Editing Closure (computer programming) (section)

====Nested function and function pointer (C)====
With a [[GNU Compiler Collection]] (GCC) extension, a nested function<ref>{{cite web
|url = https://gcc.gnu.org/onlinedocs/gcc/Nested-Functions.html
|title = Nested functions}}</ref> can be used and a function pointer can emulate closures, provided the function does not exit the containing scope. The next example is invalid because <code>adder</code> is a top-level definition (depending on compiler version, it could produce a correct result if compiled with no optimizing, i.e., at <code>-O0</code>):

<syntaxhighlight lang="c">
#include <stdio.h>

typedef int (*fn_int_to_int)(int); // type of function int->int

fn_int_to_int adder(int number) {
  int add (int value) { return value + number; }
  return &add; // & operator is optional here because the name of a function in C is a pointer pointing on itself
}

int main(void) {
  fn_int_to_int add10 = adder(10);
  printf("%d\n", add10(1));
  return 0;
}
</syntaxhighlight>

But moving <code>adder</code> (and, optionally, the <code>typedef</code>) in <code>main</code> makes it valid:

<syntaxhighlight lang="c">
#include <stdio.h>

int main(void) {
  typedef int (*fn_int_to_int)(int); // type of function int->int
  
  fn_int_to_int adder(int number) {
    int add (int value) { return value + number; }
    return add;
  }
  
  fn_int_to_int add10 = adder(10);
  printf("%d\n", add10(1));
  return 0;
}
</syntaxhighlight>

If executed this now prints <code>11</code> as expected.