Trouble using opaque pointers in Objective C++

The answer to this quesion explains that opaque pointers are a good way to include C++ member variables in an Objective C++ header. I'm getting compile errors when trying to follow the example. Here's the relevant code from my header, wit开发者_StackOverflowh the corresponding compiler errors shown as comments:

struct ADSR_opaque; // error: forward declaration of 'struct ADSR_opaque'

@interface LoopyPulser : NSObject{

    float _pulseRate;
    UInt32 tickInterval;
    UInt32 step;
    InMemoryAudioFile * audioFilePlayer;
    ADSR_opaque* env; //  error: expected specifier-qualifier-list before 'ADSR_opaque'
    Pattern * pattern;
    float loopLengthRatio;
    float volume;
}

Is there something simple I'm doing wrong here?

I don't have any problem with the following minimal sample:

struct ADSR_opaque;
@interface LoopyPulser : NSObject {
    struct ADSR_opaque* env;
}
@end

If you include the header in plain Objective-C files (not Objective-C++), you have to add struct.

Alternatively use typedefs:

struct ADSR_opaque_;
typedef struct ADSR_opaque_ ADSR_opaque;
@interface LoopyPulser : NSObject {
    ADSR_opaque* env;
    // ...