WIN32 : Minimal VST 2.x host in C++11.

minimal_vst2x_host.cpp

// Win32 : Minimal VST 2.x Synth host in C++11.
//
// This is a simplified version of the following project by hotwatermorning :
//      A sample VST Host Application for C++ Advent Calendar 2013 5th day.
//      https://github.com/hotwatermorning/VstHostDemo
//
// Usage :
//      1. Compile & Run this program.
//      2. Select your VST Synth DLL.
//      3. Press QWERTY, ZXCV, etc.
//      4. Press Ctrl-C to exit.
//
// See also :
//      Steinberg Media Technologies - The Yvan Grabit Developer Resource
//      http://ygrabit.steinberg.de/~ygrabit/public_html/index.html
//
//      MrsWatson - a command-line audio plugin host.
//      https://github.com/teragonaudio/MrsWatson
//
//  Notes :
//      "vst2.x/aeffectx.h" is part of VST3 SDK. You can download VST3 SDK from
//      Steinberg SDK Download Portal : http://www.steinberg.net/nc/en/company/developers/sdk_download_portal.html
#define UNICODE
#include <windows.h>
#include <process.h>
#include <mmdeviceapi.h>
#include <audioclient.h>

#include <algorithm>
#include <atomic>
#include <iostream>
#include <map>
#include <memory>
#include <mutex>
#include <string>
#include <thread>
#include <vector>
#include <functional>

#pragma warning(push)
#pragma warning(disable : 4996)
#include "VST3 SDK/pluginterfaces/vst2.x/aeffectx.h"
#pragma warning(pop)

#define ASSERT_THROW(c,e)   if(!(c)) { throw std::runtime_error(e); }
#define CLOSE_HANDLE(x)     if((x)) { CloseHandle(x); x = nullptr; }
#define RELEASE(x)          if((x)) { (x)->Release(); x = nullptr; }


struct ComInit {
    ComInit()  { CoInitializeEx(nullptr, COINIT_MULTITHREADED); }
    ~ComInit() { CoUninitialize(); }
};


class VstPlugin {
public:
    VstPlugin(const wchar_t* vstModulePath, HWND hWndParent) {
        init(vstModulePath, hWndParent);
    }

    ~VstPlugin() {
        cleanup();
    }

    size_t  getSamplePos()      const { return samplePos; }
    size_t  getSampleRate()     const { return 44100; }
    size_t  getBlockSize()      const { return 1024; }
    size_t  getChannelCount()   const { return 2; }
    static const char*  getVendorString() { return "TEST_VENDOR"; }
    static const char*  getProductString() { return "TEST_PRODUCT"; }
    static int          getVendorVersion() { return 1; }
    static const char** getCapabilities() {
        static const char* hostCapabilities[] = {
            "sendVstEvents",
            "sendVstMidiEvents",
            "sizeWindow",
            "startStopProcess",
            "sendVstMidiEventFlagIsRealtime",
            nullptr
        };
        return hostCapabilities;
    }

    bool    getFlags(int32_t m) const { return (aEffect->flags & m) == m; }
    bool    flagsHasEditor()    const { return getFlags(effFlagsHasEditor); }
    bool    flagsIsSynth()      const { return getFlags(effFlagsIsSynth); }
    intptr_t dispatcher(int32_t opcode, int32_t index = 0, intptr_t value = 0, void *ptr = nullptr, float opt = 0.0f) const {
        return aEffect->dispatcher(aEffect, opcode, index, value, ptr, opt);
    }

    void resizeEditor(const RECT& clientRc) const {
        if(editorHwnd) {
            auto rc = clientRc;
            const auto style    = GetWindowLongPtr(editorHwnd, GWL_STYLE);
            const auto exStyle  = GetWindowLongPtr(editorHwnd, GWL_EXSTYLE);
            const BOOL fMenu    = GetMenu(editorHwnd) != nullptr;
            AdjustWindowRectEx(&rc, style, fMenu, exStyle);
            MoveWindow(editorHwnd, 0, 0, rc.right-rc.left, rc.bottom-rc.top, TRUE);
        }
    }

