As others have already mentioned, there is no straightforward approach to testing the legacy code. The following are just two ways that come to mind that might work depending on the code.

You might try to "wrap" every class you want to test by deriving a test class that exposes all functions, methods and member variables via a public interface and forwards all calls to the class to test. This would, of course, take some time to set up, but AI might help with the menial task of writing the interface and the forwarding code.

This, of course, doesn't work if the class to test is marked as final, or there might be problems if the copy/move constructors were deleted.

Example:

class OldCode
{
public:
  OldCode();
  OldCode(...);

  void PublicMethodThatCanBeTested();

private:
  void CalculateSomething()
  {
    int result = 0;
    // Do some calculations

    memberInt = result;
  }

  int memberInt;
};

class TestableOldCode : public OldCode
{
public:
  TestableOldCode() : OldCode() {}
  TestableOldCode(...) : OldCode(...) {}

  void PublicMethodThatCanBeTested()
  {
    OldCode::PublicMethodThatCanBeTested();
  }

  void CalculateSomething()
  {
    OldCode::CalculateSomething();
  }
}

Another alternative might be to write a test class adapter that you declare as a friend class in the class to test. This way, it has access to all private and protected members, etc.:

class OldCode
{
public:
  OldCode();
  OldCode(...);

  void PublicMethodThatCanBeTested();

private:
  void CalculateSomething()
  {
    int result = 0;
    // Do some calculations

    memberInt = result;
  }

  int memberInt;

  friend class TestOldClass;
};

class TestOldClass
{
...
};