Properly using C++ functions from C

Question

I have been trying to setup an object loader for OpenGL this morning and have been making some progress, but now I am stuck on what seems to be the final step in getting my setup to work. My main OpenGL program is written in c (shift.c) and my object loader is written in C++ (loader.cpp). I have also written the linker for the two loader.h based on this SO thread.

I think that I have done something wrong in the translation, because everything I have done so far hasn't worked properly. My files are as followed.

loader.h

#ifndef LOADER_H
#define LOADER_H

#ifdef _cplusplus
class Model_OBJ
{
  public:
    Model_OBJ();
    float* Model_OBJ::calculateNormal(float* coord1,float* coord2,float* coord3 );
    int Model_OBJ::Load(char *filename);  // Loads the model
    void Model_OBJ::Draw();         // Draws the model on the screen
    void Model_OBJ::Release();        // Release the model

    float* normals;             // Stores the normals
    float* Faces_Triangles;         // Stores the triangles
    float* vertexBuffer;          // Stores the points which make the object
    long TotalConnectedPoints;        // Stores the total number of connected verteces
    long TotalConnectedTriangles;     // Stores the total number of connected triangles

};
#endif /* __cplusplus */

#ifdef _cplusplus
extern "C" {
#endif
    struct model_obj;

    float* calculateNormal( float *coord1, float *coord2, float *coord3 );
    int Load(char* filename);
    void Release();
    void Draw();

    float* normals;             // Stores the normals
    float* Faces_Triangles;         // Stores the triangles
    float* vertexBuffer;          // Stores the points which make the object
    long TotalConnectedPoints;        // Stores the total number of connected verteces
    long TotalConnectedTriangles;     // Stores the total number of connected triangles

#ifdef _cplusplus
}
#endif

#endif /* LOADER_H */

loader.cpp

/*
 *
 * Demonstrates how to load and display an Wavefront OBJ file.
 * Using triangles and normals as static object. No texture mapping.
 * http://talkera.org/opengl/
 *
 * OBJ files must be triangulated!!!
 * Non triangulated objects wont work!
 * You can use Blender to triangulate
 *
 */

#include "loader.h"
// Rest of my includes

class Model_OBJ
{
  public:
    Model_OBJ();
    float* calculateNormal(float* coord1,float* coord2,float* coord3 );
    int Load(char *filename);  // Loads the model
    void Draw();         // Draws the model on the screen
    void Release();        // Release the model

    float* normals;             // Stores the normals
    float* Faces_Triangles;         // Stores the triangles
    float* vertexBuffer;          // Stores the points which make the object
    long TotalConnectedPoints;        // Stores the total number of connected verteces
    long TotalConnectedTriangles;     // Stores the total number of connected triangles

};

#define POINTS_PER_VERTEX 3
#define TOTAL_FLOATS_IN_TRIANGLE 9
using namespace std;

Model_OBJ::Model_OBJ(){...}

float* Model_OBJ::calculateNormal( float *coord1, float *coord2, float *coord3 ){...}  
int Model_OBJ::Load(char* filename){...}
void Model_OBJ::Release(){...}
void Model_OBJ::Draw(){...}

In my shift.c file. I have been trying to do something like the following. But have yet to be successful.

#include "loader.h"
// Up in variable declarions
struct model_obj *obj1;

int main()
{
  obj1.Load("file.obj");
}

Any reason why you didn't go for C++ all the way ? You could probably even keep your existing C code and rename the file to `shift.cpp` that would likely make it work. C is not an exact subset of C++ but in this case it probably would act as-if it were. — ereOn, Dec 04 '14 at 18:22
I also note you need a move constructor, move assignment, and destructor. And maybe copy constructor, copy assignment. — Mooing Duck, Dec 04 '14 at 18:23
Any reason not to follow the example of the accepted answer in that post? — , Dec 04 '14 at 18:42
I did follow that answer in that post but ran into problems. That is why I posted another. — Scalahansolo, Dec 04 '14 at 18:45
Please investigate the proper use of the opaque type `struct model_obj;` (which is not an Model_OBJ). See `struct animal; // a nice opaque type` in that post. — , Dec 04 '14 at 18:52

Mooing Duck · Accepted Answer · 2014-12-04T19:43:14.537

5

With a C++ struct like yours, the C part of that header would require serious changes. Usually a C wrapper around a C++ class looks more like this. (Also you have to change class to struct.)