    void sendMidiNote(int midiChannel, int noteNumber, bool onOff, int velocity) {
        VstMidiEvent e {};
        e.type          = kVstMidiType;
        e.byteSize      = sizeof(e);
        e.flags         = kVstMidiEventIsRealtime;
        e.midiData[0]   = static_cast<char>(midiChannel + (onOff ? 0x90 : 0x80));
        e.midiData[1]   = static_cast<char>(noteNumber);
        e.midiData[2]   = static_cast<char>(velocity);
        if(auto l = vstMidi.lock()) {
            vstMidi.events.push_back(e);
        }
    }

    // This function is called from refillCallback() which is running in audio thread.
    void processEvents() {
        vstMidiEvents.clear();
        if(auto l = vstMidi.lock()) {
            std::swap(vstMidiEvents, vstMidi.events);
        }
        if(! vstMidiEvents.empty()) {
            const auto n = vstMidiEvents.size();
            const auto bytes = sizeof(VstEvents) + sizeof(VstEvent*) * n;
            vstEventBuffer.resize(bytes);
            auto* ve = reinterpret_cast<VstEvents*>(vstEventBuffer.data());
            ve->numEvents = n;
            ve->reserved = 0;
            for(size_t i = 0; i < n; ++i) {
                ve->events[i] = reinterpret_cast<VstEvent*>(&vstMidiEvents[i]);
            }
            dispatcher(effProcessEvents, 0, 0, ve);
        }
    }

    // This function is called from refillCallback() which is running in audio thread.
    float** processAudio(size_t frameCount, size_t& outputFrameCount) {
        frameCount = std::min<size_t>(frameCount, outputBuffer.size() / getChannelCount());
        aEffect->processReplacing(aEffect, inputBufferHeads.data(), outputBufferHeads.data(), frameCount);
        samplePos += frameCount;
        outputFrameCount = frameCount;
        return outputBufferHeads.data();
    }

private:
    bool init(const wchar_t* vstModulePath, HWND hWndParent) {
        {
            wchar_t buf[MAX_PATH+1] {};
            wchar_t* namePtr = nullptr;
            const auto r = GetFullPathName(vstModulePath, _countof(buf), buf, &namePtr);
            if(r && namePtr) {
                *namePtr = 0;
                char mbBuf[_countof(buf) * 4] {};
                if(auto s = WideCharToMultiByte(CP_OEMCP, 0, buf, -1, mbBuf, sizeof(mbBuf), 0, 0)) {
                    directoryMultiByte = mbBuf;
                }
            }
        }

        hModule = LoadLibrary(vstModulePath);
        ASSERT_THROW(hModule, "Can't open VST DLL");

        typedef AEffect* (VstEntryProc)(audioMasterCallback);
        auto* vstEntryProc = reinterpret_cast<VstEntryProc*>(GetProcAddress(hModule, "VSTPluginMain"));
        if(!vstEntryProc) {
            vstEntryProc = reinterpret_cast<VstEntryProc*>(GetProcAddress(hModule, "main"));
        }
        ASSERT_THROW(vstEntryProc, "VST's entry point not found");

        aEffect = vstEntryProc(hostCallback_static);
        ASSERT_THROW(aEffect && aEffect->magic == kEffectMagic, "Not a VST plugin");
        ASSERT_THROW(flagsIsSynth(), "Not a VST Synth");
        aEffect->user = this;

        inputBuffer.resize(aEffect->numInputs * getBlockSize());
        for(int i = 0; i < aEffect->numInputs; ++i) {
            inputBufferHeads.push_back(&inputBuffer[i * getBlockSize()]);
        }

        outputBuffer.resize(aEffect->numOutputs * getBlockSize());
        for(int i = 0; i < aEffect->numOutputs; ++i) {
            outputBufferHeads.push_back(&outputBuffer[i * getBlockSize()]);
        }

