type "B" is not allowed' on the 'b' lines. :anotherRolleye:

Other than the errors that aren't errors, I like this way better because it eliminates headers. Unless I'm overlooking something else.

C++ module question?

2 solutions

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Solution 2

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C++ module question?

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