How Can I Assign a Function to a Function Pointer

Question

I would like to be able to do something like this:

void test();
void (*testPointer)() = SomethingThatReturnsAFunctionPointer();
test = testPointer;

I would like to make something that functions similarly to the way openGL headers are implemented in which function prototypes are declared, and then the functions are set to a pointer. In other words, I would like to know how some openGL header files are able to both load the openGL functions, and have prototypes of the functions at the same time.

https://stackoverflow.com/questions/840501/how-do-function-pointers-in-c-work — Jack, Jun 10 '17 at 01:23
Note that there isn't a function prototype in the question — there are function declarations, but none of the declarations features a prototype. To declare a function that takes no parameters, C requires `void test(void);` with the second `void` being crucial. C++ does not need this; it is a difference between C and C++. What you've got allows any number of arguments to be passed to `test()`. — Jonathan Leffler, Jun 10 '17 at 06:02

score 2 · Accepted Answer · answered Jun 10 '17 at 01:27

Functions are not variables; they cannot be assigned to.

However, functions can be implicitly converted to pointers to those functions. The function itself still isn't a variable, but you can assign that function pointer to a variable that is suitably typed for that function pointer.

I would like to know how some openGL header files are able to both load the openGL functions, and have prototypes of the functions at the same time.

I don't know which particular header you're talking about, but the loader I created simply has the implementation of the function call the function pointer, passing it all of the parameters and returning its value (if any). The pointer is defined inside a source file, so it's not in the header itself.

Using your example:

//header
void test();

//source file
void (*testPointer)();

void test()
{
  testPointer();
}

You can even get fancy and make test load the pointer:

//source file
void (*testPointer)() == NULL;

void test()
{
  if(!testPointer)
  {
    testPointer = SomethingThatReturnsAFunctionPointer();
  }
  testPointer();
}

bobra · Answer 2 · 2017-06-10T03:28:24.337

1st:

int aplusb(int a, int b){return a + b;}
int aminusb(int a, int b){return a - b;}

int (*func)(int a, int b) = aplusb;

int some_func_caller ( int A, int B, int (*func)(int a, int b)){
   return func(A, B);
}

 int main(){
    int a_ =10, b_ = 7;
    int res1 = func(a_, b_);
    int res2 = somefunc_caller(a_, b_, aminusb); 
    return 0;
}

2nd:

(if you re using c++ compiler)

typedef int MyFuncionType(int a, int b);
typedef int (*MyFuncionType2)(int a, int b);

int aplusb(int a, int b){return a + b;}
int aminusb(int a, int b){return a - b;}

int some_function_caller1(int a, int b, MyfunctionType fc){return fc(a,b);}
int some_function_caller2(int a, int b, MyfunctionType2 fc){return fc(a,b);}

int main(){
   int a_ = 10, b_ = 7;
   MyFunctionType *func1 = aminusb, *func2 = nullptr;
   MyFunctionType2 func3 = aplusb, func4 = nullptr;

  int res1 = func1(a_, b_);
  int res2 = func3(a_, b_);

  int res3 = some_function_caller1(a_, b_, aplusb);
  int res4 = some_function_caller2(a_, b_, aminusb);

  return 0;

}

How Can I Assign a Function to a Function Pointer

2 Answers2