Question

In an Xcode project I have a C file with functions, it compiles and works OK

I want to wrap my C code in struct(s), how will I be able to call them in Objective-C?

Answer 1

Declare function pointers, add them to your structure and then call them, it’s just C.

Example:

//Typedef 2 function pointers, first takes and returns int,
// second takes and returns double
typedef int    (*FuncPtrInt)   (int);
typedef double (*FuncPtrDouble)(double);

// create structure to store function pointers
struct ABC
{
    FuncPtrInt    applyA;
    FuncPtrDouble applyB;
};

// create some functions to use with structure
int incrFuncA(int num) { return ++num; }
double decrFuncB(double num) { return --num; }
double multiplyFuncB(double num) { return num*num; }

// try it out
void testStruct()
{
    struct ABC abc;
    abc.applyA = incrFuncA;
    abc.applyB = decrFuncB;

    NSLog(@"increment: %d",abc.applyA(3));
    NSLog(@"decrement: %f",abc.applyB(3.5));

    abc.applyB = multiplyFuncB;

    NSLog(@"multiply: %f",abc.applyB(3.5));
}

Output:

2010-02-01 10:36:22.335 x[11847] increment: 4
2010-02-01 10:36:22.336 x[11847] decrement: 2.500000
2010-02-01 10:36:22.336 x[11847] multiply: 12.250000

---

If you want to have a struct with functions where functions operate on the structure you have to pass the pointer to that function by default (similar to what c++ does):

Define:

struct ClassABC;
typedef int (*FuncPtrClassABC)(struct ClassABC *);
typedef int (*FuncPtrClassABCInt)(struct ClassABC *, int);

int incrFunc(struct ClassABC * abc);
int decrFunc(struct ClassABC * abc);
int addFunc(struct ClassABC * abc, int num);
int subtractFunc(struct ClassABC * abc, int num);

struct ClassABC
{
    int i;
    FuncPtrClassABC    increment;
    FuncPtrClassABC    decrement;
    FuncPtrClassABCInt add;
    FuncPtrClassABCInt subtract;
};

As you can see these functions could be standalone, you would still pass the ClassABC in:

int incrFunc(struct ClassABC * abc) { return ++(abc->i); }
int decrFunc(struct ClassABC * abc) { return --(abc->i); }
int addFunc(struct ClassABC * abc, int num)
{ abc->i += num; return abc->i; }
int subtractFunc(struct ClassABC * abc, int num)
{ abc->i -= num; return abc->i; }

Initialization helper func:

void initClassABC(struct ClassABC * abc)
{
    abc->i = 0;
    abc->increment = incrFunc;
    abc->decrement = decrFunc;
    abc->add = addFunc;
    abc->subtract = subtractFunc;
}

Usage:

struct ClassABC cabc;
initClassABC(&cabc);

cabc.add(&cabc,4);
NSLog(@"add: %d", cabc.i);

cabc.decrement(&cabc);
NSLog(@"decrement: %d", cabc.i);

cabc.subtract(&cabc,2);
NSLog(@"subtract: %d", cabc.i);

Output:

2010-02-01 10:56:39.569 x[12894] add: 4
2010-02-01 10:56:39.569 x[12894] decrement: 3
2010-02-01 10:56:39.569 x[12894] subtract: 1

Enjoy