When to not put C function prototypes in header files

Question

Is there a reason why it might not be good practice to place C/C++ function prototypes in header files but instead place them at the top of the main .c/.cpp file?

For example, I could write a dothis.c file:

#include "funcs.h"

int main(int argc, char ** argv) {
   // some code
   int result = doThis();
   // more code

   return 0;
}

int doThis(void) {
   // code and return value
}

And in my funcs.h file, I'd write:

#ifndef FUNCS_H
#define FUNCS_H

int doThis(void);

// more function prototypes

#endif // FUNCS_H

Is there any reason why it might be better to place the prototype(s) (assuming there are many) at the top of the .c/.cpp file instead? For example:

#include "funcs.h"

int doThis(void);

// more function prototypes

int main(int argc, char ** argv) {
   // some code
   int result = doThis();
   // more code

   return 0;
}

int doThis(void) {
   // code and return value
}

In the first case, I feel it would be helpful to have many function prototypes in a separate header file plus documentation to logically separate declaration from implementation and to make it easier to concisely see what the main file is doing.

When the function is only to be used internally and is not part of the functionality exported/exposed by your header file. — Richard Critten, Jun 06 '18 at 17:27
If your function is only used inside a single file, then it is perfectly valid to put it inside the cpp file itself. If your function is meant to be reused it's better to put it in header... — AlexG, Jun 06 '18 at 17:28
Declare the internally used functions as static. Put the interface function declarations to header file. — Öö Tiib, Jun 06 '18 at 17:28
*Only* if the function is both defined and exclusively used in that file (in which case it might as well be explicitly declared `static`). But an `extern` prototype in a `.c` file, for a function defined elsewhere, pretty much demolishes the whole point of prototypes. (You weren't asking about `extern` prototypes, but I wanted to make this point. See also [this question](https://stackoverflow.com/questions/50401100/compiler-warning-for-function-defined-without-prototype-in-scope).) — Steve Summit, Jun 06 '18 at 17:29
Opinion-based. I avoid prototypes for unexposed functions, preferring to implement the functions in an order where it is not necessary. I'm lazy. I can't think of any good enough reasons to have to make two changes when I'm forced to update the signature of a function that is only used internally. — user4581301, Jun 06 '18 at 18:23

score 2 · Answer 1 · answered Jun 06 '18 at 17:30

Is there a reason why it might not be good practice to place C/C++ function prototypes in header files but instead place them at the top of the main .c/.cpp file?

The only time that makes sense to me is when the functions are implementation details of other functions. Of course, in that case, I prefer to define them in the file scope using file scoped functions using static qualifier or putting them in a namespace specific to the .cpp file.

For all other functions, I find it hard to justify not putting the declaration in a header file and #includeing the header file in the files that use the functions and in the file that defines the functions.

When to not put C function prototypes in header files

1 Answers1