Purpose of function declaration within function definition

Question

In C, one can of course declare and define functions. And this interacts with default constructor calls from C++ to yield the most vexing parse problem. Say I want to declare a function foo, define a function bar and create an instance t, I might mistakenly write this (having Python or Java in mind):

T foo();

T bar() {
    T t();
}

This will not do what I want, it will declare a function t with no arguments (or perhaps arbitrary many ones as there is no void written in there, not sure) and return type T. I would get a T instance with T t;.

Why is this syntax even allowed? What is the point of declaring a function just within a function? The linker will have to resolve this at link time anyway, and then one could just declare that function in the file scope. Is there some edge case that I am missing?

May be just because this function declaration at function scope is allowed in C (with no most-vexing-parse ambiguity) and C++ wants to accept most of C syntax? — prog-fh, Jan 31 '21 at 11:30
@prog-fh: I see! So my question should rather be whether that was allowed in C in the first place. — Martin Ueding, Jan 31 '21 at 13:11

score 2 · Accepted Answer · answered Jan 31 '21 at 13:43

Not a formal explanation but just a use-case in which I would use the function-scope declaration.

Imagine you have a C header file offering an inline static function, but in specific cases this function should rely on an internal helper function which is not part of the API.

// file utils.h
#ifndef UTILS_H
#define UTILS_H

// many other things...

inline static
int
nice_util(int i,
          int j)
{
  int k=i+2*j;
  if(i>10) // should not happen so often
  {
    void ugly_helper_function(int n); // no one else should be aware of this function
    ugly_helper_function(k);
  }
  return k;
}

#endif // UTILS_H

There is probably an implementation file coming with this header-file.

// file utils.c

#include <utils.h>
#include <stdio.h>

// many other things...

void
ugly_helper_function(int n)
{
  printf("%s(%d)\n", __func__, n);
}

Using this header-file provides the expected function but not the internal one (except if you ignore the warnings).

// file prog.c
#include <utils.h>

int
main(void)
{
  // ugly_helper_function(5678); // warning: implicit declaration...
  return nice_util(12, 34);
}

Without the ability to declare the internal function at function-scope, we would have had to declare it at global-scope in the header file, and that would have made it accessible without any warning.

Purpose of function declaration within function definition

1 Answers1