Forward declaration of inline functions

Question

I have a header file that is going to contain a large amount (30+) of inline functions.

Rather than having the reader scroll or search for the definition (implementation) of the inline function, I would like to have a forward declaration section that states the function declaration with comments describing the function. This section would allow the reader to find out how to use a function or to look for a function without having to scroll down to the implementation.

Also, I would like the readers to get in the habit of using functions without having to see their implementations.

What is the syntax for a forward declaration of a stand-alone function?

{This applies to C99 and C++}

FYI, I am using IAR Workbench C compiler set to use C99.

Answer 1

No differently than a non-inline function:

void func();       // "forward" declaration

// ...

inline void func() // definition
{
    // impl
}

Typically the pattern used to "hide" the definitions from the library consumer is to put the declarations in one header (a.h) and the definitions in a second header (a_def.h), then have the former #include the latter (inclusion guards omitted for brevity):

// a.h
void func();
#include "a_def.h"

// a_def.h
inline void func()
{
    // impl
}

The library consumer would simply #include <a.h>.

Answer 2

You don't need to "forward declare" it (a term that is usually only applied to types, since we usually define them in the same place that we first declare them).

Just declare it, like normal:

#include <iostream>

void foo();            // Declaration

inline void foo() {    // Defining declaration
   std::cout << "!";
}

// ---------------------------------------------

int main() {
   foo();              // Output: foo()
}

Live demo.