Class is not defined in child file after include, why?

Question

C++ / C#

There are two files,

LicenseKey.h

#include "LicensesProduct.h"

namespace Machinarium {
  namespace Licenses_System {

    #warning "LICENSE_KEY Class creation"
    class LicenseKey : IValidationInterface {

...

LicensesProduct.h

#include "LicenseKey.hpp"
#include "IValidationInterface.h"

namespace Machinarium {
  namespace Licenses_System {

    class LicensesProduct : public IValidationInterface, Machinarium::BaseClasses::Nameable {

    private:
      Cryptographic::CryptographicKeyPair cryptographicKeyPair;
      LicenseKey activeLicense;
...

==== Building licenses (release) ====
LicenseKey.cpp
In file included from src/LicenseKey.cpp:1:
src/LicenseKey.hpp:11:4: warning: #warning "LICENSE_KEY Class creation" [-Wcpp]
   11 |   #warning "LICENSE_KEY Class creation"
      |    ^~~~~~~
In file included from src/LicenseKey.hpp:6,
                 from src/LicenseKey.cpp:1:
src/LicensesProduct.h:17:4: error: ‘LicenseKey’ does not name a type
   17 |    LicenseKey activeLicense;

For what reason is it so? In essence, the LicenseKey class is defined when included in the LicenseKey.h file

Answer 1

Firstly, for some reason you have the extension h somewhere, and somewhere hpp. It needs to be brought to one.
Secondly, you have a cyclic include, this will not work. This must be decided. For example, you can solve with forward declaration and pimpl.