#ifdef _cplusplus
extern "C" {
#endif
struct Model_OBJ;

Model_OBJ* Model_OBJ_new(void);
void Model_OBJ_delete(Model_OBJ*self); 

float* Model_OBJ_calculateNormal(Model_OBJ*self,float*coord1,float*coord2,float*coord3);
int Model_OBJ_Load(Model_OBJ*self,char*filename); // Loads the model
void Model_OBJ_Draw(Model_OBJ*self); // Draws the model on the screen

#ifdef _cplusplus
}
#endif

And then in the C++ file, you'd have to also have these functions implemented.

Model_OBJ* Model_OBJ_new(void) {return new Model_OBJ();}
void Model_OBJ_delete(Model_OBJ*self) {delete self}


float* Model_OBJ_calculateNormal(Model_OBJ*self,float*coord1,float*coord2,float*coord3)
{return self->calculateNormal(coord1,coord2,coord3);}
int Model_OBJ_Load(Model_OBJ*self,char*filename)
{return self->Load(filename);}
void Model_OBJ_Draw(Model_OBJ*self)
{return self->Draw();}

And then the C code can use it:

int main() {
    Model_OBJ* obj1 = Model_OBJ_new();
    Model_OBJ_Load(obj1, "file.obj");
    Model_OBJ_delete(obj1);
}

edited Dec 04 '14 at 19:43

answered Dec 04 '14 at 18:22

Mooing Duck

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`new_Model_OBJ()` should be `Model_OBJ_new()` in that last snippet. – John Bollinger Dec 04 '14 at 18:24
@JohnBollinger: Thanks. renamed at last second to be more consistent and missed one. – Mooing Duck Dec 04 '14 at 18:25
After making these changes, when I compile I am getting errors in `loader.h` like the following. `error: unknown type name ‘Model_OBJ’ void Model_OBJ_delete(Model_OBJ*self);` – Scalahansolo Dec 04 '14 at 18:36
@SeanCallahan: Does the C part of the header contain the line `struct Model_OBJ;`? That needs to be in the C part first. The C++ header contains the full struct definition, the C merely has a declaration. – Mooing Duck Dec 04 '14 at 18:40
It does. But it isnt even getting that far into the C file during compiling. It's erroring out in the .h file. – Scalahansolo Dec 04 '14 at 18:41
I think an `extern "C"` needs to be placed about `Model_OBJ_...()` family to get C function signatures. – chux - Reinstate Monica Dec 04 '14 at 18:47
@SeanCallahan: Uh, the `struct Model_OBJ;` should be the first line in the C part _of the header file_. Is that not where you put it? – Mooing Duck Dec 04 '14 at 19:04
@chux: he already had that in his code, but I added it to my sample for clarity. – Mooing Duck Dec 04 '14 at 19:05
@Mooing Duck Right you are! I had jumped right to your fine answer. – chux - Reinstate Monica Dec 04 '14 at 19:15
1

Minor: Should `Model_OBJ* Model_OBJ_new();` be `Model_OBJ* Model_OBJ_new(void);` So C does not see this as `Model_OBJ* Model_OBJ_new(...);` – chux - Reinstate Monica Dec 04 '14 at 19:16
1

@chux: Good call, I forgot that quirk. – Mooing Duck Dec 04 '14 at 19:43

Properly using C++ functions from C

loader.h

loader.cpp

1 Answers1