Question

I am writing an Objective-C class that needs to make function calls on a C++ class instance. I found this suggestion, but if I try it I get an error for having an incomplete definition of type ‘struct MyCPlusPlusClass’

struct MyCPlusPlusClass;

typedef struct MyCPlusPlusClass MyCPlusPlusClass;

@interface MyBridgeClass() {
    MyCPlusPlusClass *my_CPlusPlus;
}

...

- (id)initWithMrGame:(MyCPlusPlusClass *)cPlusPlus
{
    self = [super init]; 
    if (self) {
        my_CPlusPlus = cPlusPlusClass;
        my_CPlusPlus->p_favorite_integer = 0; // Compiler error
    }
    return self;
}

The actual definition occurs in a .mm file that’s generated by a pre-compiler, just to add another layer of challenge.

How might I get this to work?

EDIT: Interpreting Adam’s answer

// in MyCode.h

struct MyCPlusPlusClass;  // Forward declaration, no need for #include/#import

@interface MyBridgeClass() {
    struct MyCPlusPlusClass *my_CPlusPlus;
}


// in MyCode.m
#include MyCode.h


// in BigGenerateFile.mm
class MyCPlusPlusClass;

class MyCPlusPlusClass { ... }

My goal is to be able to use MyCPlusPlusClass in MyCode.m, but I can’t include the .mm file because the compiler gets very unhappy. It may be that the way this thing is architected is going to make me go a different route.

Answer 1

You can’t access member variables of incomplete structures/classes. To do so, you need to the full definition. Typically you use forward declarations in header files so that anything that includes that header doesn’t pull in lots of unnecessary other header files it won’t need, but for source files you usually need the full definitions.

So I’d suggest changing you code to something like this:

// Header file (.h)
struct MyCPlusPlusClass;  // Forward declaration, no need for #include/#import

@interface MyBridgeClass() {
    struct MyCPlusPlusClass *my_CPlusPlus;
}

// Source file (.mm)
#include "MyCPlusPlusClass.h"
...
// Can now access my_CPlusPlus->p_favorite_integer etc.