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Compatibility with VST 2.x or VST 1

Q: How can I update my VST 2 version of my plug-in to a VST 3 version and be sure that Cubase will load it instead of my old one?

You have to provide a special UID for your kVstAudioEffectClass and kVstComponentControllerClass components, based on its VST 2 UniqueID (4 characters) and its plug-in name in the following way:

static void convertVST2UID_To_FUID (FUID& newOne, int32 myVST2UID_4Chars, const char* pluginName, bool forControllerUID = false)
{
    char uidString[33];
 
    int32 vstfxid;
    if (forControllerUID)
        vstfxid = (('V' << 16) | ('S' << 8) | 'E');
    else
        vstfxid = (('V' << 16) | ('S' << 8) | 'T');
 
    char vstfxidStr[7] = {0};
    sprintf (vstfxidStr, "%06X", vstfxid);
 
    char uidStr[9] = {0};
    sprintf (uidStr, "%08X", myVST2UID_4Chars);
 
    strcpy (uidString, vstfxidStr);
    strcat (uidString, uidStr);
 
    char nameidStr[3] = {0};
    size_t len = strlen (pluginName);
 
    // !!!the pluginName has to be lower case!!!!
    for (uint16 i = 0; i <= 8; i++)
    {
        uint8 c = i < len ? pluginName[i] : 0;
        sprintf (nameidStr, "%02X", c);
        strcat (uidString, nameidStr);
    }
    newOne.fromString (uidString);
}

Note that if you are developing a new plug-in and if you are using the VST 2 wrapper included in the SDK you do not need to use convertVST2UID_To_FUID, a VST 2 specific vendor call allows the host (Steinberg hosts since Cubase 4.0) to get from a VST 2 version a VST 3 UID.

// extracted code from vst2wrapper.cpp
//-----------------------------------------------------------------------------
VstIntPtr Vst2Wrapper::vendorSpecific (VstInt32 lArg, VstIntPtr lArg2, void* ptrArg, float floatArg)
{
    switch (lArg)
    {
        case 'stCA':
        case 'stCa':
            switch (lArg2)
            {
                case 'FUID':
                    if (ptrArg)
                    {
                        if (vst3EffectClassID.isValid ())
                        {
                            memcpy ((char*)ptrArg, vst3EffectClassID, 16);
                            return 1;
                        }
                    }
                break;
        // ....

Q: How can I support projects which were saved with the VST 2 version of my plug-in?

The host will call IComponent::setState() and IEditController::setComponentState() with the complete FXB/FXP stream. You have to extract your old state from that, for this the SDK provides a helper function VST3::tryVst2StateLoad.

Here the code to add in the VST 3 version when a VST 3 plug-in replaces a VST 2 plug-in in a Steinberg sequencer project:

#include "public.sdk/source/vst/utility/vst2persistence.h"
 
//------------------------------------------------------------------------
tresult PLUGIN_API MyVST3Effect::setState (IBStream* state)
{
    if (auto vst2State = VST3::tryVst2StateLoad (*state))
    {
        if ((vst2State->programs.empty ()) ||
            (static_cast<int32_t> (vst2State->programs.size ()) <= vst2State->currentProgram))
            return kResultFalse;
 
        //....
    }
    return kResultFalse;
}

For complete example source code you could check the function:
tresult PLUGIN_API Processor::setState (IBStream state)* in the file public.sdk/samples/vst/mda-vst3/source/mdaBaseProcessor.cpp

For Automation compatibility, you have to ensure that VST 3 parameter IDs have the same value than the indexes of their associated parameters in VST 2. Only with this condition the host can play back the automation. The parameter value has the same meaning in VST 2 and VST 3.

Q: I've already released a VST 3 plug-in with a different UID than its VST 2 equivalent. How can I declare that the VST 3 plug-in can replace its VST 2 plug-in counterpart?

You can use the compatibility array of the moduleinfo.json file. There, you use the UID of your VST 3 audio effect class as the "New" UID and put the UID of your VST 2 plug-in in the "Old" array.

If you cannot use the moduleinfo.json file, you can create a class that implements the IPluginCompatibility interface which you must provide to the host via your plug-in factory.

Q: In VST 2 the editor was able to access the processing part, named effect, directly. How can I do this in VST 3?

You cannot and more importantly must not do this. The processing part and user interface part communicate via a messaging system.
See Q: How should I communicate between the 'Processing' and the 'User Interface'? for details.

Q: Does VST 3 implement methods like beginEdit and endEdit known from VST 2?

Yes and it is essential to support this for automation. For details, please see Parameters and Automation

Q: Does VST 3 include variable Input/Output processing like processVariableIo of VST 2?

Not in version 3.1.0, we plan something in this direction later. (Note: this variableIO processing was for example for time stretching plug-ins).

Q: What is the equivalent to the VST 2 kPlugCategOfflineProcess?

VST 3 doesn't support offline processing like it did in VST 2 (this interface was exclusively used by WaveLab). But it is possible to use VST 3 plug-ins in an offline context (this means that the process function can be called faster than real time: for example, during an export or a batch processing). If the plug-in doesn't support faster than realtime, it should add kOnlyRealTime to its category.