220124_c_declaration.c

// C declaration

// tested with gcc 11.2

#include <stdio.h>

///////////////////////////////////////////////////////
// 1.
// function cannot return a function,
// but instead, it can return a pointer to a function

// compile error
//int (f_error())(); // function returning a function

int i = 42;
int *f1(int param) { i += param; return &i; };
int*(*f2())(int) { return f1; } // function returning a pointer to a function

///////////////////////////////////////////////////////
// 2.
// function cannot return an array
// but instead, it can return a pointer to an array

// compile error
//int (fa_error())[]; // function returning an array

int a[2] = {42, 239};
int (*fa())[] { return &a; } // function returning a pointer to an array

///////////////////////////////////////////////////////
// 3.
// array cannot contain functions
// but instead, array can contain pointers to function

// compile error
//int (af_error[])(); // array of function

int af1(int param) { return 42 - param; }
int af2(int param) { return 42 + param; }
int (*af[])(int) = { af1, af2 }; // array of pointers to function

// test! check that declaration shape and call shape is the same
int main()
{
    printf("%i\n", *  f2 ()(239));
    printf("%i\n", *(*f2)()(239));
    //          int*(*f2())(int)
    //               ^
    //               we can omit this

    printf("a[0] is %i\n", (*fa())[0]);
    printf("a[1] is %i\n", (*fa())[1]);
    //                 int (*fa())[ ]

    printf("af1(239) is %i\n",   af[0] (239));
    printf("af2(239) is %i\n", (*af[1])(239));
    //                     int (*af[ ])(int)
    //                          ^
    //                          we can omit this
}