        dispatcher(effOpen);
        dispatcher(effSetSampleRate, 0, 0, 0, static_cast<float>(getSampleRate()));
        dispatcher(effSetBlockSize, 0, getBlockSize());
        dispatcher(effSetProcessPrecision, 0, kVstProcessPrecision32);
        dispatcher(effMainsChanged, 0, 1);
        dispatcher(effStartProcess);

        if(hWndParent && flagsHasEditor()) {
            WNDCLASSEX wcex { sizeof(wcex) };
            wcex.lpfnWndProc    = DefWindowProc;
            wcex.hInstance      = GetModuleHandle(0);
            wcex.lpszClassName  = L"Minimal VST host - Guest VST Window Frame";
            RegisterClassEx(&wcex);

            const auto style = WS_CAPTION | WS_THICKFRAME | WS_OVERLAPPEDWINDOW;
            editorHwnd = CreateWindow(
                  wcex.lpszClassName, vstModulePath, style
                , 0, 0, 0, 0, hWndParent, 0, 0, 0
            );
            dispatcher(effEditOpen, 0, 0, editorHwnd);
            RECT rc {};
            ERect* erc = nullptr;
            dispatcher(effEditGetRect, 0, 0, &erc);
            rc.left     = erc->left;
            rc.top      = erc->top;
            rc.right    = erc->right;
            rc.bottom   = erc->bottom;
            resizeEditor(rc);
            ShowWindow(editorHwnd, SW_SHOW);
        }

        return true;
    }

    void cleanup() {
        if(editorHwnd) {
            dispatcher(effEditClose);
            editorHwnd = nullptr;
        }
        dispatcher(effStopProcess);
        dispatcher(effMainsChanged, 0, 0);
        dispatcher(effClose);
        if(hModule) {
            FreeLibrary(hModule);
            hModule = nullptr;
        }
    }

    static VstIntPtr hostCallback_static(
        AEffect* effect, VstInt32 opcode, VstInt32 index, VstIntPtr value, void *ptr, float opt
    ) {
        if(effect && effect->user) {
            auto* that = static_cast<VstPlugin*>(effect->user);
            return that->hostCallback(opcode, index, value, ptr, opt);
        }

        switch(opcode) {
        case audioMasterVersion:    return kVstVersion;
        default:                    return 0;
        }
    }

    VstIntPtr hostCallback(VstInt32 opcode, VstInt32 index, VstIntPtr value, void* ptr, float) {
        switch(opcode) {
        default:                                break;
        case audioMasterVersion:                return kVstVersion;
        case audioMasterCurrentId:              return aEffect->uniqueID;
        case audioMasterGetSampleRate:          return getSampleRate();
        case audioMasterGetBlockSize:           return getBlockSize();
        case audioMasterGetCurrentProcessLevel: return kVstProcessLevelUnknown;
        case audioMasterGetAutomationState:     return kVstAutomationOff;
        case audioMasterGetLanguage:            return kVstLangEnglish;
        case audioMasterGetVendorVersion:       return getVendorVersion();

        case audioMasterGetVendorString:
            strcpy_s(static_cast<char*>(ptr), kVstMaxVendorStrLen, getVendorString());
            return 1;

        case audioMasterGetProductString:
            strcpy_s(static_cast<char*>(ptr), kVstMaxProductStrLen, getProductString());
            return 1;

        case audioMasterGetTime:
            timeinfo.flags      = 0;
            timeinfo.samplePos  = getSamplePos();
            timeinfo.sampleRate = getSampleRate();
            return reinterpret_cast<VstIntPtr>(&timeinfo);

        case audioMasterGetDirectory:
            return reinterpret_cast<VstIntPtr>(directoryMultiByte.c_str());

        case audioMasterIdle:
            if(editorHwnd) {
                dispatcher(effEditIdle);
            }
            break;

