"cyclic dependency" error using c++20 Modules

Question

Im trying to include a module within another module, but i can't compile due to the following error:

"Cannot build the following source files because there is a cyclic dependency between them: Module1.ixx depends on Module2.ixx depends on Module1.ixx."

Code i tried:

// Module1.ixx
export module Module1;
import Module2;
export class modClass1_
{
public:
    modClass1_() {};
    ~modClass1_() {};
    int getV() { return V; };
    int getOtherModuleV() { modClass2_ mc2; return mc2.V; };
    int V = 1;
};

// Module2.ixx
export module Module2;
import Module1;
export class modClass2_
{
public:
    modClass2_() {};
    ~modClass2_() {};
    int getV() { return V; };
    int getOtherModuleV() { modClass1_ mc1; return mc1.V; };
    int V = 2;
};

Any help/suggestion will be appreciated.

Environment: Visual Studio 2019 | MSVC-2019 | C++20 | Windows 10 Pro

Answer 1

Create module interface units that do not have circular dependencies.

A module interface unit is somewhat similar in purpose to a header file.

A module interface unit has export module in it. The primary module interface unit has no partition name.

Remove the implementation of { modClass2_ mc2; return mc2.V; }; from export Module1; primary module interface unit, do that in the module implementation unit of Module1, and include import Module2; there.

// Module1.ixx
export module Module1;
export class modClass1_
{
public:
    modClass1_() {};
    ~modClass1_() {};
    int getV() { return V; };
    int getOtherModuleV();
    int V = 1;
};

// Module1.mxx
module Module1;
import Module2;

int modClass1_::getOtherModuleV() { modClass2_ mc2; return mc2.V; };

and the circular dependency is broken.