partial forward declaration in c++

Question

I have a project (A) that uses a class from another (B), but inluding B's header makes the compilation of A very messy, while separate compilation works fine (but in that case I can't use B's class in A).

B is compiled with VS2019's compiler v142, A is compiled with cuda 10's nvcc.

How can I forward declare only few members eg., a constructor, a property and a method ?

In A, I would like to uses this code without B's header:

DX12Cuda = new DX12CudaInterop(ImUtil.ImFeatures.Width, ImUtil.ImFeatures.Height, L"DX12Cuda", funcMessage);
DX12Cuda->CuSurfaceUpdater = [&](cudaSurfaceObject_t o) {UpdateCuSurface(o); };
OnRenderDX12 = [&]() {DX12Cuda->OnRender(); };
DX12Cuda->OnInit(hwnd);

In B:

class DX12CudaInterop
{
public:
    DX12CudaInterop(UINT width, UINT height, wstring name, MessageChangedCallback managedDelegate);
    function<void(cudaSurfaceObject_t)> CuSurfaceUpdater;
    OnInit(HWND hwnd);
    OnRender();
}

void DX12CudaInterop::OnInit(HWND hwnd) {/*...*/}
void DX12CudaInterop::OnRender() {/*...*/}

examples of errors:

Severity Code Description Project File Line Suppression State Error identifier "NPP_MIRROR_FLIP_ERR" is undefined ShCuSum path_to_solution\D3D12CudaUpdateFull\helper_cuda.h 412 Error name followed by "::" must be a class or namespace name ShCuSum path_to_solution\D3D12CudaUpdateFull\DXSampleHelper.h 3
Error identifier "byte" is undefined ShCuSum path_to_solution\D3D12CudaUpdateFull\DXSampleHelper.h 29
Error name followed by "::" must be a class or namespace name ShCuSum path_to_solution\D3D12CudaUpdateFull\DXSampleHelper.h 31
Error name followed by "::" must be a class or namespace name ShCuSum path_to_solution\D3D12CudaUpdateFull\DXSampleHelper.h 41
Error expected a ";" ShCuSum path_to_solution\D3D12CudaUpdateFull\DXSampleHelper.h 41
Error identifier "file" is undefined ShCuSum path_to_solution\D3D12CudaUpdateFull\DXSampleHelper.h 42
Error identifier "byte" is undefined ShCuSum path_to_solution\D3D12CudaUpdateFull\DXSampleHelper.h 52
Error inline specifier allowed on function declarations only ShCuSum path_to_solution\D3D12CudaUpdateFull\DXSampleHelper.h 63
Error incomplete type is not allowed ShCuSum path_to_solution\D3D12CudaUpdateFull\DXSampleHelper.h 63
Error identifier "ID3D12Object" is undefined ShCuSum path_to_solution\D3D12CudaUpdateFull\DXSampleHelper.h 63
Error identifier "pObject" is undefined ShCuSum path_to_solution\D3D12CudaUpdateFull\DXSampleHelper.h 63
Error expected a ")" ShCuSum path_to_solution\D3D12CudaUpdateFull\DXSampleHelper.h 63
Error expected a ";" ShCuSum path_to_solution\D3D12CudaUpdateFull\DXSampleHelper.h 64
Error identifier "IDXGIFactory2" is undefined ShCuSum path_to_solution\D3D12CudaUpdateFull\DX12CudaSample.h 30
Error identifier "IDXGIAdapter1" is undefined ShCuSum path_to_solution\D3D12CudaUpdateFull\DX12CudaSample.h 30
Error identifier "NPP_BAD_ARG_ERROR" is undefined ShCuSum path_to_solution\D3D12CudaUpdateFull\helper_cuda.h 323 Error identifier "NPP_COEFF_ERROR" is undefined ShCuSum path_to_solution\D3D12CudaUpdateFull\helper_cuda.h 326 Error identifier "NPP_RECT_ERROR" is undefined ShCuSum path_to_solution\D3D12CudaUpdateFull\helper_cuda.h 329 Error identifier "NPP_QUAD_ERROR" is undefined ShCuSum path_to_solution\D3D12CudaUpdateFull\helper_cuda.h 332 Error identifier "NPP_MEM_ALLOC_ERR" is undefined ShCuSum path_to_solution\D3D12CudaUpdateFull\helper_cuda.h 335 Error identifier "NPP_HISTO_NUMBER_OF_LEVELS_ERROR" is undefined ShCuSum path_to_solution\D3D12CudaUpdateFull\helper_cuda.h 338 Error identifier "NPP_INVALID_INPUT" is undefined ShCuSum path_to_solution\D3D12CudaUpdateFull\helper_cuda.h 341 Error identifier "NPP_POINTER_ERROR" is undefined ShCuSum path_to_solution\D3D12CudaUpdateFull\helper_cuda.h 344 Error identifier "NPP_WARNING" is undefined ShCuSum path_to_solution\D3D12CudaUpdateFull\helper_cuda.h 347 Error identifier "NPP_ODD_ROI_WARNING" is undefined ShCuSum path_to_solution\D3D12CudaUpdateFull\helper_cuda.h 350 Error identifier "NPP_MEMFREE_ERR" is undefined ShCuSum path_to_solution\D3D12CudaUpdateFull\helper_cuda.h 403 Error identifier "NPP_MEMSET_ERR" is undefined ShCuSum path_to_solution\D3D12CudaUpdateFull\helper_cuda.h 406 Error identifier "NPP_MEMCPY_ERR" is undefined ShCuSum path_to_solution\D3D12CudaUpdateFull\helper_cuda.h 409 Error (active) E0135 class "Microsoft::WRL::Details::EnableIf" has no member "type" D3D12CudaUpdate C:\Program Files (x86)\Windows Kits\10\Include\10.0.18362.0\winrt\wrl\client.h 379

Answer 1

You can’t. Here’s what you can do instead:

// interop.h, shared across the project
class iCudaInterop
{
public:
    virtual ~iCudaInterop() { }
    virtual void OnInit( HWND hwnd ) = 0;
    virtual void OnRender() = 0;
};
extern std::unique_ptr<iCudaInterop> createInterop( UINT width, UINT height, const std::wstring& name, MessageChangedCallback del );

// interop.cpp, only in project B
#include "interop.h"

class CudaInterop : public iCudaInterop
{
    DX12CudaInterop m_impl;

public:
    CudaInterop( UINT width, UINT height, const std::wstring& name, MessageChangedCallback del ) :
        m_impl( width, height, name, del ) { }
    ~CudaInterop() = default;

    void OnInit( HWND hwnd ) override { m_impl.OnInit( hwnd ); }
    void OnRender() override { m_impl.OnRender(); }
};

std::unique_ptr<iCudaInterop> createInterop( UINT width, UINT height, const std::wstring& name, MessageChangedCallback del )
{
    return std::make_unique<CudaInterop>( width, height, name, del );
}

For that property, the easy way is wrap into 2 more abstract methods of iCudaInterop interface, getter and setter, in the implementation get/set the field of m_impl.