        case audioMasterSizeWindow:
            if(editorHwnd) {
                RECT rc {};
                GetWindowRect(editorHwnd, &rc);
                rc.right = rc.left + static_cast<int>(index);
                rc.bottom = rc.top + static_cast<int>(value);
                resizeEditor(rc);
            }
            break;

        case audioMasterCanDo:
            for(const char** pp = getCapabilities(); *pp; ++pp) {
                if(strcmp(*pp, static_cast<const char*>(ptr)) == 0) {
                    return 1;
                }
            }
            return 0;
        }
        return 0;
    }

protected:
    HWND                editorHwnd { nullptr };
    HMODULE             hModule { nullptr };
    AEffect*            aEffect { nullptr };
    std::atomic<size_t> samplePos { 0 };
    VstTimeInfo         timeinfo {};
    std::string         directoryMultiByte {};

    std::vector<float>          outputBuffer;
    std::vector<float*>         outputBufferHeads;
    std::vector<float>          inputBuffer;
    std::vector<float*>         inputBufferHeads;

    std::vector<VstMidiEvent>   vstMidiEvents;
    std::vector<char>           vstEventBuffer;

    struct {
        std::vector<VstMidiEvent>   events;
        std::unique_lock<std::mutex> lock() const {
            return std::unique_lock<std::mutex>(mutex);
        }
    private:
        std::mutex mutable          mutex;
    } vstMidi;
};


struct Wasapi {
    using RefillFunc = std::function<bool(float*, uint32_t, const WAVEFORMATEX*)>;

    Wasapi(RefillFunc refillFunc, int hnsBufferDuration = 30 * 10000) {
        HRESULT hr = S_OK;

        hClose       = CreateEventEx(0, 0, 0, EVENT_MODIFY_STATE | SYNCHRONIZE);
        hRefillEvent = CreateEventEx(0, 0, 0, EVENT_MODIFY_STATE | SYNCHRONIZE);
        this->refillFunc = refillFunc;

        hr = CoCreateInstance(__uuidof(MMDeviceEnumerator), 0, CLSCTX_INPROC_SERVER, IID_PPV_ARGS(&mmDeviceEnumerator));
        ASSERT_THROW(SUCCEEDED(hr), "CoCreateInstance(MMDeviceEnumerator) failed");

        hr = mmDeviceEnumerator->GetDefaultAudioEndpoint(eRender, eMultimedia, &mmDevice);
        ASSERT_THROW(SUCCEEDED(hr), "mmDeviceEnumerator->GetDefaultAudioEndpoint() failed");

        hr = mmDevice->Activate(__uuidof(IAudioClient), CLSCTX_INPROC_SERVER, 0, reinterpret_cast<void**>(&audioClient));
        ASSERT_THROW(SUCCEEDED(hr), "mmDevice->Activate() failed");

        audioClient->GetMixFormat(&mixFormat);

        hr = audioClient->Initialize(
              AUDCLNT_SHAREMODE_SHARED
            , AUDCLNT_STREAMFLAGS_EVENTCALLBACK | AUDCLNT_STREAMFLAGS_NOPERSIST
            , hnsBufferDuration
            , 0
            , mixFormat
            , nullptr
        );
        ASSERT_THROW(SUCCEEDED(hr), "audioClient->Initialize() failed");

        hr = audioClient->GetService(__uuidof(IAudioRenderClient), reinterpret_cast<void**>(&audioRenderClient));
        ASSERT_THROW(SUCCEEDED(hr), "audioClient->GetService(IAudioRenderClient) failed");

        hr = audioClient->GetBufferSize(&bufferFrameCount);
        ASSERT_THROW(SUCCEEDED(hr), "audioClient->GetBufferSize() failed");

        hr = audioClient->SetEventHandle(hRefillEvent);
        ASSERT_THROW(SUCCEEDED(hr), "audioClient->SetEventHandle() failed");

