function pointers within a struct but with functions of different prototype in C

Question

Suppose I have functions:

void func1(int x)
{
    ....
}

void func2(int x, int y)
{
    ....
}

void func3(int x, int y, int z)
{
    ....
}

And say that I want to have a function pointer within a struct:

For example

typedef struct{
     char *ename;
     char **pname;
    < and here I want to have a function pointer> ??
} Example;

Example ex[3];

Now, I want to populate the ex[3] array as:

ex[0].ename = "X0";
ex[0].pname[0]="A0";
ex[0].pname[1]="B0";
ex[0].<function pointer to func1() > ??


ex[1].ename = "X1";
ex[1].pname[0]="A1";
ex[1].pname[1]="B1";
ex[1].<function pointer to func2() > ??

... and so on...

Is it possible to create something like this? Please help me with this. Thanks.

Sanity Check: When you decide to call that function, how will you "know" how many operands to pass it? — Mysticial
– Mysticial, Commented Mar 2, 2012 at 15:30

cnicutar · Accepted Answer · 2012-03-02 15:30:17Z

7

I would use a union of function pointers:

union {
    void (*fun1)(int);
    void (*fun2)(int, int);
} fptr;

You also need a field in the struct to tell which is used.

answered Mar 2, 2012 at 15:30

cnicutar

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Comments

Jonathan Leffler · Accepted Answer · 2012-03-02 16:44:10Z

You have two choices - sloppy but easy, or exact but painstaking.

Sloppy

typedef struct{
     char *ename;
     char *pname[3];
     void (*function)();   // Pointer to function taking indeterminate arguments
} Example;

Example ex[3] =
{
    { "func1", { "x",           }, func1 },
    { "func2", { "x", "y",      }, func2 },
    { "func3", { "x", "y", "z", }, func3 },
};

This doesn't pass muster if you compile with -Wstrict-prototypes in GCC. Notice that I made sure there was storage for the parameter names - your original had char **pname which would mean you have to allocate the storage for the arrays before assigning the names.

Painstaking

typedef struct{
     char *ename;
     char *pname[3];
     union
     {
         void (*f1)(int x);
         void (*f2)(int x, int y);
         void (*f3)(int x, int y, int z);
     } u;
} Example;

Example ex[3] =
{
    { "func1", { "x",           }, .u.f1 = func1 },
    { "func2", { "x", "y",      }, .u.f2 = func2 },
    { "func3", { "x", "y", "z", }, .u.f3 = func3 },
};

This uses C99 designated initializers to do the initializing. It assigns each function pointer to the correctly typed element of the union.

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