How can I call a function using a function pointer?

Question

Suppose I have these three functions:

bool A();
bool B();
bool C();

How do I call one of these functions conditionally using a function poi

Answer 1

Note that when you say:

bool (*a)();

you are declaring a of type "pointer to function returning bool and taking an unspecified number of parameters". Assuming bool is defined (maybe you're using C99 and have included stdbool.h, or it may be a typedef), this may or may not be what you want.

The problem here is that there is no way for the compiler to now check if a is assigned to a correct value. The same problem exists with your function declarations. A(), B(), and C() are all declared as functions "returning bool and taking an unspecified number of parameters".

To see the kind of problems that may have, let's write a program:

#include 

int test_zero(void)
{
    return 42;
}

static int test_one(char *data)
{
    return printf("%s\n", data);
}

int main(void)
{
    /* a is of type "pointer to function returning int
       and taking unspecified number of parameters */
    int (*a)();

    /* b is of type "pointer to function returning int
       and taking no parameters */
    int (*b)(void);

    /* This is OK */
    a = test_zero;
    printf("a: %d\n", a());

    a = test_one; /* OK, since compiler doesn't check the parameters */
    printf("a: %d\n", a()); /* oops, wrong number of args */

    /* This is OK too */
    b = test_zero;
    printf("b: %d\n", b());

    /* The compiler now does type checking, and sees that the
       assignment is wrong, so it can warn us */
    b = test_one;
    printf("b: %d\n", b()); /* Wrong again */

    return 0;
}

When I compile the above with gcc, it says:

warning: assignment from incompatible pointer type

for the line b = test_one;, which is good. There is no warning for the corresponding assignment to a.

So, you should declare your functions as:

bool A(void);
bool B(void);
bool C(void);

And then the variable to hold the function should be declared as:

bool (*choice)(void);