        BYTE* data = nullptr;
        hr = audioRenderClient->GetBuffer(bufferFrameCount, &data);
        ASSERT_THROW(SUCCEEDED(hr), "audioRenderClient->GetBuffer() failed");

        hr = audioRenderClient->ReleaseBuffer(bufferFrameCount, AUDCLNT_BUFFERFLAGS_SILENT);
        ASSERT_THROW(SUCCEEDED(hr), "audioRenderClient->ReleaseBuffer() failed");

        unsigned threadId = 0;
        hThread = reinterpret_cast<HANDLE>(_beginthreadex(0, 0, threadFunc_static, reinterpret_cast<void*>(this), 0, &threadId));

        hr = audioClient->Start();
        ASSERT_THROW(SUCCEEDED(hr), "audioClient->Start() failed");
    }

    ~Wasapi() {
        if(hClose) {
            SetEvent(hClose);
            if(hThread) {
                WaitForSingleObject(hThread, INFINITE);
            }
        }

        CLOSE_HANDLE(hThread);
        CLOSE_HANDLE(hClose);
        CLOSE_HANDLE(hRefillEvent);

        if(mixFormat) {
            CoTaskMemFree(mixFormat);
            mixFormat = nullptr;
        }

        RELEASE(audioRenderClient);
        RELEASE(audioClient);
        RELEASE(mmDevice);
        RELEASE(mmDeviceEnumerator);
    }

private:
    static unsigned __stdcall threadFunc_static(void* arg) {
        return reinterpret_cast<Wasapi*>(arg)->threadFunc();
    }

    unsigned threadFunc() {
        ComInit comInit {};
        const HANDLE events[2] = { hClose, hRefillEvent };
        for(bool run = true; run; ) {
            const auto r = WaitForMultipleObjects(_countof(events), events, FALSE, INFINITE);
            if(WAIT_OBJECT_0 == r) {    // hClose
                run = false;
            } else if(WAIT_OBJECT_0+1 == r) {   // hRefillEvent
                UINT32 c = 0;
                audioClient->GetCurrentPadding(&c);

                const auto a = bufferFrameCount - c;
                float* data = nullptr;
                audioRenderClient->GetBuffer(a, reinterpret_cast<BYTE**>(&data));

                const auto r = refillFunc(data, a, mixFormat);
                audioRenderClient->ReleaseBuffer(a, r ? 0 : AUDCLNT_BUFFERFLAGS_SILENT);
            }
        }
        return 0;
    }

    HANDLE                  hThread { nullptr };
    IMMDeviceEnumerator*    mmDeviceEnumerator { nullptr };
    IMMDevice*              mmDevice { nullptr };
    IAudioClient*           audioClient { nullptr };
    IAudioRenderClient*     audioRenderClient { nullptr };
    WAVEFORMATEX*           mixFormat { nullptr };
    HANDLE                  hRefillEvent { nullptr };
    HANDLE                  hClose { nullptr };
    UINT32                  bufferFrameCount { 0 };
    RefillFunc              refillFunc {};
};


// This function is called from Wasapi::threadFunc() which is running in audio thread.
bool refillCallback(VstPlugin& vstPlugin, float* const data, uint32_t availableFrameCount, const WAVEFORMATEX* const mixFormat) {
    vstPlugin.processEvents();

    const auto nDstChannels = mixFormat->nChannels;
    const auto nSrcChannels = vstPlugin.getChannelCount();
    const auto vstSamplesPerBlock = vstPlugin.getBlockSize();

    int ofs = 0;
    while(availableFrameCount > 0) {
        size_t outputFrameCount = 0;
        float** vstOutput = vstPlugin.processAudio(availableFrameCount, outputFrameCount);

        // VST vstOutput[][] format :
        //  vstOutput[a][b]
        //      channel = a % vstPlugin.getChannelCount()
        //      frame   = b + floor(a/2) * vstPlugin.getBlockSize()

