function pointer and function is incompatible because of arguments

Question

I have read the book Understanding and Using C Pointers and try to compile the code below. But after compiling I got the warning: assignment from incompatible pointer type.

I have checked the code and find out the function pointer fptrSet and function ShapeSetX is incompatible because the first argument of fptrSet is void * and function ShapeSetX is Shape *.

How can I fix this? Thanks!

typedef void (*fptrSet)(void*, int);
typedef int  (*fptrGet)(void*);
typedef void (*fptrDisplay)();    

typedef struct _vfunc
{
    fptrSet     setX;
    fptrGet     getX;
    fptrSet     setY;
    fptrGet     getY;
    fptrDisplay display;    
} vFunc;    

typedef struct _shape
{
    vFunc function;
    int x;
    int y;
} Shape;

void displayShape(){
    printf("Shape\n");
}
void ShapeSetX(Shape *shape, int x){
    shape->x = x;
}
void ShapeSetY(Shape *shape, int y){
    shape->y = y;
}
int ShapeGetX(Shape *shape){
    return shape->x;
}
int ShapeGetY(Shape *shape){
    return shape->y;
}    

Shape *newShape()
{
    Shape *shape = (Shape *)malloc(sizeof(Shape));
    shape->x = 10;
    shape->y = 10;
    shape->function.setX = ShapeSetX;
    shape->function.getX = ShapeGetX;
    shape->function.setY = ShapeSetY;
    shape->function.getY = ShapeGetY;
    shape->function.display = displayShape;
    return shape;
}

I think you can also change the typedef void (*fptrSet)(void*, int) to typedef void (*fptrSet)(Shape*, int) and the same for fptrGet. — wake-0
– wake-0, Commented Jun 14, 2016 at 10:49
@KevinWallis I think he wants something like virtual table from C++, so because there are no inheritance in C, he has to use void to allow "derived" struct from Shape — Garf365
– Garf365, Commented Jun 14, 2016 at 10:51
@KevinWallis @Garf365 is right, as you see this code is try to simulate the vtable from C++, so the the argument must be void * — FatMouse
– FatMouse, Commented Jun 15, 2016 at 3:33
Also : take a look at stackoverflow.com/questions/605845/… — Garf365
– Garf365, Commented Jun 15, 2016 at 7:16

Garf365 · Accepted Answer · 2016-06-14 10:47:06Z

1

You have to respect pointer definition: pointer need that first parameter is a pointer to void, so your function implementation should have first parameter as void:

void ShapeSetX(void *void_shape, int x){
    Shape *shape = (Shape*) void_shape;
    shape->x = x;
}
void ShapeSetY(void *void_shape, int y){
    Shape *shape = (Shape*) void_shape;
    shape->y = y;
}
int ShapeGetX(void *void_shape){
    Shape *shape = (Shape*) void_shape;
    return shape->x;
}
int ShapeGetY(void *void_shape){
    Shape *shape = (Shape*) void_shape;
    return shape->y;
}

edited Jun 14, 2016 at 10:47

answered Jun 14, 2016 at 10:45

Garf365

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2 Comments

FatMouse Over a year ago

@Garf365 Thanks for you answer and you got what I want. I try to use the code to simulate the 'ploymorphism' from OOP. Really thank you !

Garf365 Over a year ago

@FatMouse I think you have also to pass by a base structure, common on each instance (to access safetly to vtable). And also, in C++, vtable are static, you only store pointer to vtable in your instance. I try something to implements vtable here : ideone.com/P8dk6g any comments will be appreciate ;)

John Burger · Accepted Answer · 2016-06-14 10:53:27Z

1

I was going to say "Why don't you replace void with Shape then?", until I realised that Shape hadn't been defined yet - nor could you swap the two definitions, because Shape needs vFunc which needs the typedefs.

So, do this:

typedef struct _shape Shape; // Define _shape and Shape later

typedef void (*fptrSet)(Shape*, int);
typedef int  (*fptrGet)(Shape*);
typedef void (*fptrDisplay)();

If your compiler doesn't like that, you may need to change it to:

typedef struct _shape; // Define _shape later

typedef void (*fptrSet)(struct _shape*, int);
typedef int  (*fptrGet)(struct _shape*);
typedef void (*fptrDisplay)();

answered Jun 14, 2016 at 10:53

John Burger

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4 Comments

Garf365 Over a year ago

With this solution, you can use _vfunc with other structure than Shape. I think OP wants a solution to have "inheritance" solution in C

Garf365 Over a year ago

For example with a function int getXOfInstance(void * instance, vFunc * vfunc) { return vfund->getX(instance);}

John Burger Over a year ago

@Garf I thought of that - but with names like set/get' 'X/y and Display it all sounded rather "Shape-y"

Garf365 Over a year ago

It's not necessarily a Shape, it can be anything which have X, Y properties

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