        // wasapi data[] format :
        //  data[x]
        //      channel = x % mixFormat->nChannels
        //      frame   = floor(x / mixFormat->nChannels);

        const auto nFrame = outputFrameCount;
        for(size_t iFrame = 0; iFrame < nFrame; ++iFrame) {
            for(size_t iChannel = 0; iChannel < nDstChannels; ++iChannel) {
                const int sChannel = iChannel % nSrcChannels;
                const int vstOutputPage = (iFrame / vstSamplesPerBlock) * sChannel + sChannel;
                const int vstOutputIndex = (iFrame % vstSamplesPerBlock);
                const int wasapiWriteIndex = iFrame * nDstChannels + iChannel;
                *(data + ofs + wasapiWriteIndex) = vstOutput[vstOutputPage][vstOutputIndex];
            }
        }

        availableFrameCount -= nFrame;
        ofs += nFrame * nDstChannels;
    }
    return true;
}


void mainLoop(const std::wstring& dllFilename) {
    VstPlugin vstPlugin { dllFilename.c_str(), GetConsoleWindow() };
    Wasapi wasapi { [&vstPlugin](float* const data, uint32_t availableFrameCount, const WAVEFORMATEX* const mixFormat) {
        return refillCallback(vstPlugin, data, availableFrameCount, mixFormat);
    }};

    struct Key {
        Key(int midiNote) : midiNote { midiNote } {}
        int     midiNote {};
        bool    status { false };
    };

    std::map<int, Key> keyMap {
               {'2', {61}}, {'3', {63}},              {'5', {66}}, {'6', {68}}, {'7', {70}},
        {'Q', {60}}, {'W', {62}}, {'E', {64}}, {'R', {65}}, {'T', {67}}, {'Y', {69}}, {'U', {71}}, {'I', {72}},

               {'S', {49}}, {'D', {51}},              {'G', {54}}, {'H', {56}}, {'J', {58}},
        {'Z', {48}}, {'X', {50}}, {'C', {52}}, {'V', {53}}, {'B', {55}}, {'N', {57}}, {'M', {59}}, {VK_OEM_COMMA, {60}},
    };

    for(bool run = true; run; WaitMessage()) {
        MSG msg {};
        while(BOOL b = PeekMessage(&msg, 0, 0, 0, PM_REMOVE)) {
            if(b == -1) {
                run = false;
                break;
            }
            TranslateMessage(&msg);
            DispatchMessage(&msg);
        }

        for(auto& e : keyMap) {
            auto& key = e.second;
            const auto on = (GetKeyState(e.first) & 0x8000) != 0;
            if(key.status != on) {
                key.status = on;
                vstPlugin.sendMidiNote(0, key.midiNote, on, 100);
            }
        }
    }
}


int main() {
    ComInit comInit {};

    const auto dllFilename = []() -> std::wstring {
        wchar_t fn[MAX_PATH+1] {};
        OPENFILENAME ofn { sizeof(ofn) };
        ofn.lpstrFilter = L"VSTi DLL(*.dll)\0*.dll\0All Files(*.*)\0*.*\0\0";
        ofn.lpstrFile   = fn;
        ofn.nMaxFile    = _countof(fn);
        ofn.lpstrTitle  = L"Select VST DLL";
        ofn.Flags       = OFN_FILEMUSTEXIST | OFN_ENABLESIZING;
        GetOpenFileName(&ofn);
        return fn;
    } ();

    try {
        mainLoop(dllFilename);
    } catch(std::exception &e) {
        std::cout << "Exception : " << e.what() << std::endl;
    }
}

I will keep your recommendations in mind.
I am just beginning to try to host VST plugins in my audio C++ program (I use portaudio and ASIO drivers)...
Thanks T-Mat, you are very helpful!

Very nice VST host!
I'm trying to implement this in my C++ program, but I can't open two or more VSTs at the same time. The for loop with "PeekMessage" is necessary for the plugin window to work, so I only get one at a time.
Any solution?

@Israel-7, since I don't know anything about your program, I recommend you to play with this code before (re-) implement your own.

I can say that this example is able to extend to support multiple VSTs with the following modification:

/**/ indicates modified or added line.

(1) main() : Add second VST

int main() {
    ComInit comInit {};

/**/const auto dllFilename0 = []() -> std::wstring {
        wchar_t fn[MAX_PATH+1] {};
        OPENFILENAME ofn { sizeof(ofn) };
        ofn.lpstrFilter = L"VSTi DLL(*.dll)\0*.dll\0All Files(*.*)\0*.*\0\0";
        ofn.lpstrFile   = fn;
        ofn.nMaxFile    = _countof(fn);
        ofn.lpstrTitle  = L"Select VST DLL (#0)";
        ofn.Flags       = OFN_FILEMUSTEXIST | OFN_ENABLESIZING;
        GetOpenFileName(&ofn);
        return fn;
    } ();

/**/const auto dllFilename1 = []() -> std::wstring {
/**/    wchar_t fn[MAX_PATH+1] {};
/**/    OPENFILENAME ofn { sizeof(ofn) };
/**/    ofn.lpstrFilter = L"VSTi DLL(*.dll)\0*.dll\0All Files(*.*)\0*.*\0\0";
/**/    ofn.lpstrFile   = fn;
/**/    ofn.nMaxFile    = _countof(fn);
/**/    ofn.lpstrTitle  = L"Select VST DLL (#1)";
/**/    ofn.Flags       = OFN_FILEMUSTEXIST | OFN_ENABLESIZING;
/**/    GetOpenFileName(&ofn);
/**/    return fn;
/**/} ();

    try {
/**/    mainLoop(dllFilename0, dllFilename1);
    } catch(std::exception &e) {
        std::cout << "Exception : " << e.what() << std::endl;
    }
}

(2) mainLoop() : Add second VST

/**/
void mainLoop(const std::wstring& dllFilename0, const std::wstring& dllFilename1) {
/**/ VstPlugin vstPlugin0 { dllFilename0.c_str(), GetConsoleWindow() };
/**/ VstPlugin vstPlugin1 { dllFilename1.c_str(), GetConsoleWindow() };
/**/ Wasapi wasapi { [&vstPlugin0, &vstPlugin1](float* const data, uint32_t availableFrameCount, const WAVEFORMATEX* const mixFormat) {
/**/     refillCallback(vstPlugin1, data, availableFrameCount, mixFormat, false);
/**/     return refillCallback(vstPlugin0, data, availableFrameCount, mixFormat, true);
     }};

    ...

    for(bool run = true; run; WaitMessage()) {
        MSG msg {};
        ...

        for(auto& e : keyMap) {
            auto& key = e.second;
            const auto on = (GetKeyState(e.first) & 0x8000) != 0;
            if(key.status != on) {
                key.status = on;
/**/            vstPlugin0.sendMidiNote(0, key.midiNote, on, 100);
/**/            vstPlugin1.sendMidiNote(0, key.midiNote, on, 100);
            }
        }
    }
}

(3) refillCallback() : Add additive mode

/**/
bool refillCallback(VstPlugin& vstPlugin, float* const data, uint32_t availableFrameCount, const WAVEFORMATEX* const mixFormat, bool additive) {
    ...
    while(availableFrameCount > 0) {
        ...
        for(size_t iFrame = 0; iFrame < nFrame; ++iFrame) {
            ...
            for(size_t iChannel = 0; iChannel < nDstChannels; ++iChannel) {
               ...
/**/            if(additive) {
/**/                *(data + ofs + wasapiWriteIndex) += vstOutput[vstOutputPage][vstOutputIndex];
/**/            } else {
/**/                *(data + ofs + wasapiWriteIndex) = vstOutput[vstOutputPage][vstOutputIndex];
/**/            }
               ...
            }
        }
        ...
    }
    ...
}